Do Long COVID and COVID Vaccine Side Effects Involve Similar Biological Processes?

Long COVID is often categorized into distinct phases based on symptom duration: the "post-acute phase," defined by the persistence of symptoms beyond three weeks following initial SARS-CoV-2 infection, and the "chronic phase," characterized by symptoms extending beyond 12 weeks post-infection [8]. The symptoms of long COVID, like for acute COVID-19, result from damage to various organs and systems such as the respiratory, cardiovascular, neurological, endocrine, urinary, and immune systems. Several studies have documented multi-organ involvement in individuals with COVID-19. A prospective investigation conducted in individuals categorized as low-risk assessed six major organs, namely, the heart, lungs, liver, kidneys, pancreas, and spleen, and identified that 70% of the 201 participants exhibited dysfunction in at least one organ, whereas 29% showed involvement of multiple organs, indicating widespread multi-organ impairment following SARS-CoV-2 infection [9].

Long COVID has been linked to an array of pathological conditions affecting multiple systems, including cardiovascular, thrombotic, and cerebrovascular diseases [10]; metabolic disturbances such as type 2 diabetes [11]; and post-viral syndromes characterized by persistent fatigue, notably myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) [12,13], dysautonomia, and heart dysfunctions, including postural orthostatic tachycardia syndrome [14]. To date, over 200 distinct symptoms have been documented, affecting multiple organ systems and reflecting the multisystemic nature of the condition [15]. The list of long COVID symptoms includes fatigue, headaches, dyspnea, sore throat, persistent cough, and smell loss; cardiac abnormalities such as myocarditis, microvascular angina, cardiac arrhythmias, and blood pressure abnormalities; cognitive and attention impairments and severe neurological alterations; sleep disturbances; and muscle pain (Table 1) [8,16,17,18]. Multi-system complaints were also reported as long COVID symptoms, including ongoing fever and gastroenterological symptoms [8]. A recent meta-analysis identified fatigue (58%), headache (44%), cognitive impairment (27%), alopecia (25%), and dyspnea (24%) as the five most commonly reported symptoms associated with long COVID [19].

The recovery from a mild SARS-CoV-2 infection usually takes 7–10 days from the onset of symptoms, but severe sickness recovery can take between 3 to 6 months. Numerous studies have projected that individuals recovering from COVID-19 following intensive care unit (ICU) admission may experience persistent symptoms, potentially enduring for several years [51]. Additional evidence indicates that individuals with long COVID are at an elevated risk of developing serious chronic conditions over time, as reflected by post-COVID-19 diagnoses of major adverse cardiovascular complications, chronic kidney and liver pathologies, and various respiratory disorders [52,53]. As mentioned, long COVID symptoms can last for years, and particularly in cases of new-onset ME/CFS and dysautonomia, which are expected to be lifelong [54]. Long COVID has been associated with substantial reductions in work capacity, with many affected individuals unable to return to employment, which is contributing to labor shortages and strongly affects the economy [15,55].

Until today there are currently no validated effective treatments against long COVID.

Statistically significant predictors of long COVID have been proven to be significant: Arjun MC et al. (2022) have shown that several factors are independently associated with an increased risk of developing long COVID in 371 individuals with a median follow-up of 44 days. These include a greater number of symptoms during the acute infection phase (adjusted odds ratio [aOR] = 11.24; 95% confidence interval [CI]: 4.00–31.51), receipt of two doses of COVID-19 vaccine prior to infection (aOR = 2.32; 95% CI: 1.17–4.58), increased severity of the initial illness (aOR = 5.71; 95% CI: 3.00–10.89), and hospitalization during the acute phase (odds ratio [OR] = 3.89; 95% CI: 2.49–6.08) [56]. It is important to note here that receiving two doses of COVID-19 vaccine increased by 2.32 the risk of having long COVID, indicating a possible link between long COVID and vaccine side effects. Potential risk factors for long COVID also include female sex, type 2 diabetes, the presence of specific autoantibodies [57], connective tissue disorders [58], attention deficit disorders, chronic urticaria, and allergic rhinitis [23]. However, approximately one-third of individuals with long COVID do not present with any identifiable pre-existing conditions, although a third of people with long COVID have no identified pre-existing conditions [59].

In addition to these predictors, there are likely multiple potential overlapping causes which are upstream processes and biological drivers for long COVID. The following hypotheses for long COVID pathogenesis are proposed.

