Seroprevalence in Tamil Nadu through India's two COVID waves: Evidence on antibody decline following infection and vaccination (preprint)

ABSTRACT

Three rounds of population-representative serological studies through India's two COVID waves (round 1, 19 October-30 November 2020; round 2, 7-30 April 2021; and round 3, 28 June-7 July, 2021) were conducted at the district-level in Tamil Nadu state (population 72 million). State-level seroprevalence in rounds 1, 2 and 3 were 31.5%, 22.9%, and 67.1%. Estimated seroprevalence implies that at least 22.6 and 48.1 million persons were infected by the 30 November 2020 and 7 July 2021. There was substantial variation across districts in the state in each round. Seroprevalence ranged from 11.1 to 49.8% (round 1), 7.9 to 50.3% (round 2), and 37.8 to 84% (round 3). Seroprevalence in urban areas was higher than in rural areas (35.7 v. 25.7% in round 1, 74.8% v. 64.1% in round 3). Females had similar seroprevalence to males (30.8 v. 30.2% in round 1, 67.5 v. 65.5% in round 3). While working age populations (age 40-49 31.6%) had significantly higher seroprevalence than the youth (age 18-29 30.4%) or elderly (age 70+ 26.5%) in round 1, only the gap between working age (age 40-49 66.7%) and elderly (age 70+ 59.6%) remained significant in round 3. Seroprevalence was greater among those who were vaccinated for COVID (25.7% v. 20.9% in round 2, 80.0% v. 62.3% in round 3). While the decline in seroprevalence from rounds 1 to 2 suggests antibody decline after natural infection, we do not find a significant decline in antibodies among those receiving at least 1 dose of COVID vaccine between rounds 2 and 3.