Researchers have identified why Omicron sub-variants are better, than previous COVID-19 variants, at escaping detection by the human body's immune cells.

In addition to antibodies produced either by vaccines or exposure to earlier versions of the virus, these viral variants must also avoid 'killer' T cells, to sicken people.

T cells are immune cells that are unleashed when the immune system detects foreign pathogens.

To understand how Omicron breaks all these barriers, a team from the Yale University in the US, measured activity of MHC (major histocompatibility complex) molecules that present fragments of viruses for recognition by appropriate T cells.

These MHC molecules alert the T cells of foreign pathogens that then become targets for the T cells.

The researchers found that the activity of these MHC molecules was substantially lower in cells exposed to five Omicron sub-variants of SARS-CoV-2 as well as earlier versions of the virus.

But the Omicron variants, the researchers found, were particularly adept at shutting down the activity of MHC compared with earlier versions of the COVID-19 virus. Meanwhile, cells infected by a flu virus were found to have much greater MHC activity.

Reduced activity in these MHC molecules, researchers say, may make T cells less likely to locate COVID viral targets.

"The findings will help guide researchers as they investigate possible ways to overcome MHC suppression by viral infections and may help in the development of vaccines that mobilise T cells as well as antibody response against viruses," said Miyu Moriyama, a postdoctoral fellow at Yale School of Medicine.