UK Prime Minister Boris Johnson says Britain is facing a "tidal wave" of infections from the Omicron variant, and he has announced a huge increase in booster vaccinations to strengthen defences against it.
Johnson said in a televised statement that anyone 18 and older would be offered a third shot of vaccine by the end of December. The previous target was the end of January.
He said cases of the highly transmissible variant were doubling every two to three days and "there is a tidal wave of Omicron coming".
To combat it, he announced a "national mission" to deliver booster vaccines at pop-up vaccination centres and seven-days-a-week clinics.
UK scientists believe existing vaccines appear less effective in preventing symptomatic infections in people exposed to Omicron, though preliminary data shows effectiveness appears to rise to 70-75 per cent after a third vaccine dose.
The British government raised the country's official coronavirus threat level on Sunday, warning the rapid spread of the Omicron variant had pushed the UK into risky territory.
The chief medical officers of England, Scotland, Wales and Northern Ireland said the emergence of the highly transmissible new strain "adds additional and rapidly increasing risk to the public and healthcare services" at a time when Covid-19 is already widespread. They recommended raising the alert level from 3 to 4 on a five-point scale. The top level, 5, indicates authorities think the healthcare system is about to be overwhelmed.
The doctors said early evidence shows Omicron is spreading much faster than the dominant Delta variant, and that vaccines offer less protection against it. British officials say Omicron is likely to replace Delta as the dominant strain in the UK within days.
"Data on severity will become clearer over the coming weeks, but hospitalisations from Omicron are already occurring and these are likely to increase rapidly," they said.
Prime Minister Boris Johnson was due to make a televised statement about Britain's coronavirus situation and the booster vaccination campaign on Sunday evening.
Concerns about the new variant led Johnson's Conservative government to reintroduce restrictions that were lifted almost six months ago. Masks must be worn in most indoor settings, vaccine certificates must be shown to enter nightclubs, and people are being urged to work from home if possible.
Many scientists say that's unlikely to be enough, however, and are calling for tougher measures.
Johnson's government is trying to avoid that, but aims to offer everyone 18 and over a booster shot by the end of January.
Scientists in South Africa, where Omicron was first identified, say they see signs it may cause less-severe disease than Delta, but caution it is too soon to be certain.