People infected with COVID-19 in the past are likely to be protected against reinfection for several months, a Public Health England (PHE) study has found, although experts cautioned those with immunity may still be able carry the virus in their nose and throat and therefore have a risk of transmitting to others.
PHE has been regularly testing tens of thousands of health care workers across the UK since June for new COVID-19 infections as well as the presence of antibodies, which suggest people have been infected before.
SIREN study leaders are clear this first report provides no evidence towards the antibody or other immune responses from COVID-19 vaccines, nor should any conclusions to be drawn on their effectiveness. The SIREN study will consider vaccine responses later this year.
PHE scientists working on the study have concluded naturally acquired immunity as a result of past infections provide 83% protection against reinfection, compared to people who have not had the disease before. This appears to last at least for 5 months from first becoming sick.
While the SIREN study will continue to assess whether protection may last for longer, this means people who contracted the disease in the first wave may now be vulnerable to catching it again.
Between 18 June and 24 November, scientists detected 44 potential reinfections (2 ‘probable’ and 42 ‘possible’ reinfections) out of 6,614 participants who had tested positive for antibodies. This represents an 83% rate of protection from reinfection.
PHE also warned that although those with antibodies have some protection from becoming ill with COVID-19 themselves, early evidence from the next stage of the study suggests that some of these individuals carry high levels of virus and could continue to transmit the virus to others.
Professor Susan Hopkins, Senior Medical Advisor at Public Health England and the SIREN study lead said:
"This study has given us the clearest picture to date of the nature of antibody protection against COVID-19 but it is critical people do not misunderstand these early findings.
"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. Crucially, we believe people may still be able to pass the virus on.
"This means 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 that you could acquire an infection and transmit to others. Now more than ever it is vital we all stay at home to protect our health service and save lives.
"We are immensely grateful to our colleagues in the NHS for giving up their time to volunteer, and whose continued participation at a time of great stress is making this research possible."