Immune-fighting antibodies against the coronavirus are 83% effective at protecting against reinfection for at least the next five months, research suggests.
The extent to which overcoming the coronavirus prevents a second infection has been debated since the start of the outbreak, with experts and officials alike being unsure how long any protection lasts.
Concerns were raised in October when scientists from Imperial College London reported antibodies wane “quite rapidly”. Many were quick to point out, however, antibodies make up just one aspect of immunity, with so-called T cells also fighting infections.
To learn more, scientists from Public Health England (PHE) have been regularly testing tens of thousands of healthcare workers across the UK since June, looking for both coronavirus infections and antibodies.
Out of the more than 6,600 workers who “showed evidence of previous infection”, the team identified just 44 “potential” or “probable” reinfections.
Although “reassuring”, early evidence also suggests a small number of people with antibodies may still carry the virus in their nose and throat, raising the risk of transmission.
It is also unclear if the antibody response persists beyond five months, with the study’s coronavirus survivors potentially being at risk of reinfection amid the UK’s second wave.
“This study has given us the clearest picture to date of the nature of antibody protection against COVID-19 [the disease caused by the coronavirus], but it is critical people do not misunderstand these early findings,” said lead author Professor Susan Hopkins.
“We now know 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 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.”
The scientists also stressed the study only analysed the immune responses of people who overcame the coronavirus naturally, rather than those who have been vaccinated.
The ongoing one-year research, called SIREN, will look into vaccine responses later in 2021.
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Between 18 June and 24 November, more than 20,700 healthcare workers across 102 NHS trusts were tested for a coronavirus infection and antibodies every two to four weeks.
Once an infection has been overcome, “memory antibodies” circulate at low levels in the blood. If the infection is encountered again, these “ramp up”, helping to prevent it taking hold.
The PHE scientists did not measure other aspects of immunity, like T cells.
Of the healthcare workers, more than 6,600 tested positive for antibodies, while over 14,000 showed no sign of the immune-fighting proteins.
Two of the 6,600 later developed a “probable” reinfection, while 42 endured a “possible” reinfection.
“This represents an 83% rate of protection from reinfection,” wrote the scientists.
The two probable reinfection cases reported symptoms during the UK’s first coronavirus wave, but were not tested. Their symptoms were less severe the second time round.
A probable reinfection was defined as “genomic confirmation between the two episodes”, with the scientists also being sure the same worker was infected twice, rather than there being a “lab mix up”, according to Prof Hopkins.
The genome consists of all an organism’s genetic material.
Equally, none of the 42 possible reinfection cases were tested for the coronavirus during its first wave, but all showed antibodies during the PHE study.
A possible reinfection was defined as a worker having “clear symptoms” or a “serological response to the virus or genome that is circulating now but wasn’t around in the original virus”.
A serological response describes antibodies being identified in the blood.
The PHE research was carried out during the time the so-called Kent coronavirus variant is thought to have emerged, likely in September. Another variant is said to have arisen more recently in South Africa.
Immunity against these variants among people who overcame a previous version of the virus is unclear and will be studied as part of the ongoing PHE investigation.
“The [PHE] study [took] place before the UK virus variant took hold; so 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,” said Professor Lawrence Young, from Warwick Medical School.
The PHE results mimic a similar study into more than 12,000 healthcare workers by the University of Oxford, which suggests an approximate 90% protection rate around six months after overcoming the coronavirus.
The PHE scientists stressed, however, the possibility people with antibodies may transmit the virus – or even re-catch it – means everyone must adhere to the coronavirus regulations and stay home as much as possible.
Prof Young agreed, adding: “One extremely important message from this study is the antibody protection after infection lasts for five months on average, but we don’t know how long it will last beyond that time.
“It is therefore possible many people who were infected during the first wave of the pandemic may now be susceptible to re-infection.
“Evidence from previous studies with common cold coronaviruses indicates this re-infection is unlikely to result in severe disease but may mean re-infected individuals are able to spread the virus.”
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Dr Julian Tang from the University of Leicester called the results “useful” and “reassuring”, but not all that surprising.
The circulating coronavirus is one of seven strains of a virus class that are known to infect humans, with others including severe acute respiratory syndrome (Sars), Middle East respiratory syndrome (Mers) and seasonal colds.
“These results are not surprising as we already know other coronaviruses - including SARS-COV-1 [Sars] and MERS-COV [Mers], as well as seasonal common cold coronaviruses, do produce long-lasting antibodies post-infection,” said Dr Tang.
“So hopefully the results from this paper will reduce the anxiety of many healthcare worker colleagues who have concerns about getting COVID-19 twice.”
The PHE study provides no evidence on the vaccine-induced immune response.
Nevertheless, Prof Hopkins said: “Prior infection looks at least as good as vaccination at this interval.”
Dr Tang agreed, adding: “These reported protective ‘efficacy’ results for natural SARS-COV-2 [coronavirus] infection are comparable to the current COVID-19 vaccines.”
People who have overcome the coronavirus are still encouraged to get vaccinated when called up.
“Having the vaccine after recovering from COVID-19 is not an issue or contraindication, and will likely boost the natural immunity,” said Dr Tang.
“We see this with the seasonal flu vaccine.”
When asked if a former patient could forgo a vaccine due to their suspected immunity, Prof Hopkins said: “I think that would add complexities.”
The PHE study was carried out among working-age adults.
As well as the elderly being more vulnerable to coronavirus complications, they tend to have a less robust immune response to vaccines and infections, pointed out Prof Hopkins.
This suggests jabs will particularly be required among older people, regardless of whether they have been infected before.
“It will be interesting to know the degree to which previously infected individuals who are subsequently vaccinated are susceptible to reinfection,” added Young.
“It could be these individuals have an even longer-lived protective immune response.”
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