Japanese health authorities are concerned that variants of the coronavirus are driving a nascent fourth wave in the pandemic with just 109 days remaining until the Tokyo Olympics.
The variants appear to be more infectious and may be resistant to vaccines, which are still not widely available in Japan. The situation is worst in Osaka, where infections hit fresh records last week, prompting the regional government to start targeted lockdown measures for one month from Monday.
A mutant Covid-19 variant first discovered in Britain has taken hold in the Osaka region, spreading faster and filling up hospital beds with more serious cases than the original virus, according to Koji Wada, a government adviser on the pandemic.
"The fourth wave is going to be larger," said Wada, a professor at Tokyo's International University of Health and Welfare.
"We need to start to discuss how we could utilize these targeted measures for the Tokyo area."
Japan has twice declared a state of emergency that covered most of the country in the past year, most recently just after New Year as the pandemic's third and most deadly wave struck. Officials are now opting for more targeted measures that allow local governments to shorten business hours and impose fines for noncompliance.
Osaka cancelled Olympic Torch relay events there, but Prime Minister Yoshihide Suga has insisted Japan will carry out the Games as scheduled. Suga said on Sunday (local time) that measures employed in Osaka could be expanded to Tokyo and elsewhere if needed.
"This 'escape mutation' sometimes helps the virus flip past the body's immune senses." - Dr Jemma Geoghegan, New Zealand virologist
There were 249 new infections in Tokyo on Monday, still well below the peak of over 2500 in January. In Osaka, a record 666 cases were reported on Saturday.
The true extent of the mutant cases is unknown, as only a small fraction of positive COVID-19 cases undergo the genomic study necessary to find the variants.
A health ministry report last week showed 678 cases of mutant variants from Britain, South Africa, and Brazil had been discovered nationwide and at airports, with the biggest clusters in Osaka and nearby Hyogo prefecture.
But another strain, known as E484K, may be more widespread. Around 70 percent of coronavirus patients tested at a Tokyo hospital last month carried the mutation that was different from the British and South African varieties, Japanese public broadcaster NHK said on Sunday.
Concerns over the E484K variant have been echoed by one of New Zealand's leading virologist Dr Jemma Geoghegan, who told First Up it was not a new variant in and of itself, but a mutation that had independently involved within multiple different variants.
"It was first recognised in the South African variant B1351 and then it became recognised in other variants too," she said
"This 'escape mutation' sometimes helps the virus flip past the body's immune senses and therefore, some earlier preliminary studies show there might be a weakened vaccine response, as well as some people who could be reinfected with this type of variant."
She said given the mutation was in circulation it was worrying a big event like the Olympics was going ahead.
The rebound in cases came within weeks of the government lifting state of emergency measures, and the priority measures being rolled out now are intended to halt an unexpected rise in mutant cases, Makoto Shimoaraiso, a Cabinet Secretariat official for Japan's COVID-19 response said.
"We take the criticism when people say that we have not been able to detect any variants," he added.