Some authors have proposed to refer to long COVID as post-acute COVID-19 syndrome (PACS) [6]. Post-acute COVID-19 syndrome (PACS) represents a heterogeneous condition characterized by a multifactorial etiology, involving diverse and potentially overlapping pathophysiological mechanisms. Distinct PACS subtypes may be delineated based on symptom profiles, severity, and temporal onset patterns [6]. It is also recognized that post-COVID-19 vaccination syndrome (PCVS) exists as shown by thousands of publications today on COVID vaccine side effects [7,116,117,118]. It is essential to differentiate between immediate adverse events following COVID-19 vaccination, termed acute COVID-19 vaccination syndrome (ACVS), which include rapid-onset hypersensitivity reactions such as anaphylaxis and allergic responses occurring shortly after mRNA vaccine administration [6], and delayed-onset conditions classified as post-acute COVID-19 vaccination syndrome (PACVS), which may manifest days to months following either mRNA or adenoviral vector-based COVID-19 immunization [6]. Authors have reported that 3–5% of the vaccine recipients experienced side effect symptoms for longer than 1 week, and 0.2–1.4% for longer than 1 month [119,120].

Fatigue is considered a non-severe adverse event symptom [8,20]. However, ACVS can also include anaphylaxis, anaphylactic reactions, anaphylactoid reaction, anaphylactic shock, and anaphylactoid shock [121,122] as well as vasovagal syncope/presyncope [123] that can follow immediately after vaccination and in the worst case can lead to death [124]. In this review, we are using the general term post-COVID-19 vaccination syndrome (PCVS) as we focus on post-acute COVID-19 vaccination syndrome (PACVS), i.e., chronic manifestations only.

Besides myocarditis/pericarditis due to SARS-CoV-2 infection/COVID-19, long COVID, and PCVS, several other severe health conditions are observed, including neurological damages (Table 1).

SARS-CoV-2 has a strong tropism towards the nervous system and in particular towards the central nervous system (CNS), which is a key and worrying target in COVID-19 as well as in long COVID [7,151,152,153]. Various studies indicate that the virus can traverse the olfactory or vagus nerve and penetrate the blood–brain barrier (BBB) [151,154,155]. Additionally, it can access the central nervous system via the blood–cerebrospinal fluid (CSF) barrier, which is more permeable than the BBB and consists of a single layer of endothelial cells from the choroid plexus [156,157]. Consequently, neurological and neurocognitive manifestations are frequently reported in long COVID, encompassing a broad spectrum of symptoms such as deficits in memory and executive function, sensory disturbances, paresthesia, vertigo, gait instability, photophobia, phonophobia, olfactory and gustatory dysfunction (including anosmia, hyposmia, or phantosmia), and features of autonomic nervous system dysregulation, all of which can significantly impair daily functioning [16,158].

When the virus is present in the brain, inflammation is triggered through the interaction between the spike protein and ACE2 receptors, which are expressed in the human CNS, particularly in the spinal cord, spinal ganglion, brainstem nigra, choroid plexuses, hypothalamus, hippocampus, middle temporal gyrus, and posterior cingulate cortex [154,159]. Evidence from both human studies and murine models of long COVID has demonstrated widespread cellular dysregulation across multiple immune and neural lineages, persisting even after mild infections. These alterations include pronounced microglial activation, analogous to neuroinflammatory processes observed in post-chemotherapy cognitive impairment ("chemo-brain" or "chemo-fog"), accompanied by disrupted hippocampal neurogenesis, reductions in oligodendrocyte populations, and significant myelin degradation [160]. Given that activation of hippocampal microglia has been implicated in virus-associated cognitive dysfunction [161], this mechanism could potentially underlie the neuropsychiatric manifestations observed in certain individuals following SARS-CoV-2 infection, including impairments in memory, excessive daytime sleepiness, persistent fatigue, and disturbances in sleep regulation such as insomnia [162].

Moreover, both the SARS-CoV-2 spike protein and the one introduced through vaccination can reduce serotonin levels by interacting with ACE2, potentially contributing to or worsening depression and even suicidal tendencies [163]. Long COVID has been associated with a range of audiovestibular disturbances, encompassing symptoms such as persistent tinnitus, varying degrees of hearing impairment, and episodes of vertigo [16,164]. Also, neuronal atrophy and degeneration of cranial nerves, including the olfactory nerve and the neighboring olfactory bulb, have previously been reported in patients with persistent hyposmia or anosmia after acute COVID-19 [8]. In a cohort of 23 patients with persistent anosmia due to long COVID, neuroimaging revealed structural alterations in the olfactory system. Specifically, olfactory cleft opacification, particularly in the mid and posterior segments, was observed in 73.9% of cases. Moreover, 43.5% exhibited reduced olfactory bulb volumes, and 60.9% showed shallowing of the olfactory sulci, consistent with COVID-19-associated olfactory neurodegeneration [165].

