Many of the common colds circulating in daycares and schools are members of the coronavirus family, and it’s long been thought that exposure to one coronavirus will offer some shielding from another—a phenomenon called cross-immunity. That’s part of the reason why experts believe Covid infection is typically more mild in kids than in adults.
In a study published in the journal PNAS, researchers analyzed health records of more than 3 million adults who received treatment from the Kaiser Permanente Northern California healthcare system between February 2019 through January 2021, before vaccines were made available. They found that adults who had children under age 5 were less likely to have severe illness from Covid.
In contrast, adults with Covid who didn’t have kids were 49% more likely to be hospitalized with the infection and 76% more likely to stay in an intensive care unit than those with kids under 5, suggesting that coronavirus cross-immunity might be at play for those with little ones at home.
What is cross-immunity?
Cross-immunity refers to the phenomenon that an infection with one virus may lend some immunity against a similar virus. If you’re sick with a common cold caused by a coronavirus strain, experts believe that your prior exposure to that type of virus can help your immune system trigger a stronger immune response, meaning you aren’t likely to get quite as sick had you not been exposed. We can also see cross-immunity occur with routine childhood vaccinations, where one vaccine can lend some immunity against a different virus.
Because young kids are often in very close contact with their peers, they’re more likely to pick up germs and colds and bring them home, thereby exposing their adult caretakers. But when it comes to a future Covid infection, all those previous germs may actually benefit adults.
Prior exposure to a different coronavirus doesn’t mean you won’t get sick from SARS-CoV-2, the novel coronavirus that caused the pandemic, however. It’s also unlikely that you’d be able to tell whether your cold was the result of a coronavirus or another type of virus, because routine virus testing for the common cold is not readily available.
“Having small children does not confer absolute protection,” says lead researcher Dr. Matthew Solomon, a cardiologist in the research division at Kaiser Permanente Northern California in Oakland, in a statement. “Our study is just suggestive of this effect. This is one small piece of a very large puzzle that scientists are working to unravel.”
Why do some get severe Covid and others don’t?
When it comes to why some people tend to get severe forms of Covid and others are less impacted, we still have a lot of unanswered questions. Cross-immunity is a very small part of a complex issue, notes Dr. Solomon.
The Kaiser Permanente study was not without its limitations—there are several possible confounding factors the authors were unable to control for in this large-scale population study, such as whether certain occupations (like preschool teachers or pediatricians) meant higher levels of exposure or whether families had pre-existing antibodies to other coronaviruses in their blood.
Though more research is needed, the results are still promising. “It adds to the idea that the more immunity you can get, the better,” says infectious disease expert Dr. Marc Siegel, a clinical professor of medicine at NYU Langone Medical Center in New York City, who was not part of the study, to HealthDay.
Vaccines are still the best defense for kids and adults
Dr. Solomon stresses that vaccines are still the best protection from Covid for both kids and adults, however, as it’s unclear just how much protection is conferred from previous colds and if that protection carries over to the currently circulating Omicron subvariants BA.5 and BA.4. Vaccines are now available for kids ages 6 months and up, and boosters are available for kids ages 5 and up.
“Exposure to different coronaviruses may help to provide a level of immunity that decreases severity,” Siegel says. “That together with vaccination and prior infection is a good cocktail for decreasing severity. It doesn’t mean we don’t need more focused or extensive vaccines. It doesn’t mean that the current vaccine isn’t helpful—immunity is what matters no matter how you get it.”
Solomon MD, Escobar GJ, Lu Y, Schlessinger D, Steinman JB, Steinman L, Lee C, Liu VX. Risk of severe COVID-19 infection among adults with prior exposure to children. Proceedings of the National Academy of Sciences. 2022 Aug 16;119(33):e2204141119.
Yaqinuddin A. Cross-immunity between respiratory coronaviruses may limit COVID-19 fatalities. Med Hypotheses. 2020;144:110049. doi:10.1016/j.mehy.2020.110049