4th Serosurvey of Tamil Nadu: Infection and Vaccine Generated Hybrid Immunity

Abstract

Salient findings of the fourth serological survey of SARS-CoV-2 infections conducted among the age cohort above 10 years in Tamil Nadu (India) have been described. The overall seroprevalence of 87% was reported in the survey done in December 2021, whereas in the first, second, and third surveys, the seropositivity was 32%, 29%, and 70%, respectively. The % share of the antibodies formed due to vaccination in the third and fourth surveys have the contributions of 6.2% and 28.3%, respectively as per the vaccination record of the state since the above given proportion of the state’s population (full population) was fully vaccinated (two-dose) by the time the two surveys were conducted. Age-wise break up of seropositivity of the fourth survey showed that the cohort of 10-18 years had the least (68.4%) seroprevalence and adults above 60 had the highest at 84.5%. The rate of the spread of the virus decreased due to the hybrid immunity developed by infection and vaccination. Using the vaccination and serological data, the % enhancement of the antibodies formed due to vaccination has been worked out in the real world on the ground.