In agreement with these findings, patients with long COVID exhibited lower metabolic activity in their brain [158]. Vagus nerve stimulation mediating anti-inflammatory responses highlights the relationship between SARS-CoV-2 infection and the parasympathetic nervous system (PNS) and suggests the autonomic nervous system (ANS) as a therapeutic target. Of note, vagus nerve dysfunction has also been reported in SARS-CoV-2 infection and proposed as a key pathophysiological hallmark of long COVID [63].

Also, signs of peripheral nerve and muscular system dysfunction were also reported after SARS-CoV-2 infection. Persistent manifestations such as muscle pain, reduced physical endurance or exercise intolerance, abnormal sensory perceptions including paresthesia and neuropathic pain, as well as autonomic nervous system dysfunction have been consistently associated with long COVID in affected individuals [166].

After receiving COVID-19 vaccines, a significant number of healthy individuals develop PCVS with a variety of neurological complications and long-lasting neuropsychiatric symptoms similar to those experienced by a large proportion of COVID-19 survivors [22,167,168,169]. As with the SARS-CoV-2 virus, it has been demonstrated that both COVID vaccine mRNA and free spike proteins could enter the brain via the bloodstream and by penetrating the BBB [170,171]. Bahl et al. had already shown that the mRNA/LNP platform could reach the brain, as also specified by the European Medicines Agency (EMA) for up to 2–4% of the mRNA plasma concentration [172,173,174].

Neurological complications post-COVID vaccination include fatigue; headache; immune encephalitis; cerebral sinus venous thrombosis (VST); transverse myelitis (TM); Guillain–Barré syndrome and optic neuritis; ischemic stroke; intracerebral bleeding; hypophysitis; epilepsy; hyperactive encephalopathy; and acute, disseminated encephalomyelitis [21,22]. Transverse myelitis (TM) has been documented after COVID vaccination [21,175,176,177,178] as well as in COVID-19 and long COVID [179,180,181,182]. A review encompassing 28 studies identified 31 cases of post-COVID-19 vaccination-associated TM involving 17 females and 14 males with a mean age of 52 ± 19 years. Most cases followed the first vaccine dose (n = 24), and the Oxford-AstraZeneca (ChAdOx1 nCoV-19) vaccine was most frequently implicated (12 cases), followed by Pfizer [8], Moderna [7], Sinopharm [3], and Janssen [1,6]. Clinical outcomes were favorable (Modified Rankin Score < 3) in 21 patients. Notably, the incidence of post-vaccination TM appears considerably lower than that reported after SARS-CoV-2 infection, which is approximately 0.5 cases per million per year. Interestingly, a comparative analysis of 30 articles detailing 65 unique cases were included, of which 48 (73.8%) were infection TM and 17 (26.2%) revealed no substantial differences in latency to symptom onset, clinical or imaging characteristics, or recovery profiles between TM following SARS-CoV-2 infection and that occurring after COVID-19 vaccination, implying a potentially shared pathogenic mechanism [182]. Notably, about 80% of affected individuals experienced favorable neurological outcomes.

Venous sinus thrombosis (VST) represents a neurological complication associated with SARS-CoV-2 vaccination [183]. It is identified as the third most common adverse event related to vaccination, primarily attributed to vaccine-induced hypercoagulable states [22,183]. The underlying mechanisms proposed include direct platelet activation or indirect endothelial activation, both promoting a pro-thrombotic state [184].

Neurological adverse events involving the central and peripheral nervous systems (CNS and PNS) have been reported more frequently than complications affecting the cardiovascular, endocrine, renal, or dermatological systems [22]. Within the peripheral nervous system, skeletal muscle involvement has been observed in some cases [185], with manifestations such as myositis [186], dermatomyositis, immune-mediated necrotizing myopathy [185], rhabdomyolysis, and polymyalgia rheumatica.

Emerging evidence suggests that individuals with long COVID exhibit molecular signatures resembling those observed in Alzheimer's disease [187], along with the presence of neurotoxic amyloidogenic peptides capable of forming insoluble aggregates [188]. In addition, widespread neuroinflammatory responses have been documented [189], together with region-specific reductions in cerebral and brainstem metabolic activity that correlate with distinct clinical manifestations [83,190]. Furthermore, cerebrospinal fluid abnormalities have been detected even in non-hospitalized long COVID patients, with younger individuals showing a tendency toward delayed onset of neurological symptoms [191].

To summarize, possible mechanisms for the neuropathologies present in both long COVID and PCVS include neuroinflammation with microglia activation, autoimmunity, damage to blood vessels induced by coagulopathy and endothelial dysfunction, and neuron injury [7,87,158,192]. The potential link between neurodegenerative diseases and long COVID, as well as the long-term effects of COVID-19 vaccination, warrants close monitoring in the global population [168].

