This Is Why Some People Don't Have COVID Symptoms, New Study Shows

That cold you had last year may be helping to protect you from COVID-19.

For months, we have been cautioned about the dangers of asymptomatic COVID spread, but we haven't gained much of an understanding about why some people don't exhibit any symptoms while others experience life-threatening complications. Experts estimate that around 40 percent of coronavirus cases are asymptomatic, but why is this number so high? According to The New York Times, a recent study published by Nature cites "a level of pre-existing immunity" as a potential factor in why some people don't have coronavirus symptoms.

According to the study, you can thank memory T cells for mitigating coronavirus symptoms in many people. Memory T cells protect against previously encountered pathogens (like viruses), according to Nature. The study looked at three different groups and found that "in one, each of 36 people exposed to the new virus had T cells that recognize a protein that looks similar in all coronaviruses. In another, 23 people infected with the SARS virus in 2003 also had these T cells, as did 37 people in the third group who were never exposed to either pathogen," according to The New York Times. These findings led researchers to hypothesize that immunity to COVID-19 may be more widespread than originally thought.

Man wearing a mask on the subway during the coronavirus pandemic

"A level of pre-existing immunity against SARS-CoV2 appears to exist in the general population," Antonio Beroletti, MD, one of the researchers from the study, told The New York Times. This immunity could come from previous exposure to coronaviruses that cause more mundane illnesses such as the common cold, according to the study. While these T cells may not be strong enough to prevent someone from contracting COVID-19, they could explain why so many cases are asymptomatic or only generate mild symptoms.

An earlier May study published by Cell found that there were people who possessed T cells that recognized antigens from COVID-19, although they had never had COVID-19, SARS, or MERS. This study suggested "cross-reactive T cell recognition between circulating 'common cold' coronaviruses and SARS-CoV-2," which would allow for some part of the population to have some level of pre-existing immunity. The recent study in Nature seems to further this theory.

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These new findings are helping experts understand the wildly varying coronavirus cases and symptoms we are seeing across the globe. Learning how T cells interact with the virus helps doctors and scientists move toward a deeper understanding of the virus—and the development of a vaccine. And for more on the future of coronavirus, Dr. Fauci Just Said the Words About COVID-19 You Never Wanted to Hear.

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