The BA.2 strain of Omicron is substantially more transmissible than BA.1, but evidence suggests it does not lead to a higher risk of hospitalisation or death

COVID-19 illustration_sdecoret_332181046

The BA.2 subvariant of the SARS-CoV-2 Omicron variant is substantially more transmissible than BA.1, but a preliminary analysis has found no evidence that it is linked to a greater risk of hospitalisation than its predecessor.

BA.2 differs from BA.1 in its genetic sequence, including some amino acid differences in the spike protein and other proteins.

The latest risk assessment based on community testing showed that BA.2 has outcompeted BA.1 in England. The proportion of BA.2 cases accounted for 52.3% of Omicron infections in England in the 7 days to 20 February, up from 18.7% on 6 February, the UK Health Security Agency (UKHSA) said in a technical briefing.

Cases of the BA.2 variant continued to grow faster than BA.1 in all English regions, with the highest proportion in London (63%), and the lowest in the North East (33%).

Vaccine effectiveness

The analysis found no indication that vaccine effectiveness against symptomatic disease from BA.2 [B.1.1.529.BA] was reduced compared to BA.1 [B.1.1.529]. Two weeks after a third, or ‘booster’ dose, vaccine effectiveness against the BA.2 variant was 67%, UKHSA said.

The briefing also included reports on BA.2 by technical group members. A laboratory assessment by the University of Oxford suggested that angiotensin-converting enzyme 2 binding was increased for the BA.2 receptor-binding domain compared with BA.1, while preliminary experiments with hamsters by Imperial College London reported mild disease, similar to those infected with BA.1.

Among patients admitted to an intensive care unit or high dependency unit with SARS-CoV-2, and with a valid sequencing result, the proportion of cases of Omicron increased from 12% in the week commencing 15 December 2021, to 100% in the week commencing 16 February 2022.

The risk of admission to these hospital units was found to be ‘significantly’ lower for Omicron than for Delta.

Reinfections

Similar findings have been reported in South Africa. Last week, Dr John Nkengasong, Head of the Africa Centre for Disease Control and Prevention, said of the BA.2 variant: ‘South Africa is reporting that it is more transmissible than the BA.1 variant, but interestingly and very encouragingly the severity seems to be the same.’

Also, last week, the World Health Organization suggested that infection with BA.1 appeared to provide strong protection against reinfection with BA.2.

That was also the finding of a recent Danish-led study preprint, which found that Omicron BA.2 reinfections do occur shortly after BA.1 infections but were rare, and occurred mostly in unvaccinated individuals under the age of 30 years.

The study, posted on the Medrxiv website, also found that reinfections were characterised by overall mild symptoms, comparable with those of the initial infection.

This article originally appeared on Medscape, part of the Medscape Professional Network.

Credit:

Lead image: sdecoret/stock.adobe.com

Image 1: sdecoret/stock.adobe.com