PARIS • Could the ghosts of your previous colds help protect you from Covid-19, even if you have never been infected by the new coronavirus?
Scientists are investigating a poorly understood immune mechanism in the body that they hope could help efforts to curb the pandemic.
At the moment, people who think they have had the virus might get a serological test to check for antibodies.
These proteins help fight off infection and may prevent them from getting the disease again in the future - but there are signs that with Covid-19 they could fade away within weeks.
This leaves the other instrument in the body's toolkit, T lymphocytes - a type of white blood cell responsible for the second part of the immune response.
With little yet known about how they operate against Covid-19, scientists are racing to fill in the gaps in our knowledge.
One hypothesis is that these T cells might help give people a level of cross-immunity protection from Covid-19 because they "remember" previous infections by other viruses in the same family, four of which cause common colds.
"The immune system is complex," said Professor Andreas Thiel, who co-authored a study that looked at the presence of T cells able to react to the new coronavirus, both among those with confirmed infections and healthy people.
The research, published in the past week in the journal Nature, found that at least a third of adults who had never had Covid-19 have these T cells.
"These most likely originate from previous infections with endemic coronaviruses," Prof Thiel, from the Berlin-Brandenburg Centre for Regenerative Therapies, told AFP.
But he cautioned that much more research was needed to find out whether their presence would necessarily mean immunity.
The research followed a study by a team in Singapore published in Nature earlier last month that reached a similar conclusion. Another study from the United States, published on Tuesday in the journal Science, found a number of T cells that reacted both to the new virus, Sars-CoV-2, as well as to the coronaviruses that cause colds.
"This could help explain why some people show milder symptoms of disease while others get severely sick," said co-author Daniela Weiskopf, of La Jolla Institute for Immunology, in a statement.
This study builds on research, published in the journal Cell in May by the same team, which detected these Sars-CoV-2 reacting T cells in 40 to 60 per cent of people who had never had Covid-19.
The vaccines currently in development for the new coronavirus seek to trigger both types of immune response. However, attention had in the past largely focused on the immunity conferred by antibodies
"But we must not think that nothing else exists," Prof Yonathan Freund, an emergency medicine expert at the Paris Pitie-Salpetriere hospital, told AFP.