Why might some people who have not had COVID-19 already have some immunity?

The immune system of some people who have not been exposed to the new coronavirus may have some familiarity with the pathogen, possibly helping to reduce the severity of the disease if that person contracts COVID-19, a new study suggests.

The study, published in the journal Nature on Wednesday, found that among a sample of 68 healthy adults in Germany who had not been exposed to the coronavirus, 35% had T cells in their blood that were reactive to the virus.

T cells are part of the immune system and help protect the body from infection. T cell reactivity suggests that the immune system may have had some previous experience in fighting a similar infection, and you can use that memory to help fight a new infection.

So how could your immune system have reactive T cells if they never had Covid-19? They were “likely acquired from previous endemic coronavirus infections,” the researchers, from various institutions in Germany and the United Kingdom, wrote in the new study. Using this T cell memory from another yet similar infection to respond to a new infection is called “cross reactivity.”

“The big question is … understand what the role of those T cells might be?”

The new study included analysis of blood samples from 18 COVID-19 patients, ages 21 to 81, and healthy donors, ages 20 to 64, based in Germany. The study found that coronavirus-reactive T cells were detected in 83% of COVID-19 patients.

While the researchers also found preexisting cross-reactive T cells in healthy donors, they wrote in the study that the impact those cells could have on the outcome of a COVID-19 disease is still unknown.

The study findings certainly require more research, said Dr. Amesh Adalja, principal investigator for the Johns Hopkins University Center for Health Safety, who was not involved in the new study.

“It appears in this study that there is a significant proportion of individuals who have this cross-reactive T-cell immunity against other coronavirus infections that may have some impact on how they are doing with the new coronavirus. I think the big question is trying to skip from the fact that they have these T cells to understand what the role of those T cells might be, “Adalja said.

“We know, for example, that children and younger adults are relatively safe from the serious consequences of this disease, and I think one hypothesis could be that existing pre-existing T cells may be much more numerous or more active in the younger age. ” cohorts than older cohorts, “said Adalja.

“And if you could compare people with severe and mild diseases and try to look at T cells in those people and say, ‘Are people with severe disease less likely to have cross-reactive T cells compared to people who do they have mild disease? more cross reactive T cells? I think there is a biological possibility of that hypothesis, “he said. “However, it is clear that the presence of T cells does not prevent people from becoming infected, but does it modulate the severity of the infection? That seems to be the case.”

Until now, during the coronavirus pandemic, much attention has been paid to COVID-19 antibodies and their role in developing immunity against the disease.

But infectious disease expert Dr. William Schaffner, professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine in Nashville, who was not involved in the new study, said T cells cannot be overlooked. .

“Here’s a study that suggests that there may actually be some cross-reactivity, some pump priming if you like, with normal conventional coronaviruses that cause colds in humans and there may be some cross-reactivity with the COVID virus that is causing So much damage. That in itself is intriguing because we had thought from an antibody perspective that there wasn’t much crossover at all, “Schaffner said.

“It is not entirely surprising because they are all members of a family. It is as if they are cousins ​​of the same family,” he said. “Now we have to see if this has any impact on clinical practice … Does it make it more or less likely that the person infected with COVID will actually develop a disease? Does it have any implications for the vaccine? Development?”

‘Almost everyone in the world has had an encounter with a coronavirus’

Adalja added that he was not surprised to see this cross-reactivity of T cells in study participants who had not been exposed to the new coronavirus, called SARS-CoV-2.

“SARS-CoV-2 is the seventh human coronavirus ever discovered, and four of the human coronaviruses are what we call community-acquired coronaviruses, and together those four are responsible for 25% of our common colds,” said Adalja . “Almost everyone in the world has had an encounter with a coronavirus, and since they are all part of the same family, some cross-reaction immunity develops.”

The new Nature study is not the only article suggesting a certain level of pre-existing immunity among some people to the new coronavirus.

Alessandro Sette and Shane Crotty, both from the University of California, San Diego, wrote in a comment published in the journal Nature earlier this month, that “20-50% of unexposed donors show significant reactivity to the SARS antigen- CoV-2 sets of peptides, “based on separate research, but noted that the source and clinical relevance of reactivity remains unknown.

Sette and Crotty wrote that “it has now been established that the pre-existing immune reactivity of SARS-CoV-2 exists to some extent in the general population. It is presumed, but not yet proven, that this could be due to immunity to the” cold common coronavirus.