Telomere length in peripheral blood lymphocytes of COVID-19 patients aged 29 to 85 years has been found to be decreased in association with increased disease severity [193]. This could be linked with spike-induced aging as shown in a study which measured the biological age of 117 COVID-19 survivors and 144 uninfected volunteers. The authors observed that patients who had COVID-19 showed a marked increase in biological age averaging 10.45 ± 7.29 years (5.25 years above the range of normality) compared to an average increase of 3.68 years (±8.17 years) in uninfected individuals [194].

We suggested that this phenomenon may arise not only shortly after COVID-19 infection, particularly in cases where early treatment was not administered, but also in long COVID, and potentially following COVID-19 vaccination, especially after multiple doses, and that it is associated with inflammatory and oxidative stress-induced shortening of telomeres [7,195]. Besides, many of the symptoms of long COVID are typically seen with advancing age [26]. Our hypothesis for long COVID has been shown in a study performed on a large cohort of 3335 who found lower cognitive scores in individuals with more than 12 weeks of COVID-19 symptoms. These subjects were self-defined as having "long COVID" and met NICE "Post-COVID-19 syndrome" and WHO "Post COVID-19 condition" definitions [196,197]. These cognitive deficits were detectable nearly two years post-infection and were comparable to the effect of "an increase in age of approximately 10 years, or exhibiting mild or moderate symptoms of psychological distress".

In COVID-19, hyperinflammation and coagulopathy are triggered by both N (nucleocapsid) and S (spike) proteins [7,198,199].

Meanwhile, the spike protein alone activates various key inflammatory pathways which we have proposed may be common in COVID, long COVID, and PCVS [6,7,200]. A critical factor contributing to spike protein-associated toxicity lies in the broad distribution of ACE2 receptors across multiple tissues and fluids. ACE2 is expressed in numerous anatomical sites, including the upper and lower respiratory tract (pharynx, trachea, lungs), cardiovascular system (heart, vasculature, bloodstream), central nervous system (brain, cerebrospinal fluid), gastrointestinal tract, kidneys, and male reproductive organs. Moreover, ACE2 is detectable in various body fluids, such as mucus, saliva, urine, semen, and breast milk (Figure 1) [201]. Consequently, the spike protein can trigger inflammation across multiple organs and systems. This helps explain why most COVID-19 patients experience a range of symptoms beyond respiratory issues, including cardiovascular, neurological, gastrointestinal, and renal complications [28,202,203,204], as well as in people with long COVID and who have received COVID vaccines [6].

As already mentioned, long COVID-associated pathologies and damages can be linked to the persistence of the virus, which has not been completely eradicated, at the systemic, tissue, and/or cerebral levels [8,60,61,62,63], as viral proteins and/or RNA have been found in the reproductive system, cardiovascular system, brain, muscles, eyes, lymph nodes, appendix, breast tissue, hepatic tissue, lung tissue, plasma, stool, and urine in long COVID patients [64,65,67,68,205,206]. SARS-CoV-2 spike antigen has been detected in the bloodstream of 60% of individuals with long COVID up to one year post-infection, whereas it was undetectable among SARS-CoV-2-infected individuals without persistent symptoms [66]. As already mentioned, Tejerina F et al. (2022) found in a cohort study that 45% of COVID-19 patients with persistent symptoms had detectable plasma SARS-CoV-2 RNA [64].

However, these studies also show that long COVID symptoms in association with chronic inflammation may occur in the absence of SARS-CoV-2.

Given that the spike protein is a structural component of SARS-CoV-2 (including its variants) and is also synthesized following administration of mRNA and adenoviral vector-based vaccines, any potential spike protein-associated toxicity is relevant to both natural infection and these vaccine platforms, as they are based on the production of a very similar spike protein to the virus. Indeed, very importantly, Pfizer confirmed in a report, approved 26 December 2020, "We demonstrate that the BNT162b2 RNA sequence encodes a recombinant P2 S that can authentically present the ACE2 binding site and other epitopes targeted by SARS-CoV-2 neutralizing antibodies." [207]. It was also shown in a work published in Nature with several authors from BioNTech and Pfizer companies that "Our binding and structural analyses indicate that the BNT162b2 RNA sequence encodes a recombinant S(P2) that can authentically present the ACE2-binding site and other epitopes that are targeted by SARS-CoV-2-neutralizing antibodies… The binding of expressed and purified S(P2) to ACE2 and a neutralizing monoclonal antibody further demonstrates the conformational and antigenic integrity of this prefusion-stabilized S" [208]. In consequence, the spike protein produced by these COVID vaccines may be able to interact with AE2 receptors and to trigger the associated biochemical pathways.

