Some antibodies created by the immune system during infection with common cold coronaviruses may also provide some protection against Covid-19, new research suggests.

Some people, notably children, have antibodies reactive to SARS-CoV-2 in their blood, despite not ever having the virus, researchers say.

Antibodies are created by the immune system to help fight an infection.

These antibodies remain in the blood for a period after infection and in the case of reinfection – are able to tackle the virus again.

In a paper, published in Science researchers at the Francis Crick Institute and University College London, found that some people have Covid-19 antibodies despite never having the virus.

These antibodies are likely the result of exposure to other coronaviruses, which cause a common cold and which have structural similarities with SARS-CoV-2, the scientists suggest.

The researchers made the discovery while developing highly sensitive antibody tests for Covid-19.

To see how well their assay tests were performing, they compared the blood of patients with Covid-19 to patients who had not had the disease.

They found some people who had not been exposed to SARS-CoV-2 had antibodies in their blood which would recognise the virus.

To confirm the findings, they analysed more than 300 blood samples collected before the pandemic, between 2011 and 2018.

Nearly all of the samples had antibodies that responded to common cold coronaviruses, which was expected as everyone has been exposed to these viruses at some point in their lives.

However, a small fraction of adult donors, about 1 in 20, also had antibodies that cross-reacted with SARS-CoV-2.

The researchers found that in samples taken from 50 pregnant women in May 2018, 10% had cross-reactive antibodies.

Twenty-one of 48 blood samples taken from children aged one to 16 between 2011 and 2018 contained these cross-reactive antibodies.

The researchers found that such cross-reactive antibodies were found much more frequently in blood samples taken from children aged six to 16.

Kevin Ng, lead author and postgraduate student in the retroviral immunology laboratory at the Crick said: “Our results show that children are much more likely to have these cross-reactive antibodies than adults.

“More research is needed to understand why this is, but it could be down to children being more regularly exposed to other coronaviruses.

“These higher levels we observed in children could also help explain why they are less likely to become severely ill with Covid-19.

“There is no evidence yet, however, that these antibodies prevent SARS-CoV-2 infection or spread.”

In the laboratory the researchers tested the antibodies they found in blood from uninfected people to confirm they are able to neutralise SARS-CoV-2, the virus that causes Covid-19.

They found the cross-reactive antibodies target the S2 subunit of the spike protein on the surface of the virus.

George Kassiotis, senior author and group leader of the retroviral immunology laboratory at the Crick says: “The spike of this coronavirus is made of two parts or subunits, performing different jobs.

“The S1 subunit allows the virus to latch onto cells and is relatively diverse among coronaviruses, whereas the S2 subunit lets the virus into cells and is more similar among these viruses.

“Our work shows that the S2 subunit is sufficiently similar between common cold coronaviruses and SARS-CoV-2 for some antibodies to work against both.

“It was previously thought that only antibodies to the S1 could block infection, but there is now good evidence that some antibodies to S2 can be just as effective.

“This is exciting as understanding the basis for this activity could lead to vaccines that work against a range of coronaviruses, including the common cold strains, as well as SARS-CoV-2 and any future pandemic strains.”

The researchers say it is important to stress there are still many unknowns which require further research like exactly how is immunity to one coronavirus modified by exposure to another.