Booster and anergic effects of the Covishield vaccine among healthcare workers in South India

Covishield (same as ChAdOx1) vaccine was rolled out in January 2021 against SARS-CoV2 in India. Although studies show good efficacy after two doses, there is limited data on the fate of the elicited antibody responses over time in groups with or without prior exposure to SARS-CoV2. Therefore, in this study we proposed to test naive or previously exposed healthcare workers (HCWs) longitudinally after both doses for anti-SARS-CoV2 spike antibody (ASSA) levels. Serum samples were collected from 205 HCWs at days 14 and 28 after first dose, and at days 14, 28 and 3-months after second dose. ASSA levels were quantitated by ECLIA method. Non-responder rate was 17% (35/205) on day 14 and 2% (5/205) on day 28 after the first dose. After the second dose, the responder rate was 100%. Non-responder rate was significantly higher among males (p<0.00001) and senior citizens (p=0.008). The second dose boosted a 27-fold increase in the COVID-19 naive (CN) group, but caused a 1.5-fold decline in the previously exposed groups. By three months, the antibody levels declined 3-4-folds in all the groups. In spite of high antibody levels (GM-1007 U/ml) after the second dose, 14% developed mild breakthrough infections (BTI). The booster effect was significantly higher when given 10-14 weeks later. The responder rate for Covishield was 98% after first dose and 100% after second dose. The vaccine elicited a prime-boost effect in CN HCWs and a boost-anergy effect in the previously exposed HCWs. ASSA levels began to decline proportionately by three months.