Since the pandemic began, it's been known that the severity of coronavirus illness varies widely between people. Could the common cold be the reason why?
It's still just a theory, but researchers in California suspect that if you've recently had a cold -- many of which are also caused by coronaviruses -- your immune system's T-cells might recognize SARS-CoV-2 and help fight it.
"We have now proven that in some people, preexisting T-cell memory against common cold coronaviruses can cross-recognize SARS-CoV-2, down to exact molecular structures," said study co-lead author Daniela Weiskopf, an assistant professor at the La Jolla Institute for Immunology.
"This could help explain why some people show milder symptoms of disease while others get severely sick," she said in an institute news release.
Still, Weiskopf and her team cautioned that even if that's true, it's too soon to say if immune cell memory will help you recover any faster from COVID-19.
The new research was spurred by evidence -- collected from COVID-19 patients around the world -- that immune system T-cells were reacting to fragments of SARS-CoV-2's makeup, even though these cells had never encountered the virus before.
One way that could happen was if T-cells had gained a memory of these viral components from a prior encounter with a common cold coronavirus. Study co-lead author Dr. Alessandro Sette, also of the Institute, called these cold viruses COVID-19's "less dangerous cousins."
So in the new study, the La Jolla researchers collected samples from people who'd never been exposed to SARS-CoV-2. Their analysis showed that unexposed people had a wide range of memory T-cells that were equally reactive against SARS-CoV-2 as well as four types of common cold coronaviruses.
They also found that memory T-cells that recognized the common cold also recognized key sites on the SARS-CoV-2 coronavirus.
This could mean that the fight against a common cold might be teaching T-cells to recognize at least some parts of SARS-CoV-2 -- and perhaps jump-start the fight against SARS-CoV-2 should it appear.
Some T-cells appeared to target the "spike" protein on the surface of SARS-CoV-2, the area of the virus that recognizes and binds to human cells. Other parts of the immune system's memory seemed to target other SARS-CoV-2 proteins, Weiskopf's team reported Aug. 4 in Science.
The latter point is important because most vaccines under development target the spike protein. Including other targets on SARS-CoV-2 might boost a vaccine's potency, the researchers explained.
Dr. Amesh Adalja is an infectious disease expert and a senior scholar at the Johns Hopkins Center for Health Security in Baltimore. Reading over the new report, he said it "provides more evidence that the fact that humans are exposed to other coronaviruses on a regular basis does have an impact on their immunity to the novel coronavirus."
Still, just how or to what extent infection with a common cold might affect infection with SARS-CoV-2 remains "unclear," Adalja added.
Would people who've recently encountered the common cold have no or fewer symptoms of COVID-19? According to Adalja, "the next step in the studies is understanding what the differences are among individuals who have this T-cell immunity that cross-reacts, versus others that do not."
For more on COVID-19, see the U.S. Centers for Disease Control and Prevention.