Immunogenicity of fractional and standard dose COVID-19 vaccine boosters among healthy adults in Indonesia: twenty four month follow-up from a randomised controlled trial

Sep 29, 2025Nature communications

Immune response to smaller and full COVID-19 vaccine booster doses in healthy adults in Indonesia over 24 months

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Abstract

Of 1,289 trial participants, 962 (75%) completed follow-up to 24 months after receiving booster doses.

  • Among participants initially vaccinated with CoronaVac, immune responses were lower with fractional doses of Pfizer compared to standard doses at 28 days.
  • Differences in immune responses between fractional and standard doses diminished by 12 and 24 months.
  • Participants primed with AstraZeneca demonstrated lower initial immune responses compared to those primed with CoronaVac after receiving Pfizer and AstraZeneca boosters.
  • Breakthrough infections were reported in over 40% of participants between six and 12 months, and in over 35% between 12 and 24 months.
  • Both fractional and standard doses of AstraZeneca and Pfizer boosted immune responses among participants primed with either CoronaVac or AstraZeneca.

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Key numbers

3740 BAU/ml
Initial Immune Response ()
of binding antibodies in participants boosted with Pfizer fractional doses at 28 days.
>40%
Rate
Percentage of participants experiencing breakthrough infections between 6 and 12 months.
4969 BAU/ml
Antibody Response Comparison
of binding antibodies in participants boosted with Pfizer standard doses at 28 days.

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