Even among people with relatively mild cases and similar vaccination status, coming down with COVID-19 can mean different things: for some people, it’s a day on the couch. For others, it might be three days in bed. Some have body aches while others cough more. And while some cases seem to let up as quickly as they came on, others linger.
Just why you might have gotten sicker than your brother, your friend or your partner is complicated and not fully understood. But while experts say your genes and health play an important role, some of how sick you are likely to get is somewhat within your control, between vaccinations and limiting exposure to the virus.
If you think all the way back to early 2020, when COVID-19 was relatively new in the U.S., the baseline level of immunity in the population against the virus was low — for everyone.
That’s not the case today. The most recent data show 71 percent of Minnesotans have received at least one dose of a COVID-19 vaccine — highly effective at preventing severe illness and death — and 31 percent are up-to-date on their shots, including any boosters they’re eligible for. On top of that, an untold number of people have been infected with COVID-19 at least once.
At this point, people have different histories of vaccination, boosters and previous infection — and even possibly antibodies from prior infection from other coronaviruses, adding up to different levels of underlying immunity, said Ajay Sethi, a professor of population health sciences at the University of Wisconsin-Madison School of Medicine and Public Health.
Even people with the same vaccination status or combination of vaccination and infection don’t necessarily have the same level of immunity because of the way their immune systems react, producing different antibody levels. Not only that but, everyone has different lengths of time since they were last boosted or had some immune response due to an infection, which affects how readily they react to the virus when they are exposed.
“Because we have such variability in vaccine uptake and previous infection, people are really all over the map in terms of the quality of the immune response that they have now,” Sethi said.
The role of genes and underlying conditions
There are other factors that play a role in how your body is likely to handle COVID-19 before you’re even exposed — namely, your genes.
“There’s literally hundreds of different genes that control the quality and quantity of a person’s immune response,” said Marc Jenkins, director of the Center for Immunology at the University of Minnesota Medical School. “So different people have different sets of those genes. And what that means is that there’s a huge variability in people’s immune responses function.”
And while we know genes have an important role in determining immune response, there’s a lot we don’t know about just how that works.
“Genetics are going to also play a role, but that’s relatively understudied,” Sethi said, adding that many studies looking into genetics and COVID-19 look at the genes of people who died of the virus compared to others who lived.
“That’s a relatively easy thing to count versus infection, but genetics will play a role (in infection variability) because they generally play a role for your susceptibility for picking up viral infections,” he said.
Then there are underlying health conditions. There are ones we know have the potential to make people more susceptible to COVID-19, like age and being immunocompromised.
When people have chronic conditions like diabetes that keep the immune system stimulated, it’s primed to amplify response to the virus, which can lead to more severe illness, Sethi said. On the other hand, people who are immunosuppressed can have more severe COVID-19 experiences because they don’t generate a strong immune response to control the virus once it’s started replicating.
While we know these conditions can lead to more severe COVID-19 outcomes, there are also probably many many unknown conditions that affect the way people experience infections as well, Sethi said.
“That’s why you might find (all other things being apparently equal) somebody who’s 75 years old, they end up in the hospital, but another 75-year-old doesn’t,” Sethi said.
Exposure levels matter
In addition to immunity, genetics and underlying health conditions, the amount of virus you’re exposed to interplays with on how sick you may get from COVID-19, too, further making the COVID-19 experience variable.
For example, a person with low levels of antibodies who is exposed to a high level of virus will likely get sicker than someone with strong antibodies who’s exposed to a low level of virus, Jenkins said.
“If you have more antibodies than virus, the antibodies win and you probably have no symptoms,” Jenkins said. “But if you inhale a huge dose of virus, and there’s more virus than you have antibodies, you have excess virus that’s going to now go on and replicate and cause some illness.”
According to Jenkins, when viral particles outnumber antibodies, the virus is free to replicate in cells, killing them.
“So now you’ve got virus killing these cells lining your lungs, and that causes inflammation, fluid build-up, and causes your lungs to function less well,” Jenkins said. “The ultimate extension of that is to the point where you can’t breathe and you die, but short of that, your body is trying to cough out the dead cells and the consequences of the dying cells.”
The good news is, at this point, most people have some level of antibodies knocking down the virus. Not completely or right away in all cases, which is why people get symptoms, but the immune system can deter viral replication to help the body fight the virus and ultimately win.
The other good news is that people have some control in these matters, through opting to get vaccinated and by trying to minimize exposure to the virus.
While, relative to a year and two years ago, more COVID-19 cases are relatively mild, the virus still has the potential to make people very sick or leave long-lasting symptoms — and that’s not always predictable.
Even the same variant could produce a different illness trajectory in the same person today versus next week versus a month from now, Sethi said.
“I can’t emphasize enough how much uncertainty there is in knowing where you’re going to fall,” he said.