The World Health Organization on Saturday said there was “no evidence” that people who have recovered from Covid-19 and have antibodies are protected from a second coronavirus infection, Reuters reported.

The United Nations agency warned governments against issuing “immunity passports” or “risk-free certificates” to infected people who have recovered, as their accuracy could not be guaranteed. These passports or certificates allow recovered patients to rejoin work or travel once they have been screened to determine if they have developed antibodies, which make them immune to the Covid-19 virus. Last week, the government of Chile had said that it would start handing out “health passports” to those who have recovered.

“Some governments have suggested that the detection of antibodies to the SARS-CoV-2, the virus that causes Covid-19, could serve as the basis for an ‘immunity passport’ or ‘risk-free certificate’ that would enable individuals to travel or return to work assuming that they are protected against re-infection,” the WHO said, adding that there is currently no evidence on this.

While most studies have shown that people who have recovered have antibodies to the virus, the WHO said some of them have very low levels of neutralising antibodies in their blood. “This suggests that cellular immunity may also be critical for recovery.”

The WHO is reviewing the evidence on antibody responses to the virus, which emerged in Wuhan last year. So far, 2,822,003 cases have been reported globally, and a total of 1,97,578 deaths, according to the Johns Hopkins University tracker on Saturday.