NEW DELHI — India’s third nationwide study of prevailing coronavirus infections found that 21.4% of adults had already been infected before vaccinations started in January.

Nearly one-third of the people living in India’s urban slums were found to have antibodies for the virus. The study estimated that in other urban areas, 26.2% of residents had been infected.

The survey also estimated that over one-quarter of all children between the ages of 10 and 17, and more than 25% of all health workers in India had been infected with the virus.

The results of the study were announced by India’s health ministry at a press briefing on Thursday.

Health ministry officials said the results indicate that a large section of India’s population of nearly 1.4 billion remains vulnerable. They say the study underlines the importance of vaccination and a continued focus on wearing masks and maintaining physical distance.

India has recorded the second-highest number of confirmed COVID-19 cases after the U.S., over 10.7 million, and reported more than 150,000 deaths.