As Japan faces down a fourth wave of coronavirus infections, many fear that the variant strains spreading rapidly in the country’s west may soon wreak havoc in the Tokyo area as well, with the strains making up 80% to 90% of all coronavirus cases in the Tokyo area by early May, according to the National Institute of Infectious Diseases.

According to the health ministry, genome analyses have revealed that 271 people in 21 Japanese prefectures had been infected with variant strains as of March 9. But as of last Tuesday, that figure had risen over fourfold to 1,141 people across 42 prefectures.

Japan has seen mainly four variant strains — one originating from Britain, one from South Africa, one from Brazil and one of unknown origin.

The British strain, which is behind over 94% of all domestic variant cases, contains the N501Y mutation, which enables the virus’ spike protein, used to infiltrate human cells, to bind with its target more strongly.

Meanwhile, the strain of unknown origin, which is common in eastern and northeastern Japan, has the E484K mutation, which may lower the effectiveness of existing vaccines.

The Brazilian and South African strains have both of the two mutations.

Authorities have also found variants originating from the Philippines and the United States, but these variants are not believed to be spreading in Japan.

Mutations occur due to RNA copying errors when viruses that enter human cells try to replicate. The N501Y mutation occurred after the 501st amino acid on the virus’ spike protein changed from asparagine, or N, to tyrosine, or Y. Meanwhile, the E484K mutation saw its 484th amino acid changed from glutamic acid, or E, to lysine, or K.

The highly infectious N501Y mutation is especially feared to drive a rapid spread of the coronavirus. The strain of unknown origin, which was most popular in Tokyo, is being replaced by the British variant.

According to the NIID, the proportion of infections in Tokyo and three neighboring prefectures — Saitama, Chiba and Kanagawa — involving viruses that have the N501Y mutation is expected to rise from around half now to 80% to 90% by early May.

In the Tokai region of Shizuoka, Aichi, Gifu and Mie prefectures, the Kansai region of Osaka, Kyoto and Hyogo prefectures and the southernmost prefecture of Okinawa, the rate is expected to be over 95% by early May.

“The N501Y mutation is taking over in Tokyo as well, but there is a fear that different mutations may appear,” a member of the health ministry’s expert panel said. “It is important that we monitor variant strains through genome analyses.”

Amid these concerns, Tokyo Gov. Yuriko Koike has hinted at a possible third state of emergency declaration for the capital. That the request could come in the latter half of this week. COVID-19 cases in the capital continue to rise, with 405 infections confirmed Monday.

Any declaration would come after Osaka Gov. Hirofumi Yoshimura announced Monday that he would ask the central government to impose a fresh state of emergency for his prefecture.

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