Omicron Sub-variant Dominant in England but No More Severe Than Predecessor

Peter Russell

February 28, 2022

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 people being hospitalised 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 February 20, up from 18.7% on February 6, 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%, the Agency said.

The briefing also included reports on BA.2 by technical group members. A laboratory assessment by the University of Oxford suggested that ACE2 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.

Amongst 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 December 15, 2021, to 100% in the week commencing February 16, 2022.

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

Re-infections

Similar findings have been reported in South Africa. Last week, Dr John Nkengasong, head of the Africa Centre for Disease Control & 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 Organisation 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.

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

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