Study finds past coronavirus (COVID-19) infection provides some immunity for at least 5 months, but people may still carry and transmit the virus.
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.
It is therefore crucial that everyone continues to follow the rules and stays at home, even if they have previously had COVID-19, to prevent spreading the virus to others. Remember to wash hands regularly, wear face coverings and make space from others to help reduce the likelihood of passing on the virus.
It is vital that, with cases at their highest level to date and the R number above 1 across the country, people do everything that they can to avoid the risk of transmitting the virus to other people.
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.
Background information
PHE’s SIREN (SARS-CoV-2 Immunity and Reinfection EvaluatioN) study has performed regular antibody and PCR testing on 20,787 healthcare workers, including frontline clinical staff and those in non-clinical roles, from 102 NHS trusts since the study commenced in June. 6,614 of these participants tested positive for COVID-19 antibodies upon recruitment.
Of the 44 potential reinfections identified by the study, 2 were designated ‘probable’ and 42 ‘possible’, based on the amount of confirmatory evidence available. If all 44 cases were confirmed, it would represent an 83% rate of protection from reinfection, while if only the 2 ‘probable’ reinfections were confirmed, the rate would be 99%. Further research is ongoing to clarify this range.
The study found that antibody protection after infection lasts for at least 5 months, on average, and scientists are currently studying whether protection may last for longer. This means that many people who contracted the disease in the first wave may now be vulnerable to catching it again.
Both of the 2 ‘probable’ reinfections reported having experienced COVID-19 symptoms during the first wave of the pandemic, but were not tested at the time. Both patients reported that their symptoms were less severe the second time. None of the 44 potential reinfection cases were PCR tested during the first wave, but all tested positive for COVID-19 antibodies at the point of recruitment to the study.
This analysis occurred prior to the widespread dissemination of the new variant VOC202012/01, further work is underway in the laboratory to understand whether and to what extent antibodies also provide protection from this variant and future analysis will assess the impact of VOC202012/01 on symptomatic and asymptomatic infections in healthcare workers.
The study will continue to follow participants for 12 months to explore how long any immunity may last, the effectiveness of vaccines and to what extent people with immunity are able to carry and transmit the virus.
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