Background Real-world population studies have shown waning immunity, over time, after receiving the two doses of the BNT162b2 COVID-19 vaccine. Studies reporting the long-term humoral response are important to drive future vaccination strategies like the introduction of the booster dose. Yet, available literature on long follow-up periods is scarce. Covidiagnostix is a multicenter study aiming to assess the antibody response in >1000 healthcare professionals (HCPs) who received the BNT162b2 vaccine. Methods Serum was tested at time-0 (T0), before the first dose and then at T1, T2, T3 and T4, respectively, 21, 42, 177 and 302 days after T0. Antibodies against the SARS-CoV-2 nucleocapsid-protein were measured to assess SARS-CoV-2 infections, whereas antibodies against the receptor-binding domain of the spike protein were measured to assess vaccine response. Results The antibody titer observed 10 months post-vaccination showed a decrease of approximately 80% from the peak measured at T2, yet the median titer of the seronegatives HCPs was still higher than seropositives before vaccination. We identified 12 post-vaccination infected HCPs within 6 months after receiving the first dose and another 12 post-vaccination infected HCPs between 6 and 10 months post-vaccination. Conclusion Vaccination induced a humoral response which is well detectable even 10 months post-vaccination. Yet a high anti-spike serum antibody titer does not guarantee protection from infection. Differences in symptomatology between SARS-CoV-2 infections occurred within the first 6 months post-vaccination and the following 4 months, and differences in COVID-19 prevalence and vaccination coverage observed in these two time intervals were consistent with a decrease in vaccine efficacy 6 months after receiving the first dose.
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