Photo/Illutration Health minister Katsunobu Kato speaks at a meeting of an expert panel advising the ministry on COVID-19 in Tokyo on Dec. 7. (Yuki Edamatsu)

The BQ.1 subvariant of the Omicron strain of the novel coronavirus is catching up to the BA.5 subvariant as the dominant cause of COVID-19 in Japan, health experts said.

BQ.1, first detected in Nigeria in September, is derived from the BA.5 lineage. Experts believe the spike protein of BQ.1 harbors multiple mutations that BA.5 does not have.

Such mutations could make immunity gained through vaccines or infections less effective against BQ.1.

Although BQ.1 could spread more easily, the risk of infected patients developing serious symptoms is about the same as for BA.5 patients, the experts said.

Members of the health ministry’s expert panel on the novel coronavirus said they are carefully monitoring BQ.1 because it could impact how widespread COVID-19 becomes in Japan.

According to an estimate by the National Institute of Infectious Diseases, BA.5 has been the dominant strain in Japan since early August, accounting for more than 90 percent of COVID-19 cases.

The institute said the dominance of BA.5 led to the seventh wave of infections in the country.

However, BA.5’s dominance started to wane in early October. The institute’s estimate published on Dec. 7 said BA.5 is expected to account for 54 percent of all COVID-19 patients in the week starting on Dec. 5.

The institute predicts BQ.1 will cause 36 percent of all cases in the same week.

BQ.1 has already overtaken BA.5 in Western countries.

According to the UK Health Security Agency, BQ.1 patients accounted for 50 percent of all COVID-19 patients in Britain in the week through Nov. 19.

Data from the Centers for Disease Control and Prevention (CDC) showed that BQ.1 patients made up 63 percent of all COVID-19 patients in the United States in the week ending Dec. 3.

The data in Britain and the United States listed patients infected with BQ.1.1, which derives from BQ.1, as BQ.1 patients.

However, both the United States and Britain have not reported that hospitalizations of COVID-19 patients increased during the period when BQ.1 cases rose.

Takaji Wakita, the head of the Japanese institute and chair of the ministry’s advisory panel, said at a Nov. 30 news conference that the spread of BQ.1 could be behind the surge in COVID-19 patients in Japan after the pace of increase slowed down in November.

“BQ.1 could be a factor in the spread of infections,” he said. “It can also be predicted that (if BQ.1 spreads,) the number of patients might not fall as easily as it did during the seventh wave.”

The efficacy of vaccines against BQ.1 is unclear.

However, Atsuo Hamada, a specially appointed professor of travel medicine at Tokyo Medical University, said: “We can expect vaccines to be effective against BQ.1 to some extent. It could prevent serious symptoms, for example. I believe it is useful to have vaccines.”

Another cause for concern is the small number of infections of XBB, a hybrid of two lineages of the Omicron variant, detected in Japan.

XBB, which is spreading in Singapore and other countries, has immune escape abilities like BQ.1.

(This article was written by Kai Ichino and Kenta Noguchi.)

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