The new Stratus Covid strain – and why it’s on the rise

A new strain of Covid, with an apparently telltale symptom, has reached the UK, and already accounts for a high proportion of new recorded cases in England. The arrival of the Stratus strain comes as levels of coronavirus infection reach their highest point so far this year, according to UK Health Service Agency data.

What is the Stratus strain?

This new Covid strain, a sub-variant of Omicron, is officially known as XFG, along with its own variant XFG.3, but has been nicknamed Stratus. It can trigger all the usual symptoms of Covid infection, including a cough, fever, sore throat and a change of taste and smell, but it has been, anecdotally, singled out for causing a particularly razor-blade-like sore throat.

Despite Stratus now accounting for a majority of new Covid cases in the UK, health experts are not concerned. Circulation levels are officially “low” and “current data does not indicate” that Stratus leads “to more severe illness than other variants in circulation”, said the UK Health Service Agency. Also, there is “no evidence to suggest the vaccines in current use will be less effective” against Stratus, Dr Alex Allen, the UKHSA’s consultant epidemiologist, told The Independent.

The Week

Escape your echo chamber. Get the facts behind the news, plus analysis from multiple perspectives.

SUBSCRIBE & SAVE

Sign up for The Week's Free Newsletters

From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.

From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.

Sign up

How has it become so dominant?

The World Health Organization has designated XFG a “variant under monitoring”, estimating that it now accounts for roughly 60% of Covid cases worldwide – although its global “additional public risk” is low.

Stratus “has become dominant because it is more infectious (able to bypass existing immunity to some degree) than prior variants”, Paul Hunter, a professor of medicine at the University of East Anglia, told The Telegraph. “But its relative growth advantage is only about 31%, which is not that great”, compared with the 200% growth advantage of the Omicron variant which brought much of the world to a standstill in late 2021.

Why are Covid cases rising?

Covid spikes can happen at any time of year but history has shown us that the biggest waves tend to strike in the summer or early winter.

A likely major factor in the recent uptick in cases in the UK – and also in the US and France – is that population immunity to Covid is now relatively low, as the antibody “fighter cell” protection we acquired from previous infection or vaccination starts to fade.

The return to school earlier this month could well encourage a further spike in cases, Peter Openshaw, an immunology professor at Imperial College London, told The Telegraph. “It may be that, during October, we’re going to see a lot more Covid. There was quite a peak in October 2024 and then it settled around Christmas time and was replaced by influenza.”