Single-dose BNT162b2 mRNA COVID-19 vaccine significantly boosts neutralizing antibody response in health care workers recovering from asymptomatic or mild natural SARS-CoV-2 infection

Objectives: To measure SARS-CoV-2 neutralizing antibody (NtAb) titres in previously infected or uninfected health care workers who received one or two doses of BNT162b2 mRNA COVID-19 vaccine. Methods: NtAbs were titrated as dose-inhibiting 50% virus replication (ID50) by live virus micro-neutralization. We evaluated 41 health care workers recovering from mild or asymptomatic infection at first vaccination dose (T1_inf) and 21 days later (T2_inf). Sixteen uninfected health care workers were evaluated 20 days after first dose (T2_uninf) and 20 days after second vaccine dose (T3_uninf). Results: At T2_inf, but not at T1_inf, there was a significant correlation between days from diagnosis (median 313, interquartile range 285-322) and NtAb levels (P = 0.011). NtAb titres increased at T2_inf with respect to T1_inf (1544 (732-2232) vs 26 (10-88), P < 0.001). Similarly, there was a significant increase in NtAb titres at T3_uninf compared with T2_uninf (183 (111-301) vs 5 (5-15), P < 0001). However, NtAb levels at T2_inf were significantly higher than those at T2_uninf and T3_uninf (P < 0.0001 for both analyses). Conclusions: A single vaccination in people with mild or asymptomatic previous infection further boosts SARS-CoV-2 humoral immunity to levels higher than those obtained by complete two-vaccination in uninfected subjects. (C) 2021 The Author(s). Published by Elsevier Ltd on behalf of International Society for Infectious Diseases.