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The pace of decline in the number of new COVID-19 cases in areas covered by the central government’s COVID-19 state of emergency, which has been extended until June 20, is slow due chiefly to a British mutant strain believed to be 30% more infectious than the standard type.

An expert predicted that variants first found in India, seen as more infectious than the British strain, will likely become the mainstream in Japan, while calling for stricter surveillance for the variants.

According to the National Institute of Infectious Diseases (NIID), over 90% of coronavirus cases in Japan are now attributable to mutated strains, including the one originating in Britain.

On Wednesday, an expert panel of the health ministry said Japan variants were behind the slower drop in new cases.

The British variant is 40% more likely to cause severe COVID-19 symptoms than the conventional strain. In Japan, the number of seriously ill COVID-19 patients has been on the rise.

“Mutated strains have spread across the country over the past month,” said Toho University professor Kazuhiro Tateda, a member of the panel. He added that this had a strong impact on the pace of falls in the number of new cases during the state of emergency.

There is no evidence suggesting the variants found in India trigger more severe COVID-19 symptoms than the ordinary strain does, but the L452R and E484Q variants are said to be two times more transmissible.

The L452R mutation is a change in the spike protein of the coronavirus from leucine to arginine in the 452nd amino acid position. The E484Q mutation is characterized by a shift in the spike protein’s 484th amino acid from glutamic acid to glutamine.

The L452R mutation is regarded as a reason for higher infectivity, but much is still unknown.

According to the NIID and other sources, a total of 45 Indian variant cases had been found in Japan as of Monday. At airport quarantine stations, 190 cases had been confirmed. Tokyo has reported a cluster of such cases.

Takaji Wakita, who chairs the health ministry’s expert panel and heads NIID, said there aren’t currently community-acquired infections of cases involving variants from India.

But it is highly likely that the variants will replace the British mutation, Wakita said, while noting that it is hard to forecast when the replacement will happen.

He called for the quick implementation of measures against variants, such as by strengthening the nationwide monitoring system using genome analysis.

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