The study — which has not yet been peer reviewed — found that past infection was linked to an 83% lower risk of reinfection, compared to people who have not been infected before.
But researchers warned that the protection was not absolute, meaning some people do catch the virus again, and that it was unclear how long any immunity lasts. It is also possible that those who have a degree of immunity against the virus may still be able to carry the virus in their nose or throat and therefore transmit it to others.
“We now know that most of those who have had the virus, and developed antibodies, are protected from reinfection, but this is not total and we do not yet know how long protection lasts,” Susan Hopkins, senior medical adviser at PHE and co-leader of the study, said in a statement.
“Even if you believe you already had the disease and are protected, you can be reassured it is highly unlikely you will develop severe infections. But there is still a risk you could acquire an infection and transmit (it) to others,” Hopkins said.
The SIREN research study regularly tested almost 21,000 health workers from across the UK between June and November. Among them, 6,614 people participants tested positive for antibodies against the virus, while more than 14,000 had no signs of previous infection.
But of those who had been infected, 44 developed possible new infections — representing an 83% level of protection against reinfection.
The study will continue to monitor healthcare workers for 12 months to see if protection lasts even longer, but for now the length of protection identified means that people who caught the virus in the first wave of infections may now be able to catch it again. It will also look into the impact of the new variant and the effectiveness of Covid-19 vaccines.
But early insight from the next stage of the study shows that some people with existing immunity carry high levels of virus and could transmit the virus to others.
Hopkins highlighted this during an interview with the BBC’s Today program on Thursday.
“We found people with very high amounts of virus in their nose and throat swabs, that would easily be in the range which would cause levels of transmission to other individuals.”
Hopkins stressed that people who had previously caught Covid-19 still needed to obey social distancing rules to avoid transmitting the disease.
“The SIREN study has major implications for how we can get out of the current crisis. The good news is that this study gives further weight that reinfections of Covid is rare, at least at this stage, and that having antibodies will provide protection for a meaningful amount of time, although it may not be lifelong immunity,” Dr. Simon Clarke, Associate Professor in Cellular Microbiology, University of Reading, told the Science Media Centre.
“The concerning finding is that some people who have Covid antibodies appear to still be able to carry the coronavirus and could spread it to others. This means that the vast majority of the population will either need to have natural immunity or have been immunised for us to fully lift restrictions on our lives,” he said.
Lawrence Young, Virologist and Professor of Molecular Oncology, Warwick Medical School added that because the results cover the months before the new variant took hold, “it will be important to determine whether previous infection with the old virus variant is able to offer protection from re-infection with the new virus variant.”
England is currently under a stringent national lockdown after cases surged over the holiday period. The UK has recorded more than 3.2 million cases of infection.