Around two-thirds of Indians above the age of six years have antibodies against Covid-19, the government said on Tuesday as it announced the results of the fourth countrywide sero survey, conducted by the Indian Council of Medical Research.

According to the US’ Centers for Disease Control and Prevention, serology tests are “those that look for antibodies in blood”. “If antibodies are found, that means there has been a previous infection,” it says. “Antibodies are proteins that can fight off infections. However, there is a chance a positive result means that you have antibodies from an infection with a virus from the same family of viruses (called coronaviruses), such as the one that causes the common cold.”

The ICMR sero survey showed that 67.6% of the population has been exposed to the coronavirus till now. This means that at least 40 crore people in the country are still vulnerable to SARS-CoV2 – the virus that causes the coronavirus disease.

A total of 36,227 persons, including 7,252 healthcare workers, were a part of the exercise. The survey was conducted in 70 districts in 21 states in the months of June and July.

The survey also showed that over half of the children aged between 6 and 17 years had Covid-19 antibodies. “Children can handle viral infection much better than adults,” said ICMR chief Balram Bhargava. “...Once India starts considering opening of schools, it will be wise to open primary schools first.”

Among healthcare workers, 85% were found to have antibodies, and one-tenth of them had not been vaccinated yet.

“The implications [of the fourth serosurvey] clearly show there is a ray of hope, but there is no room for complacency,” warned Bhargava. “We must maintain Covid-19 appropriate behaviour and community engagement.”

Bhargava also emphasised that the survey offers a “bird’s eye view” of the Covid-19 situation, but is not a substitute for variations at the state or district level.

Serological surveys reveal how many people may have been infected with the novel coronavirus in an area. However, it still isn’t clear how long antibodies last in infected persons and what level of antibodies is needed to protect a person from reinfection.

Steep increase in exposure to Covid-19

The fourth national sero survey shows a steep increase in the presence of antibodies in the population as compared to the third one, which was conducted in December and January. At that time, 21.5% of the country’s population was estimated to have been exposed to the coronavirus.

The second survey was conducted between August 17 and September 22, 2020. It found that 6.6% of the participants had antibodies against the coronavirus. The first survey, conducted between May 11 and June 4, 2020, had found that only 0.73% of the population had been exposed to the virus.

India on Tuesday recorded 30,093 new cases of coronavirus, taking the country’s tally of infections since the outbreak of the pandemic in January last year to 3,11,74,322. The toll went up to 41,44,82 after 374 more patients died.