Why do some people get Covid while others don’t?
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After new Spanish restrictions were implemented, a man walks with his protective mask in Vellaces. This is after Spain has seen record numbers of coronavirus (Covid-19), cases in Madrid on September 21st 2020. (Photo: 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 epidemiologists and virologists — 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.”
People who are ‘Never Covid’
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.
Imperial College London released new research that suggested that individuals with greater levels of T cells, a type of immune cell, were less likely than others to contract SARS-CoV-2 (the virus responsible for 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.”
She said that Covid-19 can be prevented by high levels of T cells in the body.
She also warned that “while this discovery is important, it’s only one form protection and I would emphasize that nobody should rely solely on this.” The best way to prevent Covid-19 from happening is to get your booster shot and be fully vaccinated.
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.”
Early data suggest that this group has naturally developed immunity to previous infection with the common cold coronaviruses, he said. He said that around 20% of all common cold viruses are common cold coronaviruses. However, it is not known why certain individuals have cross-reactive immunity.
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 importance of vaccines
Covid vaccination has become widespread in many Western countries. However, there are variations in how and when coronavirus vaccine is administered.
As governments try to prevent the spread of the less severe but still transmissible omicron variant, booster shots are widely used.
Covid vaccines can be used to prevent severe infections and hospitalizations. They are also effective against the known variations of the virus. Although they can prevent infection, their effectiveness is not always 100%. Their immunity also wanes with time. This has been partially compromised by the Omicron variant.
Andrew Freedman from Cardiff University Medical School is an academic in infectious disease. CNBC learned that Covid, which can affect people’s immunity, past infection, and/or both, was a common phenomenon.
We know of many individuals who have caught mild omicron infections despite having been fully vaccinated. [having had]A booster. But, vaccination doesn’t reduce your chance of getting omicron. Individual responses will vary. He explained that some people get it while others do not, despite having been exposed to significant amounts of the disease.
Mantra Nguyen (medical staff) installs an oxygen mask to a patient within the Covid-19 ICU at United Memorial Medical Center, Houston, Texas.
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Young of Warwick University said that Covid can have different responses. Young stated, “certainly cross reactive immunity from prior infections with the common cold coronavirus may be important, especially as individuals who were vaccinated might also contribute to an immune response.”
Young said that more research will be done into the so-called “never Covid”, which will allow for a deeper understanding of how the immune system reacts to SARS. Young also highlighted the “most important facets” of the cross-reactive responses and the ways in which this information can help to develop universal vaccines.
The genetic factor
A second question has been raised during the epidemic: why do Covid patients respond differently? One person could experience severe symptoms while the other may be more asymptomatic.
It could be in our genes.
CNBC’s Altmann, from Imperial College, said that “it’s an important question.”
He said that he and his colleagues have conducted research, to be published soon, into immunogenetics (essentially, the relationship between genetics and the immune system) and Covid-19 infection, and have found that variations between people’s immune systems “makes a difference, at least to whether or not you get symptomatic disease.”
He said that the research is about different HLA genes (Human Leukocyte Antigen). It is looking into how they can influence one’s reaction to Covid. Some HLA types are more likely to have a symptomatic or asymptomatic infection.
HLA genes refer to the genes responsible for controlling your immune response. These genes are important for your reaction to SARS-CoV-2. For example, people with the gene HLA-DRB1*1302 are significantly more likely to have symptomatic infection,” Altmann added.
It could be the test.
Professor also pointed out Wednesday’s first results. British human challenge trialImperial and many other research organizations conducted an experiment in which 36 healthy, young adults were exposed to Covid. Only half became infected.
Altmann was referring to the way the virus was introduced to participants in the study.
Essentially all the trial volunteers they were given a low dose of the virus — introduced via drops up the nose — and then carefully monitored by clinical staff in a controlled environment over a two-week period.
Out of the 18 volunteers that became infected — one of only a few of its kind in the world — 16 went on to develop mild-to-moderate cold-like symptoms, including a stuffy or runny nose, sneezing, and a sore throat.
Researchers who conducted the study claimed that it is the first time they have been able to collect detailed information about the initial phase of infection before symptoms appear. The time it took for the infection to be detected and symptoms to appear was only 42 hours. That is significantly faster than previous estimates which had a 5-6 day incubation.
The viral load, or virus level in the swabs of participants’ throats and noses increased dramatically after this period. The average infection time was five days. However, high levels of virus (viral load) were detected in laboratory tests as long as nine days and sometimes up to 12 days in some cases.
As Omicron, a variant of Covid-19, spreads to Buenos Aires, Argentina, a couple wearing masks and walking on a sidewalk is one example.
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It was also interesting to see where the virus was most prevalent. The virus was first detected in the throat and it took 40 hours to develop (compared with 58 hours for the nose), but virus levels in the throat were much lower than the nose and reached their peak earlier. There was a higher risk that the virus would be shed through the nose than from the throat.
The researchers noted that there was a chance of an “infectious virus being missed early in the course, especially if it is only tested on the nose.” However, the overall findings support continued use lateral flow tests for identifying people who are likely to have the disease.
The trial’s chief investigator Professor Christopher Chiu stated that lateral flow tests were correlated very well with the presence infectious virus. They are less sensitive in the initial days, but if used correctly, repeatedly and acted upon if positive, they can have an important impact on stopping viral spread.
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