There have been several reports of Covid cases being detected among couples, families, or groups of coworkers who have come into close contact but not all of them have gotten infected. Some people do not get Covid-19
This might be attributable to a multitude of causes, including past infection with a virus identical to the one in question or heredity.
One of the major mysteries of the Covid19 epidemic and one that infectious disease experts are still looking into — is why some individuals get Covid and others don’t, while being equally exposed to the virus.
Many of us know whole families that contracted Covid and were forced to isolate during the epidemic, but there are also numerous instances of couples, families, and coworkers who contracted the virus — but not everyone.
Indeed, according to research, the chances of being infected inside a family once one case is positive are “not as great as you’d anticipate,” according to Danny Altmann, professor of immunology at Imperial College London.
People who not get Covid-19
The reasons why some people not get Covid-19 — the so-called never Covid cohort — are the subject of an increasing amount of study.
People with larger numbers of T cells (a kind of immune cell) from common cold coronaviruses were less likely to become infected with SARS-CoV-2, the virus that causes Covid-19, according to new research released last month by Imperial College London.
“Being exposed to the SARS-CoV-2 virus doesn’t always result in infection, and we’ve been anxious to understand why,” said Dr. Rhia Kundu of Imperial College’s National Heart and Lung Institute.
“We discovered that large amounts of pre-existing T cells, which are produced by the body when infected with other human coronaviruses like the common cold,” she added.
“While this is a significant finding, it is only one kind of protection,” Kundu noted, “and I would emphasise that no one should rely on this alone.” Instead, getting completely vaccinated, including your booster dose, is the greatest method to protect yourself against Covid-19.”
“There’s great curiosity in these examples of so- called ‘never Covids’ persons who have definitely been exposed to close contacts in their home who are sick, but who are resistant to infection,” Lawrence Young, a professor of molecular oncology at Warwick University, told CNBC on Wednesday.
Early evidence shows that these people had naturally acquired protection from past infections with common cold coronaviruses, according to him. “Why some individuals maintain levels of cross-reactive immunity is unknown,” he added, “but around 20% of common cold illnesses are caused to common cold coronaviruses.”
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 for those who not get Covid-19
Covid vaccination is now widespread in most Western countries, albeit with variations among populaces in terms of which coronavirus vaccine was administered, and when.
In many countries, booster doses are also frequently used, and younger children are vaccinated, as governments strive to protect as many people as possible from the more transmissible but less clinically severe omicron variety.
Vaccines against Covid have been shown to prevent severe infections, hospitalizations, and fatalities, and they are still mainly effective against known viral strains. However, they are not 100 percent efficient in avoiding infection, and the immunity they confer wears off with time, and the omicron variety has weakened it slightly.
Why some individuals get Covid and others don’t, according to Andrew Freedman, an infectious diseases expert at Cardiff University Medical School, “is a well established phenomena and likely relates to protection from vaccination, past infection, or both,” he told CNBC.
“We know that despite being fully vaccinated, including getting a booster, several persons have nonetheless contracted (usually minor) omicron illness.” Vaccination does, however, lessen the risk of contracting omicron, and reactions differ from person to person. So, despite extensive exposure, some people catch it and some don’t,” he explained.
“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 of Warwick University said when it comes to different immune responses to Covid.
Young asserted that more research into “never Covid” people will aid in developing a better understanding of the immune response to SARS-CoV-2, as well as “what facets of the cross-reactive response are most important, and how this information can be harnessed to generate universal variant-proof vaccines.”
The genetic factor
Another question that has arisen during the pandemic is why two people with Covid may respond so differently to the infection; one could have heavy symptoms, for instance, and the other could be asymptomatic.
The answer might lie in our genes.
On Wednesday, Imperial College’s Altmann told CNBC, “It’s a tremendously crucial topic.”
He and his colleagues conducted research into immunogenetics (basically, the relationship between genetics and the immune system) and Covid-19 infection, which will be published soon, and discovered that differences in people’s immune systems “makes a difference, at least to whether or not you get symptomatic disease,” he said.
The study is looking at how various HLA (human leukocyte antigen) genes impact a person’s reaction to Covid, with some HLA types being more or less likely to have a symptomatic or asymptomatic infection, he explained.
“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,” Altmann added.
Could it be the tests?
The professor also cited the preliminary findings of a British human challenge study, conducted by Imperial College London and many other research institutions, in which 36 healthy young adults were purposely exposed to Covid, yet only half of them were sick.
“How is it that when you pipette an identical quantity of virus into people’s noses, half of them get sick and the other half don’t?” Altmann wondered, referring to the trial’s technique of virus exposure.
Essentially, all study participants were given a modest dosage of the virus through nasal drops and were then closely observed by healthcare professionals in a controlled atmosphere for two weeks.
Sixteen of the 18 infected participants developed mild to moderate cold-like symptoms, such as a stuffy or runny nose, sneezing, and a sore throat.
The study’s authors claimed that it was the first to be able to offer thorough information on the early stages of infection, before and throughout the onset of symptoms. The average duration from initial exposure to the virus to viral identification and early symptoms (that is, the incubation period) was 42 hours among the 18 infected subjects, which was much lower than previous predictions of five to six days.
Following this period there was a steep rise in the amount of virus (viral load) found in swabs taken from participants’ nose or throat.
These levels peaked at around five days into infection on average, but high levels of viable (infectious) virus were still picked up in lab tests up to nine days after inoculation on average, and up to a maximum of 12 days for some.
It was also intriguing to see where the most virus was discovered. Virus levels were lower and peaked sooner in the throat, despite the fact that it was found first in the throat and much earlier than in the nose (40 hours in the throat vs. 58 hours in the nose). Peak viral levels were substantially higher in the nose than in the throat, indicating that the virus might be shed more easily from the nose than from the mouth.
While there is a risk of “missing infectious virus early in the course of illness, particularly if just the nose is checked,” the researchers concluded that the data support the use of lateral flow testing to identify persons who are likely to be infectious.
“We found that overall, lateral flow tests correlate very well with the presence of infectious virus,” said Christopher Chiu, professor of infectious diseases from the Institute of Infection at Imperial College London and chief investigator on the trial.
“Even though in the first day or two they may be less sensitive, if you use them correctly and repeatedly, and act on them if they read positive, this will have a major impact on interrupting viral spread.”
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