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STOCKHOLM, Jan. 3 (Xinhua) -- Stockholm has reported four new cases of the mutant strains of coronavirus first found in the UK and South Africa, city authorities announced in a press release on Sunday.

The four cases include three infected with the strain first found in the UK and one with the variant first detected in South Africa.

"We followed routine contact tracing procedures as soon as the individuals tested positive for COVID-19 and they have been told to follow the current recommendations and to self-isolate," said infectious disease doctor Maria Rotzen Ostlund in the release.

Stockholm regional authorities have asked individuals who have recently visited either the UK or South Africa to self-quarantine upon arriving in Sweden and take a COVID-19 test as soon as possible plus an additional test five days later.

The mutant strain spreading in the UK has now been found in 11 out of Sweden's 21 regions, state epidemiologist Anders Tegnell told Swedish Television on Sunday.

The latest data from Sweden's Public Health Agency, published Friday, show that Sweden has recorded 437,379 COVID-19 cases over the course of the pandemic, with 8,727 deaths.

As the world is struggling to contain the pandemic, vaccination is underway in some countries with the already-authorized coronavirus vaccines.

Meanwhile, 232 candidate vaccines are still being developed worldwide -- 60 of them in clinical trials -- in countries including Germany, China, Russia, Britain and the United States, according to information released by the World Health Organization on Dec. 29. Enditem