How have some people avoided getting Covid-19?

Dozens of Brits have had four bouts of Covid-19 and thousands more have been infected three times, according to official data.

As the pandemic entered a third year, figures from the UK Health Security Agency (UKHSA) showed that 62 people had reported four positive Covid tests, each at least 90 days apart.

But while many people have been re-infected multiple times, others appear to have avoided catching the coronavirus despite having come into close contact with cases – a disparity that scientists have struggled to explain. Here are some of the more plausible theories.

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Genetic resistance

An international consortium of researchers have launched a study to investigate whether a small proportion of people might be genetically resistant to Covid-19. Identifying such protective genes could lead to the development of new treatments for the coronavirus.

The scientists made a global appeal last October to find apparently Covid-resistant people to take part in the study, with the goal of enlisting a total of at least 1,000. “Of particular interest are people who shared a home and bed with an infected partner – pairs known as discordant couples,” reported scientific journal Nature.

The team, from ten research centres worldwide, had already recruited around 500 potential candidates. And within two weeks of launching their hunt, another 600 people had reportedly stepped forward offering to sign up.

“I did not think for one second that people themselves, exposed and apparently not infected, would contact us,” said study co-author Jean-Laurent Casanova of the Rockefeller University in New York.

The possible resistance that the study participants may have “is known to exist for other diseases, including HIV, malaria and norovirus”. The Guardian’s science correspondent Linda Geddes reported.

“In these cases, a genetic defect means some people lack a receptor used by the pathogen to enter cells, so they cannot be infected,” Geddes explained.

Study leader András Spaan, a professor at the Rockefeller University in New York, said that “it could well be that, in some individuals, there is such a defect in a receptor used by Sars-CoV-2”, the virus that causes Covid-19.

However, Spaan “thinks it is unlikely that the majority of those who have avoided Covid are genetically resistant, even if they have some partial immune protection”, said Geddes.

Common cold theory

A study led by a team from Imperial College London found that people who have fought off common colds may have a lower risk of catching Covid-19.

“We found that high levels of pre-existing T cells, created by the body when infected with other human coronaviruses like the common cold, can protect against Covid-19 infection,” said study co-author Rhia Kundu of Imperial’s National Heart and Lung Institute.

The researchers tested blood taken from 52 people in September 2020 – before Covid vaccines were rolled out – who were living with others who had just tested positive for the coronavirus. A total of 26 went on to get Covid during the 28-day study period.

In a bid to determine why the other half of the group escaped infection, the researchers looked at the role played by T cells, which are “a crucial part of the body’s immune system”, said the BBC.

Some of these T cells “kill any cells infected by a specific threat”, such as a cold virus, the broadcaster explained. And following a cold infection, some “remain in the body as a memory bank, ready to mount a defence against future attacks” by the virus.

According to a study paper published in Nature in January, the 26 test subjects who did not contract Covid had “significantly higher levels” of pre-existing T cells induced by previous common cold coronavirus infections that also cross-recognise proteins of the Sars-CoV-2 virus.

However, study co-author Kundu stressed that “no one should rely on this alone”, adding: “The best way to protect yourself against Covid-19 is to be fully vaccinated, including getting your booster dose.”

The team and other scientists also pointed out that not all colds are coronaviruses. Dr Simon Clarke, of the University of Reading, who was not involved in the study, said: “It could be a grave mistake to think that anyone who has recently had a cold is protected against Covid-19, as coronaviruses only account for 10%-15% of colds.”

Blood type connection

Some research findings have suggested that people with blood types A and AB are more susceptible to contracting Covid, while those with blood type O are less likely to test positive for the virus.

Two studies published in the Blood Advances journal in October 2020 showed a possible link between blood type and vulnerability to Covid. The first, by Danish researchers, compared health registry data from more than 473,000 people tested for Covid-19 with data from a control group of more than 2.2m non-tested people.

“Among the Covid-19 positive, they found fewer people with blood type O and more people with A, B, and AB types,” reported Science Daily.

The second study looked at data from 95 critically ill Covid patients hospitalised in Vancouver, Canada. Patients with the blood types A and AB were found to be more likely to require mechanical ventilation and to require dialysis for kidney failure.

Despite those findings, however, many scientists have remained unconvinced about the alleged blood type link. A review of nearly 108,000 Covid patients in the US, outlined in a paper in the Journal of the American Medical Association in April 2021, found no such link and said that more research was needed.

Asymptomatic infections

A less scientific possible explanation for why some people appear to have avoided Covid is that they actually have been infected but showed no symptoms.

According to Reuters, government estimates for the share of asymptomatic Omicron infections in Britain have ranged from 25% to and 54%.

And while many people have continued to test for Covid routinely, a recent study by Imperial College London found that lateral flow tests may be missing a “substantial” number of infections.

An analysis by a team from the university found that the Innova brand’s tests were missing between 20% and 81% of positive cases.

Concerns have also been raised that many people doing such tests are making mistakes – such as eating or drinking within the half hour beforehand, or forgetting to blow their nose in advance – that can cause incorrect results.

As one Twitter user asked: “Anyone else wondering if they’re really testing negative or just not shoving that thing far enough up their nose?”