U.K. health officials are anxious to trace one of six individuals infected with a more contagious variant of the coronavirus, initially identified in Brazil.

Up to six cases of the new strain — dubbed P.1 and considered a "variant of concern” by global health experts — have been detected in the U.K., with three cases in Scotland and three in England.

However, what’s troubling officials is that one of the three cases found in England has not been traced. The government issued a statement Sunday noting that two of the cases in England are from one household in South Gloucestershire with a history of travel to Brazil, and "there is a third, currently unlinked case.”

The cases in South Gloucestershire, in southeast England, were rapidly followed up by a team from Public Health England, and their contacts were identified and retested, the government said. All passengers on the same flight — Swiss Air flight LX318 from Sao Paulo, via Zurich, to London Heathrow on Feb.10 — were also being traced by officials.

As a precaution, health officials are stepping up testing of asymptomatic cases in the South Gloucestershire area, and increasing the sequencing of positive samples from the area.

The mystery case

However, further investigation is underway regarding a separate, third case of the variant identified in England, with health officials appealing to anyone who did not receive a result from a Covid test carried out on Feb. 12 or 13 to come forward.

"The individual did not complete their test registration card so follow-up details are not available,” the government noted.

"We are therefore asking for anyone who undertook a test on 12 or 13 February and hasn’t received their result or has an uncompleted test registration card, to call 119 in England or 0300 303 2713 in Scotland for assistance as soon as possible.”

U.K. Health Secretary Matt Hancock will hold a meeting later Monday to update British lawmakers from all parties on the variant, the BBC reported.

Why are officials worried?

Health officials are worried because the variant first identified in Brazil is believed to be a more infectious strain of the coronavirus and could cause more severe infections. There are also concerns that it could make coronavirus vaccines less effective, however this is not confirmed, and investigations are currently being carried out to see whether this is the case.

While scientists conduct this research, vaccine makers are developing booster shots to target variants.

Britain has already contended with the spread of a far more infectious variant that was responsible for a surge in cases in the winter. The strain has since become dominant in the country, and has spread worldwide.

The World Health Organization’s latest weekly report said 101 countries have now reported cases of the variant first identified in Britain.

With regards to the strain found in Brazil, it said 29 countries had reported cases to date. This P.1 variant was first identified in four travelers from Brazil to Japan in January, during routine screening at Haneda airport outside Tokyo.

The strain has been designated "of concern” as it shares some crucial mutations with the variant first identified in South Africa.  The P.1 variant has 17 unique mutations in all, according to the U.S. Centers for Disease Control and Prevention, and was also first detected in the U.S. at the end of January.

Rules

When the first cases of the this variant were detected in the U.K., the rules required anyone travelling from abroad to self-isolate at home for 10 days.

However, this changed on Feb. 15, and now travelers to the U.K. are required to quarantine in hotels, at their own expense, for 10 days. In mid-January, the U.K. banned travelers from a range of South American countries from entering the country, unless they had residence rights