The health ministry said Tuesday that it has discovered neutralizing antibodies in samples collected from eight people who tested positive for the novel coronavirus in antibody tests.

It is the first time that antibodies that neutralize the coronavirus and make people immune to infection have been found in the country. The ministry plans to conduct research on how long the antibodies will remain effective in preventing infection in humans.

When people are infected with viruses, their bodies create antibodies, or proteins capable of blocking future infection with the same viruses. Antibody tests check whether people have such proteins.

The ministry released last month the results of mass antibody tests for the novel coronavirus conducted in Tokyo, as well as Miyagi and Osaka prefectures, using an agent manufactured by a foreign company and another from a different firm abroad.

Defining antibody holders as those who tested positive for both agents, the ministry found that one out of 3,009 test-takers in Miyagi, two out of 1,971 in Tokyo and five out of 2,970 in Osaka have antibodies against the coronavirus. The finding implies that the eight people had previously been infected with the virus.

Samples of the eight people were sent to the National Institute of Infectious Diseases, which found in an in-vitro experiment that the antibodies prevented the novel coronavirus from destroying human cells.

The experiment also covered samples from people who tested positive for only one of the two agents, but sufficient levels of neutralizing antibodies were not found in them.

“Antibody tests are being conducted also by private institutes, but neutralizing antibodies had not been confirmed,” an official of the ministry said. “We’ll consider what kinds of antibody tests can be done.”

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