Lei et al. (2021) demonstrated that exposure to a pseudovirus expressing the SARS-CoV-2 spike (S) protein, specifically the S1 subunit containing the receptor-binding domain (RBD) but lacking viral RNA, induced inflammation and tissue damage in the lungs and arteries of mice following intratracheal administration [209]. Similar effects were observed in human epithelial cells, where the spike protein alone was shown to disrupt mitochondrial function, indicating that the spike protein itself is sufficient to induce the cardiovascular injuries observed in COVID-19. In the context of vaccination, beyond the presence of free circulating spike protein, its expression on endothelial cells may further contribute to vascular complications by activating platelets and promoting coagulation, potentially resulting in thrombosis through the release of platelet factor 4 (PF4) [147,210].

Furthermore, studies have demonstrated that both mRNA and free spike proteins can enter the brain via systemic circulation and by crossing the blood–brain barrier (BBB), raising concerns about potential inflammatory responses in cerebral vasculature and neural tissue [170,171]. Bahl et al. provided evidence that vaccine-derived mRNA can localize to the brain, a finding also acknowledged in reports by the European Medicines Agency (EMA) for up to 2% of the plasmatic mRNA [172,173,174]. Of note, anosmia has been described following vaccination in COVID-19-negative subjects. This shows that symptoms identical to COVID-19 can appear and have been triggered by the spike protein alone [211]. In the context of COVID-19, long COVID, and vaccine-associated adverse events, the spike protein has been implicated in triggering central nervous system (CNS) inflammation, primarily through the activation of microglia and astrocytes, which may represent a key mechanism underlying neurological toxicity [7]. Experimental studies have demonstrated that the S1 subunit of the spike protein can induce neuroinflammation in microglia via activation of the NF-κB and p38 MAPK signaling pathways, leading to elevated production of pro-inflammatory mediators such as TNF-α, IL-6, IL-1β, and nitric oxide (NO) [212].

Moreover, the spike protein has been detected several weeks following COVID-19 vaccination, raising concerns about its potential to induce prolonged or chronic inflammation across various organs [201,213,214]. Röltgen et al. demonstrated that mRNA vaccination induces persistent germinal center reactions, with vaccine-derived mRNA and spike antigens detectable in lymphoid tissues for up to eight weeks post-vaccination in some individuals [214]. Additionally, the concentration of free spike proteins in circulation after vaccination has been reported to reach levels comparable to those observed during SARS-CoV-2 infection (up to 150 pg/mL), potentially activating ACE2 receptors, particularly in tissues with high spike protein accumulation (Figure 1) [215]. These findings suggest that vaccination could, in some cases, elicit symptoms similar to those seen in COVID-19 and long COVID, and may theoretically contribute to the development or exacerbation of inflammatory conditions over the medium to long term, including cardiovascular, neurological, oncological, and autoimmune disorders. Cosentino and Marino (2022) pointed out that adverse effects of the COVID-19 vaccines could be related to excess SARS-CoV-2 spike production in specific individuals "for too long and/or in inappropriate tissues and organs", while the probability of this occurrence "is at present unpredictable, since systemic biodistribution and disposition of the COVID-19 mRNA vaccine has so far never been considered an issue, and as a consequence it has never been studied as it would have actually deserved." [215]. Indeed, Ota N et al. (2025) provided novel findings demonstrating the presence of mRNA vaccine-derived spike protein in the cerebral vasculature of 43.8% of individuals who experienced hemorrhagic stroke, detected as long as 17 months following vaccination, in the absence of documented SARS-CoV-2 infection [44].

Regarding the importance of spike proteins lasting in the body and their long-term effects even after their removal, we present here the three immuno-inflammatory pathways which could be common to severe COVID, long COVID, and COVID vaccine side effects. We suggest that these three pathways could be responsible for long COVID and COVID side effects and explain in large part their strong similarities: the renin–angiotensin–aldosterone system, the kininogen–kinin–kallikrein (KKK) system (KKS), and the lectin complement pathway.

Various authors have summarized the impact of COVID on several organs and tissues, with a special focus on the significance of the renin–angiotensin–aldosterone system (RAAS) in the disease pathogenesis (Figure 2) [7,8,216]. The renin–angiotensin–aldosterone system (RAAS) is a key regulator of blood pressure and also contributes significantly to inflammatory processes [217]. Within this system, ACE2 exerts protective effects, including vasodilation, antioxidant activity, and anti-inflammatory actions. In contrast, angiotensin-converting enzyme (ACE) promotes vasoconstriction, oxidative stress, and pro-inflammatory responses. ACE2 mediates its protective effects by modulating levels of angiotensin II, a potent vasoactive peptide generated from angiotensin I through ACE activity. Due to the downregulation of ACE2 induced by viral binding and replication, the balance tilts in favor of the inflammation produced by angiotensin II, which is no longer degraded through the numerous mediators that result from it, especially in the lungs. This inflammation occurs through the interaction between angiotensin II and the angiotensin II type 1 receptor (AT1R) in the kidney and the vascular system [218,219]. Overall, over-activated AT1R results in vasoconstriction, hypertension, inflammation, oxidative stress, heart hypertrophy, tissue fibrosis (heart, lungs, kidneys, and liver), ageusia (loss of taste), anosmia (loss of smell), neurological dysfunctions, obesity and diabetes, and lesions in the skin [8,61].

