Community prevalence of antibodies to SARS-CoV-2 and correlates of protective immunity in five localities in an Indian metropolitan city

ObjectivesTo assess seroprevalence of anti-SARS-CoV-2 antibodies in a densely populated urban Indian settings and its implications for disease trends and protective immunity. DesignCross-sectional sero-epidemiological survey linked with administrative reporting of COVID-19 testing data. SettingsPune city in western India Main outcome measurePrevalence of anti-SARS-CoV-2 spike protein antibodies were estimated and along with correlates of virus neutralisation and other immune and inflammatory markers. ResultsSeropositivity was extensive (51{middle dot}3%; 95%CI 39{middle dot}9 to 62{middle dot}4) but varied widely in the five localities tested, ranging from 35{middle dot}8% to 66{middle dot}4%. Seropositivity was higher in crowded living conditions in the slums (OR 1{middle dot}91), and was lower in those 65 years or older (OR 0{middle dot}59). The infection-fatality ratio was estimated at 0.21%. Post survey, COVID-19 incidence was lower in areas noted to have higher seroprevalence. Substantial virus-neutralising activity was observed in seropositive individuals, but with considerable heterogeneity in the immune response and possible age-dependent diversity in the antibody repertoire. ConclusionDespite crowded living conditions having facilitated widespread transmission, the variability in seroprevalence in localities that are in geographical proximity indicates a heterogenous spread of infection. Declining infection rates in areas with high seropositivity suggest population-level protection. It is also supported by substantial neutralising activity in asymptomatically infected individuals. This is the first report of a significantly high proportion of protective immune response among asymptomatic individuals in the population. The heterogeneity in antibody levels and neutralisation capacity indicates the existence of immunological sub-groups of functional interest. Trial registrationRegistered with the Clinical Trials Registry of India (CTRI/2020/07/026509)