Safety and immunogenicity of different booster vaccination schemes for COVID-19 used in El Salvador

Abstract

Purpose. The effectiveness of COVID-19 vaccination schemes and the combination of vaccines of various platforms for administering booster doses is still being studied since it will depend on the population response to vaccines. We aimed to evaluate the safety, protection and immunogenicity of the Salvadorean population´s third dose booster COVID-19 vaccine and the potential benefit of homologous vs heterologous regimens.Materials and methods. This is an analytical observational cohort study in a population aged 18 to 65 that was primarily vaccinated with AstraZeneca, Sinovac or Pfizer/BioNTech. Volunteers were recruited (n=223) and followed up for 3 months after receiving the 3rd vaccine (BNT162b2) as a booster. Adverse reactions were monitored, serum anti-Spike IgG was assessed by chemiluminescence and PCR was carried out when subjects developed clinical signs. Results. The cohorts finally included 199 participants and we observed only mild adverse effects in all cohorts. A significant increase in specific IgG levels was found after the booster dose in all cohorts. The heterologous scheme with Sinovac showed the greatest increase in antibody titer and a decrease was observed in all participants after 3 months. During the follow-up period, 30 participants showed symptomatology compatible with COVID-19, but only 4 were laboratory-confirmed and they showed mild clinical signs. Conclusion. These findings indicate that the booster dose used were safe and promoted an immediate increase in immunogenicity that decreased over time. The heterologous regimen showed stronger immunogenicity than the mRNA-based homologous scheme.