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The pace of growth in the number of new COVID-19 infections resulting from the delta coronavirus strain is accelerating in Japan.

According to the health ministry, 87 cases of the variant first discovered in India had been confirmed in 12 of the country’s 47 prefectures as of Monday.

The number of new delta variant cases in the week through Monday stood at 34, up from 24 in the preceding week and 21 the week before.

With cases of the delta strain projected by one estimate to account for over half of new infections in Japan as early as mid-July, experts are calling for authorities to strengthen measures to check for the variant.

The variant is characterized by the L452R mutation, indicating a change in the spike protein of the coronavirus from leucine to arginine in the 452nd amino acid position.

Tokyo and Kanagawa Prefecture have reported clusters of the delta variant

The L452R variant is said to be 50% and two times more transmissible, respectively, than the alpha strain, first detected in Britain, and the original virus, although some experts disagree over its infectiousness.

The delta variant is said to weaken the effects of COVID-19 vaccines, but much remains unknown. It has yet to be proven that people infected with the variant are at higher risk of developing severe symptoms.

One report said the L452R mutation evades immune cells created by the human leukocyte antigen-A24, which is found in 60% of the Japanese population. But some experts reject that analysis.

Kyoto University professor Hiroshi Nishiura, a theoretical epidemiology expert, said that the delta variant is 80% more infectious than the original virus and is expected to account for more than 50% of all domestic infections in mid-July and 80% at the end of July.

“Now is the time to find (variant cases) quickly and control the spread by taking measures against clusters of infections and through active epidemiological surveys,” National Institute of Infectious Diseases head Takaji Wakita said, while noting the difficulty of predicting how the variant will spread.

In its analysis based on an international database, a team led by Keio University professor Kenjiro Kosaki, who is well-versed in clinical genetics, said that the delta variant was brought into Japan not only directly from India but also via Britain or the United States.

“Not all coronavirus variants come directly from the initial hot spots,” Kosaki said, calling for strengthened border controls.

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