This process leads to substantial cytokine production, including transforming growth factor-beta (TGF-β). Angiotensin II subsequently activates the nuclear factor kappa B (NF-κB) pathway, a key regulator of inflammation, which is also stimulated by other cytokines implicated in COVID-19, such as IL-1β, IL-6, TNF-α, IL-10, MCP-1, AT1, and platelet-derived growth factor beta (PDGF-B) [220,221,222].

In contrast, the ACE2/Ang-(1–7)/Mas receptor (MasR) axis represents the counter-regulatory arm of the RAAS, opposing the ACE/Ang II/AT1R axis (Figure 2). Angiotensin-(1–7) binding to MasR exerts vasodilatory, anti-proliferative, anti-fibrotic, anti-inflammatory, anti-angiogenic, and vasoprotective effects [223]. This axis suppresses inflammatory mediators such as NF-κB, IL-6, TNF-α, and IL-8 [198,224,225]. However, viral binding with ACE2 impairs this protective pathway, weakening its anti-inflammatory function.

Individuals with long COVID frequently exhibit a range of cardiovascular dysfunctions including myocardial inflammation, coronary microvascular impairment, arrhythmogenic disturbances, and blood pressure irregularities, conditions that have been mechanistically associated with disturbances in the RAAS [226]. Various authors have proposed to manage these complications with antagonists such as Telmisartan, which could re-establish cardiovascular homeostasis by blocking the actions of angiotensin II on AT1 receptors (AT1R) [216,226]. Also, other authors have proposed that pathophysiological mechanisms resulting from an unbalanced RAAS including a systemic hyperinflammatory state and coagulopathy, which are exacerbated by inflammation and oxidative stress, as well as fibrosis, may contribute to long COVID symptoms [227]. Accordingly, sustained stimulation of the angiotensin II type 1 receptor (AT1R) has been implicated in the pathophysiology of long-term sequelae following SARS-CoV-2 infection (long COVID), potentially contributing to adverse effects such as vasoconstriction, elevated blood pressure, chronic inflammation, oxidative stress, cardiac hypertrophy, fibrosis affecting multiple organs (including the heart, lungs, kidneys, and liver), as well as sensory impairments (such as anosmia and ageusia) and neurological dysfunctions [8,61]. One of the principal mechanisms proposed to underlie cardiovascular involvement in long COVID thus includes the interaction between the SARS-CoV-2 spike protein and the ACE2 receptor, subsequent ACE2 downregulation, and the cascade of tissue injury mediated by persistent immune activation and inflammatory processes [7,216].

Long COVID symptoms, whether arising after SARS-CoV-2 infection or following COVID vaccination, appear to involve intricate immune-inflammatory mechanisms, including dysregulation of the RAAS and an imbalance between bradykinin B1 and B2 receptor pathways [228]. The already mentioned ability of mRNA vaccine sequences encoding a recombinant spike protein to interact with the ACE2 binding site may permit for the vaccine-induced spike protein to subsequently destabilize RAAS in the whole body [207,208]. As previously mentioned in this review, free spike proteins found in circulation can reach every organ, even the brain by crossing the BBB, and induce inflammation in the whole body [170,171,201]. For these reasons, several other authors have assumed that the spike produced after vaccination could have a similar affinity for ACE2 as the virus, which could lead to similar undesirable effects as with COVID-19 and long COVID [213,214,229] and activate inflammation by imbalancing RAAS [230].

Infection with SARS-CoV-2 by depleting ACE2 increases the levels of des-Arg⁹-bradykinin (DABK) which is a known pulmonary inflammatory peptide factor. DABK, a bioactive bradykinin metabolite, is implicated in lung inflammation and injury via activation of bradykinin receptor 1 (B1R) on pulmonary endothelial cells. ACE2 degrades DABK, but this protective mechanism is disrupted by ACE2 downregulation following viral infection, resulting in uncontrolled activation of this pro-inflammatory pathway [231]. Notably, the interplay between the renin–angiotensin system, the kallikrein–kinin system, and the inflammatory and coagulation cascades is a key driver of COVID-19 pathophysiology.

