Six months after acquiring SARS-CoV-2, the blood of individuals who had mild, moderate, or asymptomatic COVID-19 still contained immune cells that recognize the virus, a new study finds.

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To date, more than 55 million people worldwide have acquired SARS-CoV-2, the virus that causes COVID-19.

However, confirmed cases of people reacquiring the infection remain extremely rare. This is despite evidence that the number of antibodies against the virus in the blood of most people steeply declines after they contract a mild or asymptomatic infection.

The new research suggests that people reacquiring the infection may be rare because helper T cells, a type of immune cell, continue to protect most people against the virus for at least 6 months.

When they recognize a virus, helper T cells recruit other immune cells to make antibodies or kill the infected cells.

“While our findings cause us to be cautiously optimistic about the strength and length of immunity generated after SARS-CoV-2 infection, this is just one piece of the puzzle,” says study co-lead Prof. Paul Moss, a specialist in hematology at Birmingham University in the United Kingdom.

“There is still a lot for us to learn before we have a full understanding of how immunity to COVID-19 works,” he adds.

Importantly, however, the research does not provide direct evidence that the T cells identified by the scientists will protect people from reacquiring the infection.

The study, which has not yet been peer-reviewed or published in a journal, is available online at bioRxiv.

Between March and April 2020, 100 individuals who had recently recovered from COVID-19 agreed to participate in the study. Their symptoms had been mild or moderate, or they were asymptomatic, with none requiring hospital treatment.

The oldest participant was 65 years and the youngest was 22 years, while 77 were female.

They gave blood samples once a month, which allowed the researchers to track changes in antibody levels targeted at three different viral proteins.

After 6 months, all the participants had helper T cells remaining in their blood that responded to the virus. One of the key ways the cells responded was by producing IL-2, an immune-signaling molecule that is particularly important for combating viral infections.

Compared with individuals who experienced no symptoms, the T cell responses of those who had symptoms were 50% stronger.

Higher cellular immunity may give people who experienced symptoms better protection against reacquiring the virus in the future, say the authors.

Nevertheless, they also write that:

“[I]t is also possible that the quality of the T cell response at the time of initial infection [in asymptomatic patients] was sufficient to provide clinical protection. The relative susceptibility of patients with initial asymptomatic disease to episodes of reinfection, either clinically silent or symptomatic, will therefore need to be assessed over time.”

The strength of T cell responses to the three different viral proteins correlated with peak levels of the respective antibodies to these proteins during the preceding six months.

This suggests that both of these branches of the immune system — antibody-producing cells and T cells — were collaborating in the response, says Prof. Paul Morgan, Director of the Systems Immunity Research Institute at Cardiff University in the U.K., who was not involved in the research.

The study offers hope that vaccines against SARS-CoV-2, which aim to recreate the immune response that would occur with someone contracting the virus, will provide lasting immunity.

“This is all very good news and bodes well for the long term, in terms of both vaccine development and the possibility of long-term protection against reinfection, although it is important to stress that we don’t yet know whether the people in this study are protected from reinfection,” says Eleanor Riley, professor of immunology and infectious disease at the University of Edinburgh in the U.K., who was not involved in the research.

Among the study’s limitations were its small size and the limited number of older adults, males, and severely ill people in the sample.

Still, “These findings increase confidence that contracting the SARS-CoV2 infection can induce robust T cell immunity, at least in this relatively young, mild or asymptomatic disease cohort, and that this immunity persists even when antibody levels are falling,” says Prof. Morgan.

“The authors make a strong case for measuring T cell immunity more widely and including this in assessments of response to vaccines as these become available,” he adds.

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