COVID immunity: Why some people are never infected – Torrington Register Citizen

There are many people who have never gotten COVID-19, despite repeated exposure. At the other extreme, there are lots whove gotten sick from the coronavirus more than once, despite being vaccinated and even boosted.

A lot of factors are at play, experts said, including genetics and variations in immune response. Scientists are working towards finding a clear answer.

Theres a difference between people who can never catch COVID, despite exposure, and those who are able to clear it out of their systems without ever knowing they had it. If this second category of people tested regularly, they might show positive for a day or two. Different factors are likely at play between the two groups.

Its so messy, Dr. Julie Parsonnet, an infectious disease specialist at Stanford University. We dont know how to differentiate between whether youre just lucky, really careful or if genetically something is different.

In one study, conducted in early 2021 and known as the human challenge study, 36 healthy young adult volunteers none of whom had been vaccinated or previously infected were deliberately given the virus that causes COVID-19 in their noses and tested frequently afterward to track infection. Remarkably, nearly half never got infected at all.

Researchers dont yet know why some of those participants didnt get COVID, and it remains to be seen whether the pattern holds for more infectious but milder variants like omicron.

People have different immune responses to COVID: Despite exposure, some dont seem to catch the coronavirus at all, while others, even vaccinated people, are getting infected several times. A Yale New Haven Health nurse administers a COVID-19 vaccine in this file photo.

Dr. Monica Gandhi, an infectious disease expert at UCSF, said its important to note that with many infectious diseases, including HIV, which she studies and treats, there are instances where people are exposed over and over but never get the virus.

In the case of HIV, this is because these people lack the receptors for the virus. That means no matter how much they are exposed, theyll never become infected because its physically impossible. Scientists say this is likely true with COVID, as well.

But for those who seem to be able to clear the virus so quickly they never knew they had it, differences in the behavior of the immune system could be the reason.

One study on health care workers who repeatedly tested negative found that these people had high levels of memory T cells the long-lasting cells of the immune system that remember a virus and protect against severe disease from previous, non-COVID coronavirus infections, which may have helped them fight off infection more quickly. But another study found that, especially in older patients, high levels of memory T cells can actually be a risk factor for severe COVID-19, as they can trigger a more severe immune response to the virus that overwhelms their systems.

Both situations could be true, and scientists dont yet know exactly why cross-reactive T cells may be helpful for some and detrimental for others, Gandhi said, though this mechanism is less important for people who are vaccinated, as vaccines generate COVID-specific T cells.

Part of what makes COVID so weird is our own immune system, Gandhi said. Were all just really different.

A prominent example of that is the way the virus behaves in children, she said. Children tend to have a different immune response to new pathogens than adults, she said, which means their lungs and immune system are less likely to get inflamed.

Another mystery, she said, is that only some people, especially children, seem to be able to produce effective cross-immunity T cells from other, non-COVID coronavirus infections. Doctors still dont know why some peoples systems do this and others dont.

We need to still look for other genetic factors, Gandhi said.

There also could be physiological factors, like the shape of the nose, how people breathe and what kind of mucus they produce, Parsonnet said, which have not yet been widely studied.

On the flip side, scientists dont yet understand why some people end up getting COVID several times, even after being vaccinated. Some people may have never fully cleared the virus, even if symptoms go away, causing them to test positive repeatedly. Others may just have more reactive immune systems.

Gandhi said that outside factors such as stress or pregnancy can play a role in how likely you are to become infected and sick from the virus, in much the same way that college students get ill around the time of stressful final exams.

People have different immune responses to COVID: Despite exposure, some dont seem to catch the coronavirus at all, while others, even vaccinated people, are getting infected several times.New Haven Fire Department Chief John Alston, Jr., gets the first dose of a Moderna COVID-19 vaccine from APRN Grace Grajales in front of the New Haven Health Department in this file photo.

We can be more or less susceptible throughout our lives, she said. Its multifactorial. All of these reasons could be at play.

But really understanding the nuances between whats genetic, whats a quickly cleared virus and whats just lucky will take intensive scientific studies, Parsonnet said. One such effort to discover exactly how genes affect the severity of COVID infection or whether you get it at all is underway, through a collaborative project called the COVID Human Genetic Effort. But the research is still in its early stages.

Right now, we just dont know the answer, she said. But people are really interested in looking at it.

Danielle Echeverria is a San Francisco Chronicle staff writer. Email: danielle.echeverria@sfchronicle.com Twitter: @DanielleEchev

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COVID immunity: Why some people are never infected - Torrington Register Citizen