Background: Since serum anti-SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) IgA antibodies correlate with secretory anti-SARS-CoV-2 IgA and contribute to virus neutralization, we planned an observational study to measure serum anti-SARS-CoV-2 IgAs kinetics throughout a 6-month period in coronavirus disease 2019 (COVID-19) vaccine recipients.Methods: The study sample consisted of 97 baseline SARS-CoV-2 seronegative healthcare workers (median age 42 years and IQR 31-52 years; 52 females), who underwent vaccination with Pfizer/BioNTech Comirnaty mRNA-based vaccine (two 30 µg doses, 21 days apart). Serum samples were collected at baseline, before the second vaccine dose (i.e., day 21), and then 51, 111 and 201 days after enrolment (i.e., 1, 3 and 6 months after the second vaccine dose). Serum anti-SARS-CoV-2 spike S1 subunit IgA were measured with Anti-SARS-CoV-2 ELISA IgA (Euroimmun, Lübeck, Germany).Results: Anti-SARS-CoV-2 spike S1 subunit IgA displayed a peak at 1 month after the second vaccine dose, but then progressively waned afterwards. The 6-month serum anti-spike S1 subunit IgA concentration was 71% lower than the peak concentration. The rate of subjects with positive IgA values was 0% at baseline, 80.4% at day 21, 97.9% at day 51, but then declined to 73.2% and 53.6% at 3 and 6 months after the second vaccine dose. Serum anti-spike S1 subunit IgAs measured at 111 and 201 days was significantly lower than at the 51-day peak (both p<0.001). Significant inverse correlation was found between anti-SARS-CoV-2 IgA antibodies decline at 6 months/ and recipients’ age (r=-0.24; p=0.019).Conclusion: These findings may provide possible explanation to decreased efficacy of COVID-19 vaccines in preventing SARS-CoV-2 infection 6 months after vaccination.