Children bring home colds from day care and school and give them to their parents, and it's thought those lower-level infections may ultimately defend Mom and Dad from the worst of COVID. Both common colds and COVID-19 are coronaviruses, so the theory goes that getting one might offer some protection from the other, researchers said.
"One hypothesis that people batted around was maybe people that had a lot of common colds in the past few years may have some built-up immunity to cope with COVID-19, and then either not get an infection at all or get only a mild infection and not a severe one," said lead researcher Dr. Matthew Solomon, a cardiologist in the research division at Kaiser Permanente Northern California in Oakland.
"This idea of the kind of built-up immunity really resonated with a lot of people. And we thought, well, maybe we can look in our database and see if we can identify a signal of that," Solomon said.
This study can't prove that having a common cold protects you from severe COVID-19, only that it may confer some immunity. But the research team said the concept merits further exploration.
For the study, Solomon and his colleagues scoured the medical records of more than 3 million adults seen at Kaiser Permanente Northern California from February 2019 through January 2021.
They found adults without kids who had COVID-19 were 49% more likely to be hospitalized and 76% more likely to stay in an intensive care unit than COVID patients who had children ages 5 and under.
The study was done before COVID vaccines were available, so the researchers can't tell what effect vaccination might have on any possible immunity that colds may confer.
Also, Solomon said that just because you've caught colds from your kids doesn't mean that either you or they won't get COVID-19. Vaccination remains the best protection, he said.
"Having small children does not confer absolute protection," Solomon said. "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 people get COVID very badly and others do not? This is just one small piece of a very complex issue."
Infectious disease expert Dr. Marc Siegel said the notion that one coronavirus can protect you from another isn't new, and this study provides some evidence it might be true.
He too stressed the study doesn't show that you won't get COVID-19, only that it might not be severe. "It adds to the idea that the more immunity you can get, the better," said Siegel, a clinical professor of medicine at NYU Langone Medical Center in New York City. He was not part of the study.
Still, it's not clear if any immunity conferred by common colds applies to all strains of COVID, especially the current more contagious strains, Siegel said. These include the Omicron subvariants BA.5 and BA.4, which are spreading in the United States.
The best protection is getting vaccinated against COVID-19 and having your kids vaccinated, too, he said.
"Exposure to different coronaviruses may help to provide a level of immunity that decreases severity," Siegel said. "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."
The report was published online July 27 in the Proceedings of the National Academy of Sciences.
The U.S. Centers for Disease Control and Prevention has more on COVID-19.Proceedings of the National Academy of Sciences