Although it doesn’t provide a conclusive answer, a study published yesterday (May 14) in Cell appears to be good news on the immunity front.

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Even as researchers around the world rush to develop a vaccine against the virus that causes COVID-19, and some pin their hopes on the idea that enough people will recover from infections to achieve herd immunity in the meantime, questions about whether exposure to the virus induces immunity to it have lingered. If the virus itself does not prompt immunity, a vaccine against it might not either. 

Researchers at the La Jolla Institute for Immunology in California took blood from 20 adults who’d recovered from COVID-19 and exposed the samples to proteins from the SARS-CoV-2 virus. All of the patients had CD4⁺ helper T cells that recognized the virus’s spike protein, and 70 percent of them had CD8⁺ killer T cells that responded to the same protein. “Our data show that the virus induces what you would expect from a typical, successful antiviral response,” says coauthor Shane Crotty in an institute press release. 

The authors also tested blood samples collected between 2015 and 2018 to see whether people who were never exposed to SARS-CoV-2 might nevertheless have some immunity to it. They detected CD4⁺ T cell responses to SARS-CoV-2 in about half of those samples, which they suggest could be due to exposure to other coronaviruses that cause a cold. 

Science notes that the results align with those of another study, led by researchers at Charité University Hospital in Berlin and reported in a preprint last month, that found CD4⁺ T cells that recognized the spike protein in blood from 83% of COVID-19 patients and 34% of healthy people tested.  

“This is encouraging data,” Columbia University virologist Angela Rasmussen, who was not involved in either study, tells Science. Although not conclusive, the T cell response “bodes well for the development of long-term protective immunity” among people who have recovered from COVID-19, she says, and could be useful in designing vaccines. 

The results suggest that “one reason that a large chunk of the population may be able to deal with the virus is that we may have some small residual immunity from our exposure to common cold viruses,” viral immunologist Steven Varga of the University of Iowa tells Science. But neither study tested whether that is the case.