Delta is now the dominant coronavirus variant in Japan

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Delta dominates in areas around Tokyo

The highly transmissible Delta variant of the coronavirus is widespread in Japan and around the globe. People are worried that it could cause more serious symptoms – and want to know whether vaccines are effective against it.

Japan’s National Institute of Infectious Diseases examined the prevalence of the L452R mutation – which is mostly found in Delta – among recently confirmed cases. It used data from seven private testing providers and disclosed its estimates to an expert panel of the health ministry on August 18.

The study found that almost all new infections in Tokyo and neighboring Kanagawa, Saitama and Chiba prefectures are caused by viruses with the mutation found in Delta. The rate was 98 percent.

The Delta variant is also well entrenched in other parts of Japan. In Osaka, Kyoto, and Hyogo prefectures it accounts for 92 percent of cases. The figure stands at 99 percent in Okinawa, 97 percent in Fukuoka, 94 percent in Aichi, and 85 percent in Hokkaido.

The numbers show that Delta has almost entirely replaced the original coronavirus variant across Japan. Its spread has led to a surge in the number of infections and hospitalizations, causing a critical situation for the medical system in Tokyo and other areas.

Delta is highly infectious

Experts say Delta is much more infectious than the Alpha variant that was first detected in the United Kingdom, and the original virus. Studies in Japan and other countries have found it has double the infectious capacity of the original virus, and 1.5 times that of the Alpha variant.

The World Health Organization has also cited a Chinese study that found people infected with the Delta variant had a viral load more than 1,200 times that of people with the original. Experts point out the detail is linked to infectiousness.

Seriousness of symptoms

So far, there is no definitive answer about whether the Delta variant causes more serious symptoms. Studies are underway around the world, but in the meantime the WHO says hospitalization rates are rising.

A study at the University of Toronto released in August, not yet peer-reviewed, found the risk of hospitalization is 2.08 times more than that of the original virus. The study also found that Delta carries 3.34 times more risk of ICU admission, and 2.32 times more risk of death. Researchers analyzed a sample size of 200,000 coronavirus patients.

Vaccine efficacy

A document issued by the WHO on July 27 states that laboratory studies have shown vaccines produce fewer antibodies against the Delta variant. However, it also says that doesn’t mean vaccine effectiveness is lowered – and current vaccines are proving to be highly effective.

A British study published in the New England Journal of Medicine in August found that the effectiveness of preventing symptoms against the Delta variant was 35.6 percent with the first Pfizer dose, and 88.0 percent with the second. That compared with 30.0 percent with the first AstraZeneca dose, and 67.0 percent with the second. The researchers concluded two vaccine doses are highly efficacious against the Delta variant.

This information is accurate as of August 19, 2021.