T-Cell Immunity Likely To Prevent Severe COVID-19 from Omicron Infections

Vaccination and prior infection induce a strong second line of immunological defense, finds South African study.

Vaccinations and prior infections with earlier COVID-19 virus variants strongly boost T-cell immunity against the omicron variant, according to a just-released preprint study by a team of researchers led by South African virologists Wendy Burger and Catherine Riou. The omicron variant often evades our bodies' first line of defense against infections: the antibodies induced by infections from earlier COVID-19 variants and vaccines that aimed at those same variants. While fast-reacting antibodies can prevent infections entirely, the number of antibodies produced in response to vaccinations and infections fall over time. That's where our bodies' second line of immunological defense—called T cells—comes into play.

Created by vaccinations and infections, T cells lurk in our bodies waiting to be activated when they are invaded by disease microbes. Some T cells directly attack and kill cells that have been infected, as these infected cells reproduce the virus; other T cells help B cells to rev up the production of additional antibodies to defend against the infection.

The South African researchers wanted to know how well T cells, which they collected from the blood samples of 50 patients who had been vaccinated or infected by earlier COVID-19 variants, would react to the omicron variant. Their concern was that the highly mutated omicron variant would not be recognized by the T cells that resulted from vaccinations against or infections from earlier COVID-19 variants. Happily, they report that this is not the case.

"Despite extensive neutralization escape against Omicron, 70-80% of the T cell response is preserved," they report. Neutralization escape means that waning antibodies are not enough to prevent infections by the omicron variant in a significant proportion of those vaccinated or previously infected by earlier variants. Nevertheless, T-cells remain 70-80 percent effective at defending against omicron infections.

Consequently, the researchers conclude:

The limited effect of Omicron's mutations on the T cell response suggests that vaccination or prior infection may still provide substantial protection from severe disease. Indeed, South Africa has reported a lower risk of hospitalisation and severe disease compared to the previous Delta wave. Cross-reactive T cell responses acquired through vaccination or infection may contribute to these apparent milder outcomes for Omicron. The resilience of the T cell response demonstrated here also bodes well in the event that more highly mutated variants emerge in the future.

If this research proves out, it means that while the omicron variant has a greater chance to cause infection by breaching our bodies' first line of defense, a second line established through vaccination or prior infection is likely to fight it off with only modest harm to our health. In a Twitter thread, virologist Burger further notes, "We still need to see how long T cell immunity lasts, but after SARS1 it was still detectable after 17yrs."