Omicron subtype has apparent transmission advantage

The BA.2 subtype of the Omicron coronavirus variant appears to have a substantial growth advantage over the currently predominant BA.1 type, Britain's UK Health Security Agency (UKHSA) said on Friday.

UKHSA said that there was an increased growth rate of BA.2 compared with BA.1 in all regions of England where there were enough cases to compare them, and that "the apparent growth advantage is currently substantial".

"We now know that BA.2 has an increased growth rate which can be seen in all regions in England," said Dr. Susan Hopkins, Chief Medical Advisor for the UKHSA.

The agency said there was no data on the severity of BA.2 compared to BA.1, but reiterated that a preliminary assessment did not find a difference in vaccine effectiveness against symptomatic disease between the two Omicron subtypes.

The rapid spread of BA.1 fuelled an Omicron wave which pushed cases to record highs in Britain in December, displacing the previously dominant Delta variant.

However, hospitalisations did not rise to the same extent, owing to population immunity through vaccination and previous infection, as well as Omicron's lower severity.

The UKHSA said that a separate analysis showed that between November 24 and January 19, the majority of intensive care admissions from COVID-19 had Delta infections, even as Omicron was growing to dominate the number of cases.

It also found that a rise of cases of Omicron in care homes had not been associated with an increase in hospital admissions.

"Our findings suggest the current wave of Omicron infections is unlikely to lead to a major surge in severe disease in care home populations with high levels of vaccine coverage and/or natural immunity," UKHSA said, noting that the findings were based on BA.1 due to limited numbers of BA.2 cases in the study.