India reported over 10 million COVID-19 cases and 149,000 deaths in 2020 . To estimate exposure and the potential for further spread, we used a SARS-CoV-2 transmission model fit to seroprevalence data from three serosurveys in Delhi and the time-series of reported deaths to reconstruct the epidemic . The cumulative proportion of the population estimated infected was 48.7% (95% CrI 22.1% - 76.8 %) by end-September 2020 . Using an age-adjusted overall infection fatality ratio (IFR) based on age-specific estimates from mostly high-income countries (HICs), we estimate that 15.0% (95% CrI 9.3% - 34.0 %) of COVID-19 deaths were reported . This indicates either under-reporting of COVID-19 deaths and/or a lower age-specific IFR in India compared with HICs . Despite the high attack rate of SARS-CoV-2, a third wave occurred in late 2020, suggesting that herd immunity was not yet reached . Future dynamics will strongly depend on the duration of immunity and protection against new variants.