Some people who have never been exposed to the new coronavirus Sars-CoV-2 may have an immune system prepared to respond to it because of previous infections from other coronaviruses, according to a new study that can potentially offer clues to building immunity against Covid-19.

Published in the journal Nature on July 15, the study by researchers from Singapore reports some people have T-cells that recognise both proteins on the virus called structural or nucleocapsid protein, and non-structural viral proteins (NSP) produced within an infected cell. T-cells are a type of white blood cell that recognise infections within a cell and kill it.

“We characterised Sars-CoV2 specific T-cells in Covid-19 and Sars (the 2003 outbreak) convalescents and uninfected healthy individuals,” said study co-author Dr Antonio Bertolleti, a professor of emerging infectious diseases at Duke-Nus Medical School in Singapore, in an email. Those who recovered from Covid-19 had T-cells that recognised structural proteins more efficiently than NSPs. Additionally, those infected with severe acute respiratory syndrome (Sars), which is also caused by a coronavirus, 17 years ago also had T-cells that reacted to these proteins, suggesting induced cross reactive cellular immunity.

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“In our study, more than 50% of (37) uninfected healthy individuals demonstrate the presence of Sars-CoV2 specific T cells. Importantly, these T cells recognize more frequently sequences of non-structural proteins found only in animal coronaviruses. Thus, a large part of the population possess Sars-Cov2-specific T cells, likely induced by other coronaviruses,” said Dr Bertoletti. “One possible implication of our findings is that many subjects might have a level of progress T-cell reactivity that might partially protect them.”

Experts, however, say the results are not surprising. “On average, 20% of the common cold is because of coronaviruses already endemic in the human population. Sars-CoV-2 shares about 80% sequence identity with Sars-CoV-1 and 50% with other coronaviruses. So there are proteins made by all of these viruses that have common sequences,” said Shahid Jameel, virologist and CEO of Wellcome Trust/DBT India Alliance.

The Sars outbreak in 2003 largely spared India but more research is needed. “There may have been coronaviruses, maybe innocuous, in the past in the country, but we don’t have research. I have been asking those searching for IgG (antibodies) as to how do they know if it is of Sars-CoV-2 and not of any other coronavirus in the background?” said Sudhanshu Vrati, executive director of the regional centre for biotechnology, Faridabad.

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The study has implications on vaccine development. “People who are given inactivated virus vaccines that have fewer viral proteins, are more likely to generate neutralising antibodies and some T-cell response. T-cell response is higher and longer if you use the live virus vaccine. But it is difficult to make a live vaccine,” said Vrati.

Noted drug developer Derek Lowe, who reviewed the paper, said vaccine development may have to be driven by T-cell immunity. “(It) is perhaps the way to reconcile the apparent paradox between (1) antibody responses that seem to be dropping week by week in convalescent patients but (2) few (if any) reliable reports of actual re-infection.”