Inactivated SARS-CoV-2 vaccine dosing schedules and antibody responses in adults aged 60 to 80 years

BackgroundAdults aged ≥60 years face elevated risks of severe COVID-19 and long COVID. Although inactivated SARS-CoV-2 vaccines are safe and effective, critical gaps remain regarding optimal booster timing, durability of immune responses, and protection against emerging variants in this vulnerable population.MethodsIn this prospective cohort study in Zhejiang Province, China, 450 adults aged ≥60 years were randomized to receive an inactivated SARS-Cov-2 vaccine (CoronaVac or Covilo) under three dosing schedules. Neutralizing antibody titers, SARS-CoV-2-specific IgG, and variant-specific neutralization were evaluated using serum samples collected at multiple timepoints. Safety outcomes included local and systemic adverse reactions and long COVID symptoms.ResultsParticipants (age: 60–80 years; 50.9% male) with balanced baseline demographics were stratified into six subgroups by vaccine type and schedule (S1/S2/S3; n=75 each; all P>0.05). Adverse reaction incidence was low (0.0%–8.0%), with CoronaVac-S3 recipients experiencing significantly more local reactions than Covilo-S3 recipients (8.0% vs. 0.0%; P = 0.0124). Immunogenicity varied markedly at 1 month post vaccination, with the highest geometric mean titers (GMTs) of neutralizing antibodies in both S3 groups (Covilo: 57.2; CoronaVac: 59.4; P