Why do some people get Covid while others don’t?

One of the great mysteries that has emerged from the Covid-19 pandemic — and one that’s still being investigated by infectious disease specialists — is why some people catch Covid and others don’t, even when they’re equally exposed to the virus.

Many of us know entire households who caught Covid and had to isolate over the pandemic, but there are also multiple anecdotes of couples, families and colleagues where some people caught the virus — but not everyone.

Indeed, Danny Altmann, professor of immunology at Imperial College London, told CNBC that studies indicate the likelihood of becoming infected within a household once one case is positive is “not as high as you’d imagine.”

An increasing amount of research is being devoted to the reasons why some people never seem to get Covid — a so-called “never Covid” cohort.

Last month, new research was published by Imperial College London suggesting that people with higher levels of T cells (a type of cell in the immune system) from common cold coronaviruses were less likely to become infected with SARS-CoV-2, the virus that causes Covid-19.

Dr Rhia Kundu, first author of the study from Imperial’s National Heart & Lung Institute, said that “being exposed to the SARS-CoV-2 virus doesn’t always result in infection, and we’ve been keen to understand why.”

“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,” she said.

However she also cautioned that, “while this is an important discovery, it is only one form of protection, and I would stress that no one should rely on this alone. Instead, the best way to protect yourself against Covid-19 is to be fully vaccinated, including getting your booster dose.”

Lawrence Young, a professor of molecular oncology at Warwick University, told CNBC Wednesday that, “there’s much interest in these cases of so-called ‘never Covid’ – individuals who have clearly been exposed to close contacts in their household who are infected, but who themselves are resistant to infection.”

He said that early data suggests these individuals have naturally acquired immunity from previous infections with common cold coronaviruses. Around 20% of common cold infections are due to common cold coronaviruses, he said, “but why some individuals maintain levels of cross-reactive immunity remains unknown.”

As well as a degree of immunity provided by prior exposure to coronaviruses — a large family of viruses that cause illness ranging from the common cold to more severe diseases or infection — one’s Covid vaccination status is also likely to be a factor as to whether some people are more…