WHO advisory group calls for Covid boosters targeting XBB variants

The World Health Organization (WHO) Covid vaccine composition advisory group has recommended that vaccine makers drop the ancestral strain and switch to a monovalent (single-strain) vaccine that contains an XBB.1 descendant lineage such as XBB.1.5.

As of May 2023, XBB.1 descendent lineages predominate SARS-CoV-2 circulation globally. The WHO's Technical Advisory Group on Covid-19 Vaccine Composition (TAG-CO-VAC) said the current vaccines continue to provide substantial protection against severe illness and death, but new formulations are needed to protect against symptomatic disease.

They recommended a switch to a monovalent vaccine because the index virus no longer circulates in people, it prompts -- at best -- very low levels of antibodies against current strains, it reduces the concentration of the new target antigen, and it may induce immune imprinting.

"In order to improve protection, in particular against symptomatic disease, new formulations of Covid-19 vaccines should aim to induce antibody responses that neutralise XBB descendent lineages," the WHO TAG-CO-VAC said in a statement.

They noted that XBB descendant lineages, including XBB.1.5 and XBB.1.16, are highly immune-evasive, with XBB.1.5 as one of the variants with the highest magnitude of immune escape from neutralising antibodies. They acknowledged, however, that vaccine efficacy estimates against circulating strains are limited, with some studies showing efficacy similar to BA.5 and others showing reduced efficacy against XBB.1 lineages.

Studies involving blood samples of people who had two to four mRNA vaccine doses showed substantially lower neutralisation against XBB.1 lineages.

Preclinical data on XBB.1 candidate vaccines, shared confidentially by vaccine makers, showed a higher neutralising antibody response to current subvariants compared with currently approved vaccines.

Meanwhile, in its weekly epidemiological update, the UN health body said that globally nearly 2.6 million new cases and over 17 000 deaths were reported in the last 28 days, a decrease of 14 per cent and 26 per cent, respectively.

The situation is mixed at regional levels, with increases in reported cases seen in the South-East Asia and Western Pacific regions and increases in deaths in South-East Asia. As of May 14, over 766 million confirmed cases and over 6.9 million deaths have been reported globally.