The case for updating covid-19 vaccines for the Omicron variant

VARIANTS VARY, but how much? Since SARS-CoV-2 was first sequenced at the beginning of 2020 dozens of strains have been identified. And five have been designated “variants of concern” by the World Health Organisation (WHO). The latest of these is Omicron, which was given its name in November last year. As Omicron becomes dominant around the world, working out how to protect people from it is becoming more pressing. To do this, scientists must study how different this strain is from those that came before and what that means for immunity, from both previous infection and vaccination.

One question occupying scientists and politicians is whether vaccines would work even better if they were updated to deal with new strains. Up to and including the Delta variant, which was first identified in India and was designated a variant of concern in May 2021, the answer has been “no”. But new research, which has mapped differences between all major versions of SARS-CoV-2, suggests that, although administering existing vaccines is still useful, Omicron is so different from other strains that the answer might now be “yes”.

Researchers at universities and government agencies in the Netherlands acquired blood samples collected from 51 unvaccinated people shortly after they had been infected with various strains of SARS-CoV-2. These were the original, referred to as the “ancestral strain”, as well as the later Alpha, Beta, Gamma and Delta variants. (Omicron samples were not available.) The researchers then assessed the neutralising capacity, or antibody response, of those samples against different versions of the virus, again using the ancestral strain, Alpha, Beta, Gamma and Delta and, this time, Omicron.

By measuring the antibody responses of the serum of each individual against the various variants, the researchers were able to position both the viruses and the serum samples on an “antigenic map”, which is used to study how mutations make viruses more or less different from the ancestral strain over time, in a process known as “antigenic drift”. In such maps a one-unit distance is equivalent to a two-fold change in neutralisation titre (a measure of the concentration of antibodies in the blood needed for neutralisation of a virus). Samples plotted closer together are more similar. Going by a measure conventionally used to group influenza viruses—a distance of three or fewer antigenic units—the researchers found that Omicron would count as the first of a new group of the SARS-CoV-2 pathogen. Whereas all widely circulating variants before it cluster in a single group, being less than three units apart from each other, Omicron is more than five units away from all others.

A preprint of this research was uploaded to medRxiv, a research portal, on January 3rd but has not yet been peer-reviewed. And the authors of the study have also cautioned that their methodology varied slightly from that conventionally used to map influenza. Nevertheless they conclude that “the change in neutralisation between Omicron and other variants of SARS-CoV-2, including the ancestral strain, is striking.” (A separate investigation, published on the same day in Cell, a journal, reaches a similar conclusion.) In correspondence with The Economist, Colin Russell, one of the authors of the study, cautioned that its findings should not be interpreted to mean that existing vaccinations, designed for the ancestral strain, are ineffective. “Omicron’s substantial reduction in cross-reactivity with previous variants doesn’t mean that there is no reactivity. This is probably why boosting with current vaccines provides some protection,” Dr Russell says.

As more people catch Omicron or are vaccinated, a higher share of the population will have some protection than in the past. Other studies have also shown that another vital part of the body’s immune system engaged by natural infection and vaccines, the T-cells, appear to be less impaired by Omicron’s mutations. That is likely to be one reason Omicron rarely causes severe disease and death in those who have acquired immunity through infection or vaccination. The other reason is that Omicron itself seems to be inherently less lethal. But, just as twice a year the WHO organises expert reviews of influenza viruses and issues recommendations on how to tweak vaccine compositions, Dr Russell suggests something similar for vaccines against covid. For maximum efficacy, he suggests, vaccines should be updated as soon as possible. ■

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