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

A man with her protective face mask walks in Vellaces neighborhood after new restrictions came into force as Spain sees record daily coronavirus (Covid-19) cases, in Madrid, Spain on September 21, 2020. (Photo by Burak

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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.”

‘Never Covid’ people

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 susceptible to Covid than others.

The role of vaccines

Covid vaccination is now widespread in most Western countries, albeit with variations among populaces in terms of which coronavirus vaccine was administered, and when.

Booster shots are also being deployed widely, and younger children are being vaccinated in many countries, as governments race to protect as many people as possible from the more transmissible, but less clinically severe, omicron variant.

Covid vaccines have been proven to reduce severe infections, hospitalizations and deaths and remain largely effective against known variants of the virus. However, they are not 100% effective in preventing infection and the immunity they provide wanes over time, and has been somewhat compromised by the omicron variant.

Andrew Freedman, an academic in infectious diseases at Cardiff University Medical School, told CNBC that why some people get Covid and others don’t “is a well recognized phenomenon and presumably relates to immunity from vaccination, previous infection or both.” 

“We know that many people have still caught (mostly mild) omicron infection despite being full vaccinated, including [having had] a booster. However, vaccination does still reduce the chance of catching omicron and responses do vary from person to person. So some people catch it and others don’t despite very significant exposure,” he said.

Medical staff member Mantra Nguyen installs a new oxygen mask for a patient in the Covid-19 intensive care unit (ICU) at the United Memorial Medical Center in Houston, Texas.

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Warwick University’s Young said, when it comes to different immune responses to Covid, “certainly cross-reactive immunity from previous infections with common cold coronavirus is likely to be a major contributor, particularly as these individuals may have additional immune benefits from also having been vaccinated.”

Further studies into so-called “never Covid” individuals will help in…

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