Rufino, a patient who has recovered from the coronavirus disease (COVID-19), gestures next to his son as he leaves from the Juarez Hospital to go to his house in Mexico City, Mexico, July 27, 2020.

The human body’s immune response to the coronavirus remains a mystery, even in the months immediately following infection. It’s still unclear how long people who recover from the virus are immune to reinfection.

Research on the subject is so inconclusive that the CDC has hesitated to define that time period. The agency seemed to do that for the first time in early August when it updated its quarantine guidelines to exclude people who have had the virus in the last three months. But on Friday the CDC issued a statement clarifying that it was not saying those people have immunity.

“On August 3, 2020, CDC updated its isolation guidance based on the latest science about COVID-19 showing that people can continue to test positive for up to three months after diagnosis and not be infectious to others,” read a statement the agency issued on Friday. “Contrary to media reporting today, this science does not imply a person is immune to reinfection with SARS-CoV-2, the virus that causes COVID-19, in the three months following infection.”

The August 3 update had seemed to imply that people would probably be safe from reinfection for three months. Those guidelines say that people who have had COVID-19 in the last three months don’t need to quarantine or get tested as long as they don’t develop symptoms again — even if they have been in close contact with someone who has COVID-19, which seems to imply that they are not susceptible to reinfection.

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Instead, a CDC spokesperson told CNN, the new guidance was “based on the latest science about COVID-19 showing that people can continue to test positive for up to three months after diagnosis and not be infectious to others.”

People who recover from COVID-19 probably gain some immunityScientist Linqi Zhang shows a tube with a solution containing COVID-19 antibodies in his lab where he works on research into novel coronavirus disease (COVID-19) antibodies for possible use in a drug at Tsinghua University in Beijing

The Friday statement pointed to the results of 15 recent studies on how and when people can spread the virus. The research suggested that the amount of live virus in a person’s nose and throat (which can then get into the air and spread to other people) drops significantly soon after they show symptoms. The science also shows that most people are infectious for no longer than 10 days after their first symptoms — 20 days for people who are severely ill or immunocompromised.

Other research has shown that antibodies can disappear quickly after infection. But many experts believe that people who recover from the coronavirus do gain some immunity.

That’s because the immune response doesn’t just come from antibodies. Other agents which don’t show up in antibody tests mount an immune response to the virus if it re-enters the body. White blood cells can identify and kill infected cells, as well as create new antibodies to fight back the pathogen. Early research has shown that recovered patients do retain some of these white blood cells, which last much longer than the antibodies themselves.

“There’s a lot that we are still learning,” Maria Van Kerkhove, an epidemiologist serving as the World Health Organization’s technical lead on the coronavirus, said in a press briefing on Thursday. “We expect that individuals infected with the SARS-CoV-2 virus develop an immune response. We don’t know for how long that immune response lasts. We don’t know how strong it is.”

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