The research, published in the journal Science, noted that the immune system's memory T cells keep track of the viruses they have seen before, giving the cells a headstart in recognising and fighting off repeat invaders.

However, the scientists, including those from La Jolla Institute (LJI) in the US, cautioned that it is too soon to say whether such pre-existing immune cell memory affects COVID-19 clinical outcomes. 

"We have now proven that, in some people, pre-existing T cell memory against common cold coronaviruses can cross-recognise SARS-CoV-2, down to the exact molecular structures," said Daniela Weiskopf, a co-author of the study from LJI. "This could help explain why some people show milder symptoms of disease while others get severely sick," Weiskopf said.

Alessandro Sette, another co-author of the study from LJI, noted that the reactivity of the immune system may translate to different degrees of protection. "Having a strong T cell response, or a better T cell response may give you the opportunity to mount a much quicker and stronger response," Sette said.

An earlier study by Sette and his team had shown that 40 to 60 per cent of people who were never exposed to the novel coronavirus SARS-CoV-2 had T cells that reacted to the virus. 

According to the study, the immune systems in these individuals recognised fragments of the virus it had never seen before -- a finding which was also reported among people in the Netherlands, Germany, Singapore, and the UK.

In the current research, the scientists assessed samples collected from study participants who had never been exposed to SARS-CoV-2. They defined the exact parts of the virus that are responsible for the cross-reactive T cell response. Their analysis showed that unexposed individuals can produce a range of memory T cells that are equally reactive against SARS-CoV-2, and four types of common cold coronaviruses.

Based on the finding, the scientists said fighting off a common cold coronavirus could teach the T cell compartment to recognise some parts of SARS-CoV-2 as well. They believe this process provides evidence for the hypothesis that common cold viruses can, in fact, induce cross-reactive T cell memory against SARS-CoV-2. 

"We knew there was pre-existing reactivity, and this study provides very strong direct molecular evidence that memory T cells can 'see' sequences that are very similar between common cold coronaviruses and SARS-CoV-2," Sette said.

The scientists found that while some cross-reactive T cells targeted the SARS-CoV-2's spike protein -- the region of the virus that recognises and binds to human cells -- pre-existing immune memory was also directed to other SARS-CoV-2 proteins. Sette noted that the finding is relevant since most vaccine candidates target the spike protein. 

The findings, according to the researchers, suggest the hypothesis that inclusion of additional SARS-CoV-2 targets might enhance the potential to take advantage of this cross reactivity, and could further enhance vaccine potency.