The researchers from the National Institute of Allergy and Infectious Diseases (NIAID) found that one key player in the immune response to SARS-CoV-2 -- the CD8+ T cell -- remained active against the virus.

To investigate this possibility, the research team analysed blood cell samples from 30 people who had contracted and recovered from Covid-19 prior to the emergence of virus variants.

The investigators asked whether CD8+ T cells in the blood of recovered Covid-19 patients, infected with the initial virus, could still recognize three SARS-CoV-2 variants -- B.1.1.7, which was first detected in the UK; B.1.351, originally found in South Africa; and B.1.1.248, first seen in Brazil.

The researchers said that each variant has mutations throughout the virus, and, in particular, in the region of the virus' spike protein that it uses to attach to and enter cells.

Mutations in this spike protein region could make it less recognisable to T cells and neutralise antibodies, which are made by the immune system's B cells following infection or vaccination, the researchers said.

Although details about the exact levels and composition of antibody and T-cell responses needed to achieve immunity to SARS-CoV-2 are still unknown, the researchers assume that strong and broad responses from both antibodies and T cells are required to mount an effective immune response.

CD8+ T cells limit infection by recognising parts of the virus protein presented on the surface of infected cells and killing those cells.

In their study of recovered Covid-19 patients, the researchers determined that SARS-CoV-2-specific CD8+ T-cell responses remained largely intact and could recognise virtually all mutations in the variants studied.

While larger studies are needed, the researchers note that their findings suggest that the T cell response in convalescent individuals, and most likely in vaccinees, are largely not affected by the mutations found in these three variants, and should offer protection against emerging variants.