XBB.1.5, the most transmissible COVID sub-variant yet

PARIS, AFP: The Omicron sub-variant XBB.1.5 is the most transmissible of the COVID-19 sub-variants to emerge so far and has been spreading quickly across the United States.

Here’s what we know ─ and don’t ─ about XBB.1.5.

The sub-variant was first detected in the United States in October, and now represents more than 27% of the country’s infections, according to the CDC’s variant tracker.

38 countries have reported XBB.1.5 cases, of which 82% are in the US, 8% in Britain and 2% in Denmark, the World Health Organisation said in a rapid risk assessment on Wednesday.

The European Union’s ECDC health agency said this week that XBB.1.5 accounted for less than 2.5% of cases in Europe.

The sub-variant is very similar to its predecessor XBB.1 but has an additional mutation to its spike protein ─ the key that allows the virus into the body’s cells ─ said Grace Roberts, a virologist at Leeds University in Britain.

WHO’s technical lead on COVID Maria Van Kerkhove said this week that XBB.1.5 was the most transmissible form of COVID and clearly has a “growth advantage” over other forms of the virus.

The most likely explanation for this advantage is the XBB.1.5’s spike protein mutation, in combination with the already very transmissible XBB, the ECDC said.

XBB.1.5 has a growth advantage over other Omicron variants of 109% in North America and 113% in Europe, the ECDC added, though it cautioned that these figures came with “significant uncertainty”.

It added that the proportion of XBB.1.5 cases had doubled every nine days in the United States.

But Roberts told AFP “there’s no data to suggest XBB.1.5 is any more harmful ─ in terms of severe disease and death ─ than previous variants”.

The WHO is still assessing data on the subject but said XBB.1.5 did not carry any mutations that were known to increase severity.

Omicron’s XBB variants, alongside BQ.1, are the most resistant to antibodies built up from vaccination and previous infections, according to the WHO.

A study published in the journal Cell last month found that XBB.1 is 63 times less likely to be neutralised by existing antibodies than the BA.2 sub-variant.

It is also 49 times more resistant than the BA.4 and BA.5 sub-variants, which are currently dominant in the UK and numerous other countries.