A DNA variation that affects the immune system can boost a person’s odds of avoiding Covid-19 symptoms, a study found. The work, published Wednesday in the journal Nature, helps explain why some people infected with the virus that causes Covid-19 don’t fall ill.

The T cells of some people with the variation can find and kill the virus without having seen it before, researchers said. That is because the part of the virus their T cells home in on is similar enough to common coronaviruses they have already encountered.

“There are people out there that have got strong prior immunity from their common coronavirus exposures,” said Danny Altmann, a professor of immunology at Imperial College London, who wasn’t involved in the study. The work could help researchers design better vaccines, Altmann said.

People with a copy of the genetic variation were more than twice as likely to avoid symptoms than people without it, the researchers found. For people with two copies, the chance of avoiding symptoms increased more than eightfold.

Weekly reported Covid-19 deaths and hospitalizations in the U.S. are at record lows, federal data show. Concentrations of the virus detected in wastewater have ticked up recently from a low base. Health officials and hospitals have pulled back on reporting and surveillance.

Yet researchers are still exploring mysteries about the virus including why some people get infected and transmit it without getting sick. Age, underlying health problems, vaccination status, and healthcare quality all influence how a person fares against infection.

Jill Hollenbach, senior author of the study, wanted to focus on what are known as HLA genes that create human leukocyte antigens, immune-signaling molecules that sit on the surface of cells. These molecules hold up bits of foreign protein for patrolling T cells to inspect. A person’s HLA type determines which pieces of pathogen are flagged to T cells.

“There’s a whole lot of variation in that mix,” said Hollenbach, a professor of neurology, epidemiology and biostatistics at the University of California, San Francisco.

Hollenbach and her colleagues enrolled nearly 30,000 volunteers from a national registry of bone-marrow donors that matches them with transplant recipients based on HLA-type. From February 2020 through April 2021, more than 1,400 unvaccinated donors reported via a smartphone app that they had tested positive for the virus and 136 said they were asymptomatic.

The researchers found that a variant called HLA-B*15:01 was associated with asymptomatic infections. About a fifth of participants who stayed symptom-free had at least one copy, while 9% of people with symptoms had it, the researchers found. The finding was reproduced with data from two other groups of patients.

“It’s not saying that if you have this, you don’t have to worry,” said Paula Cannon, a professor of molecular microbiology and immunology at the Keck School of Medicine of USC, who wasn’t involved in the research. “But honestly, I’d like to have these genes. I’d sleep better at night if I did.”

The researchers also looked at the T cells of people who carried the HLA variant but hadn’t been exposed to the virus. Their T cells still responded to a piece of the Covid-19 virus that is similar to one found in other common cold coronaviruses.

For some people with HLA-B*15:01, the piece of virus that gets presented to T cells happens to look like a section from common coronaviruses they were already familiar with, making it easier to fight, Cannon said: “It’s just kind of good luck.”

The final analysis for the study was limited to people who said they were white because the numbers of participants from other groups weren’t large enough to confirm a link, Hollenbach said.

The researchers found that another HLA variation enhanced the protective effect when a person had both. Several factors, including genetics, are at play when the body contends with an infection, said Ludmila Prokunina-Olsson, director of the Laboratory of Translational Genomics at the National Cancer Institute, who wasn’t involved in the study.

“A combination of all these factors will determine how it will go,” Prokunina-Olsson said.