Immunologic assessment of the impact of SARS-CoV-2 vaccine booster doses on humoral immunity: a cross-sectional study in morocco

To assess the impact of the SARS-CoV-2 booster dose on the immune response against COVID-19, we conducted a cross-sectional study in the Casablanca-Settat region of Morocco. The study included 2,802 participants from 16 provinces, all of whom had received three doses of a SARS-CoV-2 vaccine. IgG antibodies targeting the S1 RBD subunit of the SARS-CoV-2 spike protein were quantified using the SARS-CoV-2 IgG II Quant assay and measured on the Abbott Architect i2000SR instrument. Adjusted seroprevalence of anti-RBD antibodies showed that participants who received two doses of ChAdOx1-S followed by a BBIBP-CorV booster had a seroprevalence rate of 99.68% (95% CI: 99.39-99.83%), while those who received a BNT162b2 booster had a rate of 99.67% (95% CI: 99.38-99.82%). Both rates were higher than those observed with other vaccination regimens. The evaluation of booster dose effects revealed significant differences in anti-RBD antibody levels across various vaccination regimens: two doses of BBIBP-CorV compared to three doses of BBIBP-CorV (P < 0.0001), two doses of BNT162b2 versus three doses of BNT162b2 (P = 0.023), two doses of ChAdOx1-S versus two doses of ChAdOx1-S with a BNT162b2 booster (P = 0.023), and two doses of BBIBP-CorV versus two doses of BBIBP-CorV with a BNT162b2 booster (P < 0.0001). However, no significant difference was found between two doses of ChAdOx1-S and three doses of ChAdOx1-S (P = 0.23). Participants with prior SARS-CoV-2 exposure who received two doses of ChAdOx1-S followed by either a BBIBP-CorV or BNT162b2 booster showed higher levels of anti-RBD IgG antibodies (P = 0.042 and P = 0.001, respectively). Interestingly, individuals with comorbidities who received the BNT162b2 booster dose exhibited a significantly stronger humoral response (P < 0.05). In conclusion, our findings highlight the effectiveness of the BNT162b2 booster dose in eliciting a strong immune response. The high seroprevalence rates achieved with both BNT162b2 and BBIBP-CorV boosters reflect the country's robust vaccination strategy.