Although the BA.2 strain of Omicron may be even more transmissible than the original variant, current vaccines appear to offer the same level of protection against symptomatic disease
The latest data from the UK Health Security Agency (UKHSA) suggest that the BA.2 strain of Omicron may be even more transmissible than the original variant. The good news is that current vaccines appear to offer the same level of protection against symptomatic disease.
According to the agency’s latest technical briefing on novel severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) variants, a total of 1072 genomically confirmed cases of BA.2 have been identified in England as of 24 January 2022, with the largest numbers in London (34%) and the South East (26.5%).
Because of the relatively small number of confirmed cases, the UKHSA cautions that any conclusions are provisional. However, early analyses do suggest an increased growth rate compared with BA.1.
Professor Jonathan Ball, Professor of Molecular Virology at the University of Nottingham, said: ‘It’s still early days, but the evidence so far suggests that BA.2 may be more transmissible than its close relative Omicron. However, the key issues are whether this variant is associated with more severe disease, and if it can escape immunity delivered by vaccines.
‘Early indicators suggest that the vaccines will provide similar levels of protection as we have seen for Omicron, so this is good news. Whether or not it causes more severe disease will become apparent as more data is collected.’
Dr Meera Chand, COVID-19 Incident Director at the UKHSA, said: ‘So far, there is insufficient evidence to determine whether BA.2 causes more severe illness than Omicron BA.1, but data is limited and UKHSA continues to investigate.’
How does BA.2 differ from Omicron?
BA.2 is a subvariant of the original BA.1 Omicron SARS-CoV-2 strain currently dominant in the UK and around the world. It shares many of the same mutations as its ancestor strain, but with a further 28 mutations not previously seen in the Omicron variant.
The new subvariant was first detected in genome sequences from the Philippines in November 2021. Since then, it has been found in at least 40 countries worldwide, and has already become the dominant SARS-CoV-2 strain in Denmark.
BA.2 was designated a variant under investigation by the UKHSA on 19 January 2022. As is routine for any new variants under investigation, the UKHSA is carrying out laboratory and epidemiological investigations to better understand the characteristics of this variant.
However, although the original BA.1 variant was relatively easy to track because of a spike deletion (H69/V70), which provided a convenient target for testing, BA.2 does not contain this mutation. This means that it is no longer possible to quickly distinguish between Omicron and other COVID-19 variants using polymerase chain reaction testing. Instead, monitoring requires additional genomic sequencing.
Is BA.2 more transmissible?
The latest figures from the UKHSA suggest that BA.2 has an increased growth rate compared with BA.1 in all regions of England where there are enough cases to assess it. Although the authors caution that growth rates can be overestimated in early analyses of a new variant, they add that ‘the apparent growth advantage is substantial’.
Analysis of routine contact tracing data suggests that transmission is also higher among household contacts of BA.2 cases (13.4%) than for those of other Omicron cases (10.3%).
Dr Susan Hopkins, Chief Medical Advisor for the UKHSA, said: ‘We now know that BA.2 has an increased growth rate, which can be seen in all regions in England. We have also learnt that BA.2 has a slightly higher secondary attack rate than BA.1 in households.’
Are current vaccines effective against BA.2?
Preliminary analysis by the UKHSA suggests that our current vaccines are still effective against symptomatic disease following BA.2 infection.
Vaccine efficacy was analysed in a test-negative case-control design combining all vaccines. After two doses of a vaccine, effectiveness at preventing symptomatic disease was 9% (95% confidence interval [CI] 7–10%) for BA.1 compared with 13% (95% CI -26–40%) for BA.2.
After a third vaccine, effectiveness increased to 63% (95% CI 63–64%) for BA.1 and 70% (95% CI 58–79%) for BA.2.
The UKHSA report did not look at whether previous Omicron infection provides immunity against the new subvariant; more data from sequenced cases will be needed to address this question.
However, Professor Francois Balloux, Director of the UCL Genetic Institute, pointed out: ‘Only a minority of the approximately 20 mutations distinguishing BA.1 from BA.2 fall within regions of the genome important for antibody immune recognition. As such, it is anticipated that infection by either sublineage should provide robust immunity against the other one, as well as against itself.’
More data will become available over the next few weeks. Until then, it is difficult to be certain of the implications for the current wave of infections in the UK.
Professor Ball remains optimistic: ‘Of course, it is important to keep monitoring the situation and try to gain a better understanding of how this variant behaves, but so far there is nothing in these early analyses to worry us unduly.’