Heterologous versus homologous COVID-19 booster vaccination in previous recipients of two doses of CoronaVac COVID-19 vaccine in Brazil (RHH-001): A phase 4, non-inferiority, single blind, randomised study

The LancetClemens SAC, Weckx L, Clemens R, et al. | January 25, 2022


There has been a wide use of the inactivated whole-virion SARS-CoV-2 vaccine (CoronaVac, Sinovac) in a two-dose schedule. Researchers herein examined boosted immune responses following receipt of a third dose of the homologous or a different vaccine.

  • Brazilian adults (18 years and older) in São Paulo or Salvador who had received two doses of CoronaVac 6 months previously were enrolled in RHH-001, which is a phase 4, participant masked, two center, safety and immunogenicity study.

  • A recombinant adenoviral vectored vaccine (Ad26.COV2-S, Janssen), an mRNA vaccine (BNT162b2, Pfizer–BioNTech), or a recombinant adenoviral-vectored ChAdOx1 nCoV-19 vaccine (AZD1222, AstraZeneca) was the third heterologous dose used, compared with a third homologous dose of CoronaVac.

  • A RedCAP computer randomization system was used to perform random assignment of participants (5:6:5:5) stratified by site, age group (18–60 years or 61 years and over), and day of randomization, with a block size of 42.

  • Of 1,240 participants randomly assigned to one of the four groups, 1,239 were vaccinated and 1,205 were eligible for inclusion in the primary analysis.

  • There were low antibody concentrations at 6 months after previous immunization with two doses of CoronaVac.

  • However, significant increase in binding and neutralizing antibodies occurred after all four vaccines administered as a third dose, which could improve protection against infection.

  • Following heterologous boosting, more robust immune responses were observed when compared with those observed following homologous boosting and hence heterologous boosting might increase protection.

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