Visitors to the UK must take PCR test after arrival: Omicron cases confirmed

If you're planning to travel soon to the U.K., you should know about some ongoing developments. All travelers arriving in the U.K. must take a PCR test for COVID-19 before the end of day two post-arrival (Day zero is when you arrive.) They must then self-isolate until receiving a negative result. The new rules were announced today by Prime Minister Boris Johnson at a Downing Street news conference.

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The PM's announcement comes after officials confirmed two cases of the new Omicron variant have been identified in the United Kingdom. The cases were discovered in Nottingham and Chelmsford in persons who had been to southern Africa recently.

Related: Omicron is now in UK and possibly Germany

The World Health Organization has deemed Omicron a "variant of concern," its most severe category. Omicron contains several spike protein mutations that it is feared may make it less responsive to COVID-19 vaccines. There are also signs it may be more contagious than other variants. Health officials around the world are scrambling to conduct further research on Omicron, as well as contain its spread.

"We don't know how effective our vaccines will be" against Omicron, the PM said at the press conference.

Related: UK and Israel impose restrictions over new COVID-19 strain from Africa

The worrisome new variant was first reported by South Africa on Nov. 24.Thus far, it has also been detected in Botswana, Belgium, Hong Kong, Israel and now the U.K. Additionally, German officials on Saturday announced a suspected case in that country. The Netherlands is also conducting tests on passengers who recently entered the country on a flight from South Africa. Several have tested positive for COVID-19, though it is not yet known if they have the Omicron variant.

Describing the need to "buy time" to conduct further studies, Johnson announced that mandatory face coverings will be required on public transport and in shops in the U.K. Additionally, he said that anyone in contact with a suspected Omicron case must self-isolate for 10 days, even if they have been vaccinated.

"We need to slow down the spread of this variant here in the U.K., because measures at the border can only ever minimize and delay the arrival of a new variant rather than stop it altogether," Johnson said.

As of Friday, the U.K.'s travel red list included South Africa, Namibia, Zimbabwe, Botswana, Lesotho and Eswatini. As of Sunday, the list will expand to add Angola, Mozambique, Malawi and Zambia.

Officials also noted that the government is considering a heightened push for booster shots.

This is a developing story and will be updated.