Antibody response following the fourth SARS-CoV-2 vaccine dose during the Omicron wave in Brazil.

We investigated the impact of the fourth dose with ChAdOx1 nCoV-19 (AstraZeneca) in the humoral immune response to SARS-CoV-2 during a 9-month follow-up period in which Omicron was the predominant variant in Brazil. IgG for the SARS-CoV-2 spike protein (S) and nucleocapsid (N) proteins were analyzed in samples collected before and after the fourth dose. All participants were tested monthly for SARS-CoV-2 infection by RT-qPCR. The antibody response induced by the fourth dose of the coronavirus disease 2019 vaccine was evaluated and compared with the response induced by the second and third doses. The additional antibody response to the viral S protein after the fourth dose was smaller than those after the third vaccine dose. In contrast, an increase in the N IgG levels could be observed after the fourth dose compared to other vaccine doses. In the comparison of the antibody response before and after the fourth dose, an increase in both S-and-N IgG was noted, mainly in the positive qPCR group. We did not observe a significant decline in IgG levels after the fourth dose, as observed after the second and third doses, therefore, a sustained humoral response to both S and N proteins seems to be achieved.

Comparison of Adverse Events and Antibody Responses Among Different COVID-19 Vaccination Schedules.

Global investment in developing COVID-19 vaccines has been substantial, but vaccine hesitancy has emerged due to misinformation. Concerns about adverse events, vaccine shortages, dosing confusion, mixing vaccines, and access issues contribute to hesitancy. Initially, the WHO recommended homologous vaccination (same vaccine for both doses), but evolving factors led to consideration of heterologous vaccination (different vaccines). The study compared reactogenicity and antibody response for both viral protein spike (S) and nucleocapsid (N) in 205 participants who received three vaccination regimens: same vaccine for all doses (Pfizer), two initial doses of the same vaccine (CoronaVac or AstraZeneca), and a Pfizer booster. ChAdOx1 and BNT162b2 vaccines were the most reactogenic vaccines, while CoronaVac vaccine was the least. ChAdOx1 and BNT162b2 achieved 100% of S-IgG seropositivity with one dose, while CoronaVac required two doses, emphasizing the importance of the second dose in achieving complete immunization across the population with different vaccine regimes. Pfizer recipients showed the highest S-IgG antibody titers, followed by AstraZeneca recipients, both after the first and second doses. A third vaccine dose was essential to boost the S-IgG antibodies and equalize the antibody levels among the different vaccine schedules. CoronaVac induced N-IgG antibodies, while in the Pfizer and AstraZeneca groups, they were induced by a natural infection, reinforcing the role of N protein as a biomarker of infection.