Immunological memory in infected & exposure näive individuals one year post SARS-CoV-2 vaccination.

Background & objectives COVID-19 has affected millions and wreaked havoc on global healthcare systems as well as a devastating impact on the economies of various nations. Vaccines are highly pivotal in promoting an appropriate immune response. Understanding the effectiveness and stability of the vaccines is essential as these may differ across populations. Hence, this study explored the durability and efficacy of the COVISHIELD (ChAdOx1 nCoV-A) vaccine among healthcare workers (HCWs). Methods In this study, 84 HCWs who received two doses of COVISHIELD and had no breakthrough infections or precautionary doses were assessed. Participants were categorised into four groups (A, B, C, and D) based on pre-immunisation antibody status and SARS-CoV-2 RT-PCR results. Group A had prior infection and IgG antibodies, group B had RT-PCR positivity without detectable antibodies, group C had IgG antibodies without RT-PCR testing, and group D was exposure-näive. Humoral immunity was assessed by measuring IgG antibodies to the spike protein, while cell-mediated immunity was evaluated through SARS-CoV-2-specific T-cell markers and immunophenotyping. Results Over one year, a significant decrease in anti-spike IgG levels was observed in all groups (P<0.0001). It was observed that memory B cells declined significantly among all the groups over a period of one year [group A (P<0.0001), group B (P=0.0080), group C (P=0.0158), and group D (P=0.0004)]; no significant decrease inactivated T-cells was observed over a year. The mean of anti-spike IgG levels in samples from exposed participants (group A, B, C) versus non-exposed (group D) was significantly higher (P<0.0001). Interpretation & conclusions These findings indicate the need for regular booster doses of vaccination due to the waning of immunity by the vaccine. We also demonstrate that hybrid immunity (a combination of immune response post-natural infection and vaccination) provides better protection than vaccination alone.