Several studies have examined how SARS-CoV-2 infection affects cardiovascular function, emphasizing the potential disruption of the kinin–kallikrein system (KKS) as a contributing factor. This dysregulation, potentially triggered by viral interactions with host pathways, has been proposed as a therapeutic target, with implications for the development of future pharmacological strategies (Figure 3) [7,198,216,226]. Hyperactivation of pro-inflammatory pathways, including excessive cytokine release and dysregulated bradykinin signaling, has been associated with severe clinical manifestations and poor outcomes in COVID-19 patients.

A machine learning-driven proteomic investigation revealed 119 proteins significantly associated with long COVID among outpatient individuals (Bonferroni-adjusted p < 0.01). Further model optimization identified two minimal protein panels, comprising nine and five proteins, that demonstrated perfect discriminatory capacity between long COVID and control groups, with both models achieving an area under the curve (AUC) of 1.00 and F1 score of 1.00, indicating outstanding diagnostic accuracy. The identified proteins (CXCL5, AP3S2, MAX, PDLIM7, FRZB) reflected widespread organ and cell type expression [232]. Reduced ACE2 function is linked to activation of the des-Arg⁹ bradykinin (DABK)/bradykinin receptor B1 (BKB1R), potentially increasing neutrophil infiltration and the release of proinflammatory cytokines such as CXCL5 [231,233]. This latter mechanism is consistent with the elevated CXCL5 in long COVID outpatients found by this proteomic study [232]. Moreover, in a murine model of long COVID (K18-hACE2 mice), Sriramula S et al. (2023) found that elevated B1R expression may drive the long-lasting inflammatory response in the brain, driving chronic inflammation and ultimately cognitive and neuropsychiatric symptoms [234]. It is also noteworthy that several symptoms commonly reported in COVID-19, such as fatigue, nausea, gastrointestinal disturbances, and headaches, are similarly observed in disorders characterized by elevated bradykinin levels and increased vascular permeability, such as pulmonary angioedema [216,235,236].

Also, one hypothesis for the activation of KKS in long COVID is that a dormant SARS-CoV-2 infection of endothelial cells could lead to low-level expression and release of COVID-19 proteins on the cell surface, thereby perpetuating KKS and complement system activation [237]. It is also possible that this pathway remains overactivated by a prolonged AE2/ACE imbalance even in the absence of the spike protein.

As for RAAS activation, the ability of vaccine-induced spike proteins to bind with ACE2 receptors and fuse with them [207,208,213,214,229] together with the ability of mRNA and free spike proteins to reach several key organs via blood circulation make it possible for this spike protein, produced in a not fully controlled manner by COVID vaccines, to deregulate the ACE2/bradykinin B1R/DABK axis and produce DABK, subsequently resulting in inflammation similar to that observed in COVID and long COVID [170,171,201]. Interestingly, Cugno M et al. (2021) noted that after COVID vaccination, the physiopathological basis of the higher risk of urticaria/angioedema in subjects taking ACE inhibitors may be the increase in bradykinin, a vasoactive peptide that induces vascular permeability and that is catabolized by ACE [238].

Pharmacological agents modulating components of the RAAS and the KKS have been proposed as potential therapeutic strategies to mitigate long COVID manifestations and may apply as well for COVID vaccine side effects, in particular with C1 inhibitors (C1-INH) [226,236,239].

A central role for the complement system, in particular lectin complement, in the pathogenesis of COVID-19 has emerged since the beginning of the pandemic outbreak (Figure 4) [7,198,199,200,216,240].

A number of pathophysiological characteristics including microangiopathy and myocarditis have been attributed to dysregulated complement activation due to SARS-CoV-2 infection. Furthermore, the analysis of >6500 proteins in 268 longitudinal samples revealed dysregulated activation of the complement system in individuals experiencing long COVID [241], an inflammatory pathway that we have described as being key in COVID as well as vaccine side effects, due to both N (nucleocapsid) and S (spike) protein implication [7]. Indeed, spike proteins have been shown to activate this pathway [200,242,243,244]. Authors found significantly lower levels of MBL and lectin pathway activity in the sera of patients experiencing brain fog as compared to recovered COVID-19 patients without brain fog [240]. Also, Baillie et al. observed that markers of the classical, alternative, and terminal complement pathway were markedly elevated in patients with long COVID [245].

After COVID vaccination, hypercoagulability like in VST has been attributed to a direct activation of complement pathways, promoting thrombin generation [184]. In COVID-19, as well as in long COVID and vaccine side effects, the spike protein can initiate inflammation in CNS through microglia and astrocytes and be linked to complement, bradykinin, and RAAS pathways [7,246,247]. Moreover, microglial activation and complement system activation share pathological mechanisms in various neurodegenerative disorders, including Alzheimer's disease [248,249,250].

