Omicron BA.2 variant more transmissible, say UK and Danish studies

The new BA.2 subvariant of Omicron is around 50% more transmissible than the original BA.1 strain, and there is somewhat conflicting evidence on its ability to evade the protection from vaccines, according to new studies.

BA.2 is now the dominant strain of the COVID-19 virus in Denmark, and scientists from the country's Statens Serum Institute (SSI) have compared the two subvariants in more than 8,500 households including almost 18,000 people.

They found that that BA.2 spreads to other household members in 39% of cases, compared to 29% with BA.1, which points to "an inherent increased transmissibility" for the emerging strain.

That increase was seen regardless of the vaccination status of the secondary cases, and there was an increased risk of spread within vaccinated households with BA.2 infections compared to BA.1, which could suggest the new subvariant may be more likely to sidestep vaccine protections.

That finding is somewhat at odds with recent figures from the UK Health Security Agency (UKHSA), which found that vaccines seem to provide higher protection against BA.2 than BA.1.

The new strain has an increased growth rate compared to BA.1, said the UKHSA – with household spread of 13.4% versus 10.3% with other Omicron variants – but it found no difference between the groups in vaccine effectiveness.

After two doses, protective efficacy was 9% and 13% respectively for BA.1 and BA.2, 25 weeks or more after the last dose, and this increased to 63% for BA.1 and 70% for BA.2 at two weeks following a third shot.

As is always the case with emerging variants, initial findings are compromised by low numbers of cases, and as of 24 January the UK had recorded just 1,072 genomically-confirmed cases of BA.2, although the true number could well be much greater.

Some scientists have given it the nickname 'stealth variant', because it is more difficult to detect than BA.1 and doesn't share the genetic deletion on the spike protein that has been used to monitor the spread of Omicron and distinguish it from Delta, which fuelled the last wave of COVID-19 cases.

There is growing evidence however that BA.2 has started to outcompete BA.1 in some parts of Europe an Asia, and there isn't enough data yet to say if prior infection with the first Omicron strain will protect against the new subvariant. On the plus side, there's no evidence yet that it is more likely to cause severe disease.

"Although hospitalisations and deaths remain low, cases are still high in some areas and some age groups so it's important that we continue to act cautiously as restrictions are lifted," said UKHSA chief medical advisor Dr Susan Hopkins.