How our immune system’s killer T cells may be protecting us from severe Covid-19

The immune system of people who have been vaccinated against Covid-19 or who have already contracted the virus and recovered eliminates the omicron variant before it is able to cause serious illness, according to several preliminary studies carried out in South Africa, the United States and the Netherlands. Researchers believe this could explain why omicron is resulting in fewer hospitalizations and fatalities in many countries than in previous coronavirus waves.

All of the studies were based on the analysis of lymphocytes – white blood cells capable of remembering a pathogen and cleaning it out of the body for months, years, decades or even a lifetime. The elite of these white blood cells are natural killer cells, which identify infected cells and destroy them without mercy. This prevents a virus from prolonging the infection and causing serious illness. These lymphocytes, known as CD8 or “killer” T cells, work in conjunction with CD4 “helper” cells that help to reactivate the immune system against a new infection.

A University of Cape Town team led by virologist Wendy A. Burgers has studied the levels of these lymphocytes in the blood of 90 patients vaccinated with either Pfizer-BioNTech or Janssen, or who had previously contracted coronavirus. The results – still preliminary and not yet peer-reviewed – show that 70% to 80% of the CD4 and CD8 T-cell response was maintained compared to previous variants. The researchers also analyzed plasma from 19 patients hospitalized with the omicron strain. All of them were unvaccinated and had not previously contracted Covid-19. The results showed that the lymphocyte response was just as effective as among patients admitted to hospital with other coronavirus variants.

In the US, a team led by immunologist Alessandro Sette analyzed the white blood cells of 86 people vaccinated with either Moderna, Pfizer-BioNTech or Janssen. Preliminary results suggest that as much as 80% of the lymphocyte response remains intact against omicron. “These results mean that up to 80% of our police officers are still patrolling the body,” said Sette, of the La Jolla Institute for Immunology. “We do not yet know what level of protection is sufficient. It depends on whether we are talking about contagion or serious illness. It is probable that this level of lymphocytes will not prevent the former, but will prevent the latter.”

In the Netherlands, another preliminary study focused on 60 healthcare workers vaccinated with Pfizer-BioNTech, Moderna, AstraZeneca or Janssen. The results of this research showed that white blood cell-mediated immunity against omicron is equal to that against other variants.

For now, all that we can say is that getting vaccinated is the best thing anyone can do to avoid becoming ill with omicron or any other variant

Andrew Redd, US National Institute of Allergy and Infectious Diseases

All of this research work takes a different angle from the immunity studies that have been carried out up to now, which have focused on antibodies. These proteins are produced after vaccination or infection and can prevent the virus from entering cells. Several studies have shown that the effectiveness of antibodies against omicron is considerably weaker than recorded in previous coronavirus waves.

The new data appears to fit with what is being observed in many countries: the omicron strain is capable of infecting people who have been vaccinated or who have previously contracted Covid-19, but it is less able to escape the attention of white blood cells, which in a majority of cases can still identify and eliminate infected cells before the variant can cause serious illness. This would also explain why countries like South Africa have witnessed up to 80% fewer hospitalizations during the omicron wave than in previous ones. However, it is still too early to assess the full impact of omicron on hospital admissions and fatalities.

The South African study is based on infections recorded during the first two weeks of December, and as such does not reflect what has been seen during the entire course of the omicron wave. There is no more recent data available. “We were exhausted from working 16-hour days and furthermore the team had been depleted by omicron infections,” says Burgers. “We were forced to stop work for two weeks and we hope to resume next week,” adds the South African researcher, whose national government recently lifted its curfew after apparently overcoming the peak of the omicron wave.

Many experts now consider omicron to be the fastest-spreading virus in history. “In South Africa, deaths and the severity of the illness have been lower with omicron,” notes Burgers. “But with the vast number of cases being reported in many countries, a lot of hospitalizations are to be expected in a very short period of time during which many healthcare workers will be missing due to infection. The situation is very serious.”

A lot of what happens next will depends in large part on the level of vaccination. The studies cited also show that a third vaccine dose ramps up antibody levels and restores their ability to avoid infection. “It is still too early to give conclusive data on the seriousness of omicron infection in different parts of the world,” says Andrew Redd, a researcher at the US National Institute of Allergy and Infectious Diseases. His team was among the first to publish research showing that the capacity of lymphocytes to overcome omicron is “practically intact” despite the number of mutations it has built up. “For now, all that we can say is that getting vaccinated is the best thing anyone can do to avoid becoming ill with omicron or any other variant. Everybody should get vaccinated as soon as possible and get a booster shot if applicable,” Redd says.

Marcos López-Hoyos, president of the Spanish Society for Immunology, views all of this data with cautious optimism. “Infection is rampant because the ability of the antibodies that neutralize the virus is failing; it is at barely 20%,” he says. “On the other hand, the lymphocytes analyzed in these studies are able to prevent serious illness. With protection of between 70% and 80%, the vast majority of people who are infected will not suffer serious illness. Furthermore, there is a new study that shows that a third dose of the vaccine not only increases antibody levels, but also CD4 lymphocytes.”

These results are similar to what has been seen with other dangerous coronavirus variants. The initial studies show that omicron was able to escape the antibodies generated by vaccines or infection, but the data concerning white blood cells later indicated that immunity against serious illness remained almost entirely intact.