Why some people don’t get Covid despite contact with a patient?

Scientists are investigating why some people do not seem to get infected with Covid-19 despite staying in close proximity with those who got infected, CNBC reported.

There have been many instances among couples, families or groups of colleagues where not everyone became infected.

Researchers have attributed the term "Covid cohort" to this group of people, who seem to get away even without adhering to health safety measures such as social distancing and wearing masks.

According to the researchers, this phenomenon could be due to an array of factors, including prior infection from similar virus and genetics.

Studies suggest that the probability of everyone getting infected by Covid-19 within a household after one member is found positive is "not as high as you'd imagine," Danny Altmann, professor of immunology at Imperial College London, told CNBC.

Prior infections and vaccination

A new research published by Imperial College London last month revealed that people with higher levels of T cells (a type of cell in the immune system) from other coronaviruses that cause common cold were less likely to become infected with SARS-CoV-2, the virus that causes 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 Dr Rhia Kundu, first author of the study from Imperial's National Heart and Lung Institute.

"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," she noted.

Lawrence Young, a professor of molecular oncology at Warwick University, told CNBC on Wednesday that early data suggests many individuals have naturally acquired immunity from previous infections with common cold coronaviruses.

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Around 20% of all common cold infections are due to common cold coronaviruses, he said.

"But why some individuals maintain levels of cross-reactive immunity remains unknown," he added.

In case of different immune responses to Covid-19, "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," Young also said.

Further research into so-called Covid cohorts will help in understanding of the immune response to SARS-CoV-2, Young further said.

As vaccination is being carried out en masse across the world, including booster doses for elderly people as well as jabs for younger children, it has reduced the severity of symptoms, rate of hospitalisations and deaths, and is significantly effective against the virus across different known variants.

The vaccines are, however, not 100% effective in preventing infection and the immunity produced from vaccines wanes over time.

The vaccines are also found to be compromised by omicron variant of Covid-19 to some extent.

"We know that many people have still caught (mostly mild) Omicron 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," Andrew Freedman, an academic in infectious diseases at Cardiff University Medical School, told CNBC.

Genetics

Our genetic factors may also play a role to decide how two persons may respond to Covid-19 infection at the same time.

Imperial College's Altmann told CNBC that he and his colleagues have conducted research into immunogenetics (the relationship between genetics and the immune system) and Covid-19 infection, and have found that variations between people's immune systems "make a difference, at least to whether or not you get symptomatic disease."

They studied different HLA (human leukocyte antigen) genes and are investigating how these can affect a person's response to Covid-19, he said.

"The key genes that control your immune response are called HLA genes. They matter for determining your response on encounter with SARS-CoV-2. For example, people with the gene HLA-DRB1*1302 are significantly more likely to have symptomatic infection," he also said.

While some HLA types are more likely to experience a symptomatic infection, others may be asymptomatic, Altmann added.