More than two thirds of India’s population may have Covid-19 antibodies, according to a new serological survey released Tuesday, providing yet more evidence the virus may have spread far more widely than official figures suggest. About 67.6% of Indians surveyed above the age of 6 showed antibodies, according to the nationwide study, which was conducted between June and July by the government-run Indian Council of Medical Research (ICMR). The survey covered 70 districts in 21 states, with 28,975 participants. Our immune systems develop antibodies either induced by vaccination, or in response to infection. The majority of survey participants, 62%, had not received a vaccine; about a quarter had gotten their first dose. The study marks a dramatic jump from just a few months ago — at the start of the year, less than one quarter of participants were showing antibodies, according to the last national serological survey. But in the months between the two surveys, India was devastated by a massive second wave, which overwhelmed the medical system and left people dying outside hospitals, waiting for oxygen or an open bed. The wave lasted from April through June, infecting millions and killing tens of thousands. The study’s findings, combined with a slow vaccination rollout, raise concerns about the possibility of a third wave of infections, according to ICMR Director General Balram Bhargava. “More than half of the children (6 to 17 years old) were sero-positive, and sero-prevalence was similar in rural and urban areas,” he said — but “states, districts and areas without antibodies run the risk of infection waves,” meaning about 400 million people will still be vulnerable if a third wave hits. Active immunity, meaning protection against a disease, is often measured by the presence of antibodies — proteins in the blood, made by the immune system to help fight infections, acquired either through prior infection or vaccination.