Study: A majority of the population may have 'some degree' of preexisting immunity to COVID-19

A new study from scientists in the United States suggests that a significant majority of the population may already have some level of immunity to the coronavirus, a possible explanation for why so many individuals seem to experience few to no symptoms from the disease. 

The study, written by researchers in California, New York and North Carolina and soon to be published in the journal Cell, discovered that certain types of cells in blood samples taken from donors in 2015-2018—well before COVID-19 arose—were reactive against the COVID-19 virus. In other words, those blood samples were at least partially immune from the coronavirus even though they had never been exposed to it. 

"CD4+ T cell responses were detected in 40-60% of unexposed individuals. This may be reflective of some degree of crossreactive, preexisting immunity to SARS- CoV-2 in some, but not all, individuals," the researchers state in the paper. 

The scientists are careful to qualify their conclusions. "Whether this immunity is relevant in influencing clinical outcomes is unknown—and cannot be known without T cell measurements before and after SARS- CoV-2 infection of individuals—but it is tempting to speculate that the crossreactive CD4+ T cells may be of value in protective immunity," they write.

The research could provide an important clue for public health officials hoping to figure out why significant numbers of COVID-19 infections are either asymptomatic or else largely mild. The disease affects elderly and less healthy individuals most severely, with younger and healthy individuals for the most part spared its worst effects. 

Though the term "coronavirus" has become ubiquitous in recent weeks as a way to describe the virus causing the current pandemic, coronaviruses are actually a variegated strain of infectious agents that cause illnesses ranging from the common cold to SARS. 

The researchers in their paper suggest that the immune response seen in the uninfected blood samples could have been generated by the coronaviruses that cause the common cold.