The spread of coronavirus variants more contagious than the original strain could make the second year of the pandemic even tougher.
The warning from the World Health Organisation comes as coronavirus deaths in the US hit another one-day high at more than 4300.
Meanwhile, Japan has expanded a state of emergency to seven more prefectures as the country exceeded 300,000 infections.
Experts fear new variants of the virus that have been detected in the UK, South Africa and Brazil, are contributing to the surge.
On Thursday morning (Australian time), the WHO revealed that the UK coronavirus mutant strain – dubbed B117 – has now spread to 50 countries.
The British-identified variant was first reported on December 14 and considered to be around 70 per cent more contagious.
“Going into a second year of this could even be tougher given some of the transmission dynamics,” the WHO’s Mike Ryan said during an event on social media.
Another variant of the virus that was first identified in South Africa has now infected people in 20 countries, according to the WHO.
After two weeks of fewer cases being reported, five million new cases were reported last week, its latest epidemiological update revealed.
“After the holidays, in some countries the situation will get a lot worse before it gets better,” said Maria Van Kerkhove, WHO’s technical lead for COVID-19.
The WHO also raised concern that a third new coronavirus “variant of concern” found in Japan can affect a person’s immune response and therefore requires further investigation.
“The more the SARS-CoV-2 virus spreads, the more opportunities it has to change. High levels of transmission mean that we should expect more variants to emerge,” said the WHO.
Prime Minister Yoshihide Suga has decided to include seven more prefectures in the state of emergency that was declared in the capital, Tokyo, and three neighbouring prefectures, last week.
The state of emergency, which is set to last through February 7, now covers 55 per cent of Japan’s population of 126 million.
It is much narrower in scope than the first one last spring and focuses on combating transmission in bars and restaurants while urging people to stay home as much as possible.
The announcement comes just three days after a coronavirus variant different from the ones in Britain and South Africa was detected in four travellers from Brazil’s Amazonas state.
On Thursday, Japan surpassed 300,000 total confirmed cases, while the death toll reached 4187, public broadcaster NHK said.
Meanwhile, the US coronavirus death toll, which has stayed far above everywhere else, has climbed past 380,000, the latest data from Johns Hopkins University shows.
On Tuesday alone, the US recorded 4327 deaths, with Arizona and California among the hardest-hit states.
New cases are running at nearly a quarter-million per day on average.