The central government on Tuesday advised citizens to travel only if they were fully vaccinated against Covid-19.

Indian Council of Medical Research chief Balram Bhargava also appealed against non-essential travel. At a press briefing on India’s Covid-19 situation, the ICMR chief spoke about the fourth countrywide sero survey conducted by the medical research body.

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

The results of the fourth survey showed that 67.6%, or more than two-thirds, of Indians above the age of six years have antibodies against Covid-19. This means that at least 40 crore citizens remain vulnerable to SARS-CoV2 the virus that causes Covid-19.

Bhargava said that while the results of the survey were a “ray of hope”, there was no room for complacency. He said that all kinds of congregations should still be avoided.

The ICMR chief emphasised that the sero survey offered a “bird’s eye view” of the Covid-19 situation in the country. But the findings was not a substitute for state or district-level variations in the data. He warned that the variation in data among states indicated the possibility of more waves of infection in the future.

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.”

In the past couple of weeks, the central government has issued multiple warnings about non-essential travels.

Last week, the home ministry wrote to state governments flagging violation of Covid-19 protocols in hill stations and markets. As travel restrictions were eased in many states, photos and videos on social media showed large crowds flocking to popular mountain retreats such as Shimla, Manali and Mussoorie without following Covid-appropriate behaviour.

Meanwhile, on Wednesday, India recorded 42,015 new coronavirus infections in the last 24 hours, as the country’s tally of cases since the outbreak of the pandemic in January last year rose to 3,12,16,337. The toll rose sharply by 3,998 deaths to 4,18,480 as Maharashtra added 3,509 old fatalities in its revised data for the pandemic.