Current COVID-19 vaccines provide robust protection against severe disease and hospitalisation caused by both the Delta and Omicron variants, according to a study.

The research, published in the journal Nature on 31 January, demonstrated that the vaccines induce this protection through cellular immunity or the production of protective immune cells, such as so-called killer and memory cells. Cellular immunity continues to protect from severe COVID-19 disease despite the Omicron variant's evasion of neutralising antibodies, the researchers said.

The team at Beth Israel Deaconess Medical Center (BIDMC) in Israel assessed samples from 47 individuals vaccinated with either the Johnson & Johnson or Pfizer-BioNTech vaccines. "Our data provide immunological context for the observation that current vaccines still provide robust protection against severe disease and hospitalisation due to the Omicron variant despite substantially reduced neutralising antibody responses and increased breakthrough infection," said corresponding author Dan H Barouch.

The researchers used samples from uninfected individuals who received either the Johnson & Johnson or Pfizer vaccines. They measured CD8+ T cell and CD4+ T cell responses to the original, Delta and Omicron strains of the SARS-CoV-2 virus after one month and then again after eight months following final vaccination.

Both CD4 and CD8 cells, also known as T cells, are white blood cells that fight infection and play an important role in the immune system. The team also assessed antibody responses to the variants at one and eight months. Consistent with previous studies, the scientists observed minimal cross-reactive Omicron-specific neutralising antibodies.

In contrast, the data suggested that Omicron-specific CD8+ T cell responses were more than 80 per cent cross-reactive compared with the CD8+ T cell response to the original strain of the virus. Similarly, more than 80 per cent of Omicron-specific CD4+ T cells demonstrated cross-reactivity, although responses could vary among individuals, the researchers noted.

Given the role of CD8+ T cells in clearance of viral infections, it is likely that cellular immunity contributes substantially to vaccine protection against severe SARS-CoV-2 disease," said Barouch, whose team was involved in the development of Johnson and Johnson vaccine.

"This may be particularly relevant for Omicron which dramatically evades neutralising antibody responses," he said. A highly mutated version of SARS-CoV-2, the virus that causes COVID-19, the Omicron variant has been shown to cause breakthrough infections among the vaccinated. This is due to its ability to evade the virus-killing neutralising antibodies that the body makes in response to getting vaccinated.