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Hybrid Immunity in SARS-CoV-2: Are Antibody Responses Similar in all Infected and AZD1222 Vaccinated Persons?

Introduction: There is a diversity in population regarding the number and doses of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) vaccines and past infection status and the antibody titres may be different across various groups. The antibody titres determined in the same time-frame after the immune evoking event may give clues regarding the prioritisation for boosters and factors causing variability in titres. Aim: To compare and assess the Immunoglobulin G (IgG) antispike (S) antibody titres among the Healthcare Workers (HCWs) with history of Adenovirus vector based vaccine AZD1222 (Covishield) and infections, in different orders. Materials and Methods: An observational cross-sectional cohort study was conducted in a tertiary care centre during November 2021 to December 2021. The antibody titres of a healthy cohort of HCWs (n=178) who were either double-vaccinated with no history of SARS-CoV-2 infection or vaccinated but along with a history of SARS-CoV-2 infection were determined six weeks after the last event (infection/vaccination). They were grouped based on the order of vaccination (V) and infection (I). Results: The major groups were group 1 (V+V), group 2 (I+V+V), group 3 (V+ V+ I) and group 4 (V+I+V). The highest titres of Anti-S IgG antibody observed in vaccinated with breakthrough infection group 3-V+V+I (n=71) {20662(10853-34744)}. The group with double vaccination but with no history of infection {group 1-V+V (N=49)} had the lowest titres - {2395(844.4-7443)}. The hybrid immunity group (those who had infection which was followed by vaccination) group 2 (I+V+V) had titres 4241 (2220-7373) and group 4 (V+I+V) had titres 6542 (3772-11700) which were lower than those with breakthrough infection. Conclusion: Anti-S antibody titres are highest among vaccinated with breakthrough infections and lowest in those with two doses of vaccines but no history of previous confirmed infections and booster doses may be prioritised for the second group. The timing of previous infection can also be a criterion for further booster doses.