Video report by ITV News Science Editor Tom Clarke
People who recover from Covid-19 could have immunity for at least five months but could still carry the virus and pass it to other people, scientists warned.
The findings suggest people who had coronavirus in the first wave could now catch it again.
A study by Public Health England (PHE) found antibodies from past infection provide 83% protection against reinfection for at least five months.
But it said early evidence from the next stage of the study suggests some of these individuals carry high levels of the virus and could continue to transmit it to others.
Professor Susan Hopkins, senior medical adviser at PHE who is leading the Siren study, 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.
Dr Sarah Jarvis has more on the latest developments from the UK's Covid crisis:
“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.”
Since June, PHE has been testing tens of thousands of healthcare workers across the UK for new coronavirus infections and antibodies.
Reinfections are rare. Out of 6,614 people in the study who had antibodies, 44 were potentially reinfected between June 18 and November 24 – two probable, and 42 possible.
The study, which has not been peer-reviewed, will continue to see whether protection may last longer and consider vaccine responses later this year.
Prof Hopkins added: “It is good that it’s protecting people, but it’s not 100% protective, and therefore people still need to follow the rules, until we know more about this, on the durability of the response, and also understand better why some of these individuals have not responded, or is this particular to a certain group.”
Science Editor Tom Clarke reporting on the coronavirus vaccine rollout
She continued: “And at that time (when the study was designed), the vaccine effectiveness studies were setting out to look at ‘did vaccines have 50% to 60% efficacy in reducing future symptomatic infection?’
“And if you look at this from a symptomatic point of view, we’re seeing a very small proportion – 13 out of the 44 – have had symptomatic infection.”
Prof Hopkins added: “What’s that saying to us is that prior infection looks as good as vaccine, at least at this time interval, which is very good news for the population, and it will help alongside vaccine to give a level of immunity in the population that will start to reduce transmission.
“What we don’t yet know and what this study is setting out to do is to determine the durability of that response.
“Overall I think this is good news, it allows people to feel that their prior infection will protect them from future infections, but at the same time, it is not complete protection, and therefore they still need to be careful when they’re out and about.”
“I am strongly encouraged that people have immunity that is lasting much more than the few months that was speculated before the summer,” Prof Hopkins said.