A new study is raising questions about whether people who've been exposed to viruses that can cause the common cold have some level of immunity to the novel coronavirus—a prospect that, if proven true, could hold implications for long-term coronavirus immunity and vaccinations.

Who's immune to the new coronavirus—and for how long? Here's what the research says.

Study finds 'cross-reactive T cells' respond to novel coronavirus

For the study, which was published Wednesday in Nature, researchers from Charité-Universitätsmedizin Berlin and the Max Planck Institute for Molecular Genetics analyzed blood samples from 18 patients between the ages of 20 and 64 who were infected with the novel coronavirus, known as SARS-CoV-2, as well as samples from 68 healthy adults who hadn't been infected with SARS-CoV-2.

The scientists added synthetic fragments of SARS-CoV-2 to participants' blood samples to evaluate any responses generated by the participants' immune cells. They found that, in samples from 83% of the participants who had previously been infected with the virus, the patients' T cells were reactive to SARS-CoV-2. As CNN reports, "T-cell reactivity suggests that [a person's immune] system might have had some previous experience fighting a similar infection and may use that memory to help fight a new infection," so finding reactivity among patients who'd had confirmed cases of SARS-CoV-2 wasn't surprising.

However, the researchers also found T-cell reactivity in 35% of the samples from patients who hadn't previously been infected by SARS-CoV-2. So how could it be that individuals were able to generate immune responses to SARS-CoV-2 if they hadn't been infected by the pathogen before?

"One of the characteristics of T-helper cells is that they are not only activated by a pathogen with an 'exact fit,' but also by pathogens with 'sufficient similarity,'" explained Claudia Giesecke-Thiel, head of the Flow Cytometry Facility at the Planck Institute and a lead author of the study.

As such, the researchers hypothesized in the study that the healthy participants "probably acquired [the T cells]" that reacted to SARS-CoV-2 after fighting an infection from a similar coronavirus, such as those that can cause the common cold. The researchers said those T cells' "memory" of the similar infection likely resulted in what's known as "cross-reactivity" to SARS-CoV-2, CNN reports.

Can having a common cold make you immune to the novel coronavirus?

However, the researchers said it's not clear how effective cross-reactive T cells are at fighting or providing immunity against SARS-CoV-2—a note of caution that's been echoed by other experts.

Amesh Adalja, a senior scholar at the Johns Hopkins University Center for Health Security, who was not involved with study, said while it's clear that "a significant proportion of individuals … have this cross-reactive T cell immunity from other coronavirus infections," more research is needed to understand "what the role of those T-cells might be."

For instance, Adalja raised the possibility that those T cells may be a factor in determining whether a person develops a mild or severe case of Covid-19, the disease caused by SARS-CoV-2. "[I]f you could compare people maybe with severe and mild illness and try and look at the T cells in those individuals and say, 'Are people who have severe disease less likely to have cross reactive T cells versus people who have mild disease maybe having more cross reactive T cells?' I think that there's biological plausibility to that hypothesis," he said.

William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine who was not involved in the new study, called the findings "intriguing."

"Here's a study that suggests actually there may be some cross-reactivity … with the normal conventional coronaviruses that cause colds in humans and there may be some cross-reactivity with the [SARS-CoV-2]. …That's in and of itself intriguing because we had thought from the antibody perspective that there wasn't much cross at all," he said, asking, "Does it make it more or less likely that the person who is infected with [SARS-CoV-2] actually will develop an illness? And does it have any implications for vaccine development?"

Still, both Adalja and Schaffner said they weren't surprised by the study's findings. According to Adalja, the novel coronavirus is the seventh human coronavirus to be discovered, four of which are responsible for 25% of common colds.

"Almost every person in the world has had some encounter with a coronavirus," he said, "and since they are all part of the same family, there is some cross reactive immunity that develops" (Howard, CNN, 7/30; Braine, New York Daily News, 7/31).