The elevated levels of pro-inflammatory cytokines in the plasma aggravate systemic inflammation and lead to acute respiratory distress syndrome (ARDS), multi-organ failure, and death. Studies have shown increased levels of cytokines, particularly IL-1β, IL-6, TNF, and IL-10, in long COVID [162,251,252], which are the same or very common with key COVID cytokines [220]. It is important to note that systemic elevation of circulating cytokines may not always be detectable, as these molecules primarily exert paracrine and autocrine effects within local tissue environments rather than through widespread distribution in the bloodstream [253]. Studies have demonstrated that S1-induced neuroinflammation in microglia involves activation of the NF-κB and p38 MAPK pathways, leading to elevated production of pro-inflammatory mediators, including TNF-α, IL-6, IL-1β, and NO [212]. Sustained activation of immune-inflammatory pathways and cytokine production have been proposed as a key contributor to the prolonged clinical manifestations observed in individuals after SARS-CoV-2 infection [97,252].

In long COVID, studies have demonstrated significantly increased concentrations of Von Willebrand Factor, platelet factor 4, serum amyloid A, α-2antiplasmin E-selectin, and platelet endothelial cell adhesion molecule-1 in the soluble part of the blood [253]. The presence of microclotting, together with relatively high levels of these inflammatory molecules known to be key drivers of endothelial and clotting pathology, points to thrombotic endotheliitis as a key pathological process in long COVID like in COVID with microthrombosis and macrovascular thrombosis. This could help the research on long COVID therapies for the identification of patient cohorts by a battery of cytokine markers that have a persistent, low-level grade of inflammation in association with often two or more troubling symptoms [254].

Post-vaccination immune profiling has revealed significantly increased concentrations of several pro-inflammatory cytokines and chemokines (including IL-4, IL-6, CCL3 (p = 0.0012), CCL5, sCD40L, IL-8, and VEGF) in symptomatic individuals relative to controls, with the magnitude and duration of this inflammatory response showing a correlation with symptom persistence [255]. CCL3 has also been shown to be associated with long COVID patients as well as IL-6, TNF-α, and IL-17 [256].

The pivotal role of oxidative stress in the pathogenesis of COVID-19 and long COVID remains underappreciated and warrants further investigation to explore potential antioxidant and anti-inflammatory therapeutic strategies [7,257,258,259,260]. In addition, oxidative stress has emerged as a significant predictor of intensive care unit (ICU) admission, with particular emphasis on reduced plasma thiol concentrations (<154 µmol/L), demonstrating a sensitivity of 79.7%, specificity of 64.6%, positive predictive value of 58.8%, and negative predictive value of 78.9%. Other oxidative stress-related markers, such as advanced oxidation protein products (AOPPs), have also been associated with increased disease severity [259,261]. The main pathogenic processes of long COVID include a systemic hyperinflammatory state and coagulopathy, which are exacerbated by inflammation and oxidative stress [260]. Increased arterial stiffness, impaired endothelial function, and sustained oxidative stress have been implicated as potential mechanisms underlying persistent cardiac abnormalities observed in individuals with long COVID [262,263].

There is a vicious cycle between oxidative stress and inflammation, and long-COVID disease progression because of thrombosis and inflammation leads to the reactivation of reactive oxygen species (ROS). Inflammation is triggered by ROS, which also affects the endothelium, causes microthrombi and neuroinflammation, stimulates the production of autoantibodies, and interferes with the synthesis of neurotransmitters [227,260]. If oxidative stress and inflammation persist for months, even in the absence of spike proteins, and is not treated appropriately, it can contribute to the multitude of symptoms seen in long COVID [7,78]. Combined antioxidant and anti-inflammatory therapy have been proposed for treating chronic inflammation in long COVID patients [264,265]. An initial investigation reported that supplementation with Pycnogenol led to significant improvements in biomarkers of oxidative stress, alongside enhanced functional status and quality of life as reflected by elevated scores on the Karnofsky Performance Scale Index [266]. Also, it has been proposed that pharmacological activation of the nuclear factor erythroid 2-related factor 2 (NRF2) pathway may enhance the expression of antioxidant enzymes involved in glutathione biosynthesis, thereby augmenting intracellular antioxidant defenses and mitigating oxidative stress through the neutralization of reactive oxygen species [254]. PCSV and COVID vaccine side effects are also strongly associated with oxidative stress and glutathione (GSH), with GSH being responsible for maintaining mitochondrial function, antiviral defense, regulation of cellular proliferation, apoptosis, DNA synthesis, microtubular-related processes, and immune responses [267].

Oxidative stress in long COVID and vaccine side effects is driven by the three inflammatory pathways described in this review.