Covid mystery of why some people don't catch virus probed by scientists

Scientists are trying to find out exactly why some people seem to be “immune” to contracting Covid-19.

Researchers at Imperial College London studied those who showed resistance to catching Covid and found that of the 36 unvaccinated participants who were artificially exposed to the virus, half did not develop an infection while most of the others later tested positive for very low levels of the virus.

One participant, 22-year-old Phoebe Garrett from High Wycombe, said she has been knowingly exposed to the virus around four times – even at a party where everyone else later tested positive – yet she is still yet to get ill.

As a study participant, she had the virus dripped into her nose then had her nostrils pegged shut for several hours in order for her to be infection, but, she still did not test positive.

“We had multiple rounds of tests, and different methods of testing,” she told The Guardian. “Throat swabs, nose swabs, other types of swabs that I’d never done before like nasal wicks – where you hold a swab in your nose for a minute – as well as blood tests.

“But I never developed symptoms, never tested positive. My mum has always said that our family never gets flu, and I’ve wondered if there’s maybe something behind that.”

“Together, these findings imply that there is a struggle between the virus and host, which in our ‘uninfected’ participants results in prevention of infection taking off,” said Professor Christopher Chui from Imperial College.

Professor Chiu said a reason could be a rapid, early response from the immune system. He said: “In our previous studies with other viruses, we have seen early immune responses in the nose that are associated with resisting infection.”

For the 18 participants who had low levels of coronavirus and/or mild symptoms – such as a blocked nose, sore throat or headache – this could be explained as common issues unrelated to their exposure to the virus.

“Either way, levels of the virus didn’t climb high enough to trigger detectable levels of antibodies, T cells or inflammatory factors in the blood that are usually associated with symptoms,” Professor Chiu said.

The study also revealed unique insights into the timeline of catching Covid-19, as among the 18 positive participants, the average time from first exposure to the virus to detection and early symptoms was 42 hours.

This is significantly shorter than existing estimates and put the average incubation period at five to six days, with levels of the virus being the highest on roughly the fifth day.

Professor Sir Jonathan Van-Tam, Deputy Chief Medical Officer for England, said: “These studies offer real advantage because the timing of exposure to the virus is always known exactly, therefore things like the interval between exposure and the profile of virus shedding can be accurately described.

“This important study has provided further key data on Covid-19 and how it spreads, which is invaluable in learning more about this novel virus, so we can fine-tune our response. Challenge studies could still prove to be important in the future to speed the development of ‘next-generation’ Covid-19 vaccines and antiviral drugs.”