Impact of BNT162b2 primary vaccination and homologous booster on anti-SARS-CoV-2 IgA antibodies in baseline seronegative healthcare workers

Objectives We investigated here the response of anti-SARS-CoV-2 IgA antibodies to BNT162b2 primary vaccination followed by administration of a homologous booster dose in baseline seronegative healthcare workers. Methods The study population included 69 healthy recipients of primary BNT162b2 vaccination (two doses) followed by administration of a single homologous booster after 8 months. Blood samples were collected throughout the study, starting before the first vaccine dose, up to 1 month after the booster. The serum levels of anti-SARS-CoV-2 IgA were assayed with Euroimmun Anti-SARS-CoV-2 spike S1 ELISA IgA. Results A first peak of serum anti-SARS-CoV-2 IgA was seen 1 month after the second BNT162b2 dose, after which values gradually declined, with stabilization after 6 months. The BNT162b2 booster (third dose) elicited a second peak, comparable to that observed 1 month after the second dose (p=0.100). Highly significant correlation was found between pre- and post-booster anti-SARS-CoV-2 IgA serum values (r=0.41; p<0.001), whilst no significant correlation was observed with age (r=0.10; p=0.416) or sex (r=0.04; p=0.729). The rate of SARS-CoV-2 IgA seropositive recipients increased from 0% before vaccination to 80 and 97% after the first and second vaccine dose, but then declined becoming 74% at 3 months and 54% at 6 months, respectively, after which stabilization was reached. The BNT162b2 booster dose restored the seropositivity rate to 99%. Conclusions These results support the suggestion that vaccine boosters may be advisable after 3 months from primary vaccination to restore IgA to protective levels, especially in those at higher risk of SARS-CoV-2 infection and complications.