Explainer | BA.4 and BA.5: what we know about the new Omicron subvariants

They are now the dominant strains in the US and elsewhere, and many countries are expecting a surge in cases in the coming months
Studies have shown they can escape antibodies from vaccines and previous infections, but it’s not clear if they cause more severe disease

Coronavirus China

Josephine Ma

Published: 8:00pm, 30 Jun 2022

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Two Omicron subvariants – BA.4 and BA.5 – have become the dominant coronavirus strains in the United States, causing 52 per cent of cases last week, according to new estimates from the Centres for Disease Control and Prevention.

The new and highly transmissible strains have also gained prevalence elsewhere, including in the United Kingdom and Portugal, and many countries are expecting a surge in cases in the coming months.

Scientists are closely monitoring the new subvariants, with early studies showing that antibodies from existing vaccines and previous Covid-19 infections offer less protection against them. Here’s what we know so far.

Stealthier than ‘stealth Omicron’?

Before BA.4 and BA.5 came along, the prevailing strains in the US were also Omicron subvariants – first BA.2, then BA.2.12.1. Scientists called these earlier subvariants “stealth Omicron” because it was difficult to detect them.

The new strains, BA.4 and BA.5, were first detected in South Africa – in January and February – where they caused a wave of cases, and have rapidly spread elsewhere.

There are different theories about how they evolved – that they came from previous Omicron strains, or are offshoots of BA.2.

While they share similarities with earlier Omicron strains and are seen as close relatives, BA.4 and BA.5 are distinct from them because of mutations in two spike proteins. These mutations make it easier for the two subvariants to escape antibodies produced after Covid-19 vaccines and previous infections caused by other strains – which could be why they have spread so rapidly.

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What the studies say so far

Early studies have shown that the two subvariants can elude the antibodies produced by existing vaccines. Those vaccines were already less effective against previous Omicron strains and other variants like Delta, and protection is weaker again for BA.4 and BA.5, according to a study published in The New England Journal of Medicine last week.

But in general, scientists say the coronavirus vaccines still offer protection against severe disease.

In addition, some of the antibody treatments developed for earlier strains may be ineffective against the new subvariants because of their mutations.

It is also not clear yet whether BA.4 and BA.5 cause more severe disease.

A preprint study led by Kei Sato, a virologist at the University of Tokyo, said lab tests suggested that the earlier Omicron strain BA.2.12.1 as well as BA.4 and BA.5 replicated more efficiently in lung cells than BA.2. The study, which has not been peer-reviewed, also found that BA.4 and BA.5 were able to bind to lung cells more easily. They caused more severe disease in hamsters, but so far there is no evidence that this is the case for humans.

In South Africa, there was no rise in the death toll during the surge in cases fuelled by the two new subvariants.

Hospitalisations have risen in the United Kingdom, where BA.4 and BA.5 are now dominant, but not to the levels seen in earlier waves and the number of patients needing ventilators remains low.

Still, scientists have urged countries with a higher proportion of vulnerable elderly people to be on guard.

What does it mean for vaccines?

The rapid rise of the new subvariants is a headache for regulators deciding which vaccines should be used in winter.

In the US, the Food and Drug Administration met on Tuesday to discuss which variants new boosters should target. FDA scientists at the meeting suggested BA.4 and BA.5 should be the targets, not BA.1 which drove a surge in cases last winter, Reuters reported.

But given how fast new strains can emerge and overtake earlier ones, it is not known if these subvariants will still be dominant by the time new vaccines are ready and approved.

Kanta Subbarao, representing a World Health Organization advisory committee, told the FDA meeting that she preferred vaccines targeting BA.1. That was because they might be able to produce a wider immune response given that the subvariant is more distinct from the original coronavirus than the others, according to Reuters.

Moderna and Pfizer have both said new vaccine boosters aimed at BA.1 should be ready for roll-out soon. Moderna said it would have BA.1 vaccines ready by September, but it could take a couple more months for those targeting BA.4 and BA.5. Pfizer said it could have a BA.1 vaccine and one targeting BA.4 and BA.5 ready by October.

The FDA plans to make a decision on boosters in early July.

The US FDA met on Tuesday to discuss which variants new boosters should target. Photo: Shutterstock Images

And for zero-Covid in China?

China has thrown a huge amount of resources at testing, contact tracing and quarantine to try to stamp out “stealth Omicron”, and the emergence of more transmissible subvariants will be an even bigger challenge to its zero-Covid strategy.

On Tuesday, a researcher with an institute under the Chinese Centre for Disease Control and Prevention said imported cases of BA.4 and BA.5 were on the rise in China.

“Their transmissibility and immunity escape are relatively higher and it will be more difficult to control and prevent [the spread of the subvariants],” Wang Wenling, from the China CDC’s National Institute for Viral Disease Control and Prevention, told reporters.

She made the remarks the same day China cut quarantine time for overseas arrivals from 14 days to seven days in a government-run facility, followed by three days in home isolation – a first step towards easing the country’s border restrictions.

But President Xi Jinping on Wednesday said the zero-Covid policy would be maintained, even at the cost of economic growth.