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

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

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One of the big mysteries that has emerged from the Covid-19 pandemic – and one that is still being investigated by infectious disease specialists – is why some people catch Covid and others do not, even when they are just exposed to the virus.

Many of us know entire households that caught Covid and had to isolate themselves over the pandemic, but there are also several anecdotes about couples, families and colleagues where some people caught the virus – but not all.

In fact, Danny Altmann, professor of immunology at Imperial College London, told CNBC that studies indicate that the likelihood of being infected in a household when one case is positive is “not as high as you might 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 coronavirus were less likely to become infected with SARS-CoV. 2, the virus that causes Covid-19.

Dr. Rhia Kundu, lead author of the study from Imperial’s National Heart & Lung Institute, said that “being exposed to the SARS-CoV-2 virus does not always result in infection, and we have been eager to understand why.”

“We found that high levels of pre-existing T cells, created by the body when infected with other human coronaviruses such as the common cold, can protect against Covid-19 infection,” she said.

However, she also warned that “although 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 from Covid- 19 by being fully vaccinated, including getting your booster dose. “

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

He said early data suggest that these individuals have naturally acquired immunity from previous infections with common cold viruses. About 20% of common cold infections are caused by common cold viruses, he said, “but why some individuals maintain levels of cross-reactive immunity is still unknown.”

In addition to a degree of immunity given by previous exposure to coronavirus – a large family of viruses that cause disease ranging from colds to more serious illnesses or infections – one’s Covid vaccination status is probably also a factor in whether some people are more susceptible to Covid than others.

The role of vaccines

Covid vaccination is now widespread in most western countries, however, with variations among the population in relation to 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 serious, omicron variant.

Covid vaccines have been shown to reduce serious 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 diminishes over time, and has been somewhat compromised by the omicron variant.

Andrew Freedman, a graduate of Infectious Diseases at Cardiff University Medical School, told CNBC that why some people get Covid and others is not “a well-known phenomenon and presumably relates to immunity from vaccination, previous infection or both.”

“We know that many people still have (mostly mild) omicron infection despite being fully vaccinated, including [having had] and booster. However, vaccination still reduces the chance of catching the omicron, and the reactions vary from person to person. So some people catch it and others do not despite very significant exposure, “he said.

Medical staff member Mantra Nguyen installs a new oxygen mask for a patient at the Covid-19 Intensive Care Unit (ICU) at 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, “surely cross-reactive immunity from previous infections with the common cold coronavirus is likely to be a significant contributor, especially as these individuals may have additional immune benefits by also being vaccinated.”

Further studies of so-called “never Covid” individuals will help develop a better understanding of the immune response to SARS-CoV-2, Young insisted, and “which facets of the cross-reactive response are most important and how this information can be used to generate universal variant-safe vaccines. “

The genetic factor

Another question that has arisen during the pandemic is why two people with Covid may react so differently to the infection; one could have severe symptoms, for example, and the other could be asymptomatic.

The answer may lie in our genes.

“That’s a really important issue,” Imperial Colleges Altmann told CNBC on Wednesday.

He said he and his colleagues have conducted soon-to-be-published research 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 “make a difference, at least to whether you get symptomatic disease or not. “

The research is focused on different HLA genes (Human leukocyte antigen) and looks at how these can affect one’s response to Covid, with some HLA types being more or less likely to experience a symptomatic or asymptomatic infection, he said.

“Key genes that control your immune response are called HLA genes. They are important in determining your response when confronted with SARS-CoV-2. For example, people with the HLA-DRB1 * 1302 gene are significantly more likely to have a symptomatic infection. “added Altmann.

Could it be the tests?

The professor also pointed to the first results published on Wednesday of a British human challenge experiment conducted by Imperial and several other research bodies, in which 36 healthy young adults were deliberately exposed to Covid, but only half of them were actually infected with viruses. .

“How can it be that you pipette an identical dose of virus into people’s nostrils and 50% get infected, the other 50% do not?” asked Altmann, referring to the method used in the attempt to expose the participants to the virus.

Essentially all experimental volunteers were given a low dose of the virus – introduced via nasal drops – and then closely monitored by clinical staff in a controlled environment over a two-week period.

Out of the 18 volunteers who became infected – one of only a few of its kind in the world – 16 developed to develop mild to moderate cold-like symptoms, including a stuffy or runny nose, sneezing and sore throat.

The researchers who conducted the study said it was the first to be able to provide detailed data on the early stage of infection, before and during the occurrence of symptoms. Among the 18 infected participants, the average time from initial exposure to the virus to virus detection and early symptoms (i.e., the incubation period) was 42 hours, significantly shorter than existing estimates, which set the average incubation period at 5-6 days.

After this period, there was a steep increase in the amount of virus (viral load) found in inoculations taken from the participants’ noses or throats. These levels peaked at about five days after infection on average, but high levels of viable (infectious) viruses were still detected in laboratory tests up to nine days after inoculation on average and up to a maximum of 12 days for some.

A pair of protective masks walk down a street in the middle of a new wave of Covid-19 cases when the Omicron variant spread on December 28, 2021 in Buenos Aires, Argentina.

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It was also interesting where most viruses were found. While the virus was first detected in the throat and significantly earlier than in the nose (40 hours in the throat compared to 58 hours in the nose), the virus level was lower and peaked faster in the throat. Peak levels of virus were significantly higher in the nose than in the pharynx, indicating a potentially greater risk of the virus being excreted from the nose than from the mouth.

They noted that although there is a possibility of “lacking infectious virus early in the course of infection, especially if only the nose is tested,” the researchers said the results generally support continued use of lateral flow testing to identify people likely to be contagious.

“We found that lateral flow tests generally correlate very well with the presence of infectious virus,” said Professor Christopher Chiu, lead researcher in the experiment. “Although during the first day or two they may be less sensitive if you use them correctly and repeatedly and act on them if they read positively, this will have a big impact on interrupting viral spread.”

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