Analysis and assessment of the state of coronavirus infections in Japan is handled by a Ministry of Health, Labor and Welfare advisory board. Of particular note in the board's report this week was a graph showing the proportion of those infected with the N501Y mutation of the U.K. variant of the virus.
In the western Japan region of Kansai, the variant already accounts for 70% of infections, and by mid-May it is expected to replace almost all the original strain. Meanwhile in Tokyo and the neighboring prefectures of Kanagawa and Chiba, while the variant was believed to account for around 10% of infections at the beginning of April, there are indications that by early May, the figure could reach around 75%. It indeed seems that today's Osaka is tomorrow's Tokyo.
High transmissibility of the variant compared to the original strain has played a part in the change. After the U.K. variant was detected last autumn, it spread around the world rapidly, and had become the dominant strain in the United States by mid-March this year.
In the U.K., the effective reproduction number of the variant, indicating how many other people will get the virus from a single infected individual, is about 1.4 to 1.9 times higher than that of the original strain. Latest analysis in Japan puts the figure at an average 1.32 times. We ought to expect that what happened in Britain and the U.S. will occur in Japan as well.
The number of infections by age is another unsettling point. With the original strain, fewer children were infected compared to other age groups. But analysis of the U.K. variant showed that there was no significant difference between the infection rates of those under age 15 and those in their 30s or older. If the variant spreads in Japan, we may no longer be able to say that it is harder for children to get infected.
Of course, the U.K. variant of the virus is not the only one. The World Health Organization (WHO) divides virus variants into two categories: variants of interest (VOI) and variants of concern (VOC). A variant of concern is one which has traits such as increased transmissibility and increased cluster infection risk, greater threat of serious symptoms, and not being as effectively curbed by vaccines.
In Japan, in addition to the U.K. variant, variants from South Africa, Brazil and the Philippines have been defined as variants of concern. For the strains other than the U.K. variant, in addition to the N501Y mutation there are concerns of so-called "immunoescape" reducing the effectiveness of vaccines.
Mutations themselves are not rare, but will the quick succession of changes we are seeing in the virus's nature turn out to be its trait? Virologist Yoshihiro Kawaoka of the Institute of Medical Science at the University of Tokyo comments, "Compared to influenza viruses, variants (of the coronavirus) requiring alteration of the vaccine strain are appearing more quickly."
Indeed, in the case of the new influenza virus that appeared in 2009, it was eight years later that the vaccine strain was changed.
Fatigue from and acclimation to the coronavirus crisis are evident across the world, but the virus itself still has many unknown elements. People need to renew their awareness to prevent an explosion of infections.
(Japanese Original by Yuri Aono, Expert Senior Writer)