Explaining the 'immortal' cases in the F0 world

More than two years after Covid-19 emerged, most Americans have developed immunity to the virus — either through vaccination, past infection, or a combination of both. But there are still rare cases where some unvaccinated people seem to have avoided the virus despite repeated exposure.

This has raised the question of whether those people have natural immunity against Covid-19?

Scientists are trying to understand whether and how this ability exists. Studying these cases could help develop new vaccines and treatments for Covid-19.

Professor Shane Crotty, a virologist at the La Jolla Institute for Immunology (USA), said one possibility is that people who have not been infectedCovid-19were lucky. It could also be due to their behavior, like wearing masks properly or avoiding situations that put them at risk of infection.

Conducting screening for children before Covid-19 vaccination. Photo: document

“I think a more plausible explanation is that some people have been infected with Covid-19 but have no symptoms. So they don’t know they have it,” said Catherine Troisi, an infectious disease epidemiologist at the University of Texas School of Public Health.

Scientifically, there are two possible explanations for why some people are much more resistant to SARS-CoV-2 than others, according to Professor Crotty. First, this group may clear the virus quickly, before it reaches detectable levels, due to immunity to other coronaviruses such as colds.

There are more than 200 common cold viruses — four of which are coronaviruses, which account for about 30% of all common colds. According to the Centers for Disease Control and Prevention (CDC), most people get infected with one or more seasonal coronaviruses at some point in their lives.

Antibodies, acquired from vaccines or previous infections, attack new viruses that enter the body. Meanwhile, T cells prevent viruses from replicating and causing serious disease. T cells appear to be effective against different coronaviruses, similar to cross-reactivity.

Using extracorporeal membrane oxygenation (ECMO) - Artificial heart-lung to save the life of a post-Covid-19 patient. Photo: Tat Ngoc

A study of healthcare workers in the UK published in November 2021 found similar results. The analysis looked at people who had been exposed to the virus but did not test positive for SARS-CoV-2. The presence of T cells contributed to the rapid clearance of SARS-CoV-2 and the diseases caused by coronaviruses.

Another explanation is that some people have innate immunity with genetic factors that protect them from SARS-CoV-2 infection.

Associate Professor Neville Sanjana, New York University, has studied the underlying genetic factors that are responsible for Covid-19 resistance. To enter the human body, the SARS-CoV-2 virus needs to combine with the ACE2 receptor. Mutations in ACE2 can make it difficult for the virus.

A similar situation has been demonstrated in AIDS. “With HIV, the virus that causes AIDS, the entry receptor is CCR5. Some people have mutations that eliminate CCR5 and they are virtually immune to HIV,” Associate Professor Sanjana said.

Scientists have also looked at other genetic variations across the human genome.

“Humans have about 20,000 genes and we really don’t know which genes might affect cells in the airway epithelium or in the lungs – the entry route for SARS-CoV-2,” Associate Professor Sanjana shared.