Timing, lowered COVID guard among factors for severe flu season in Japan this year: experts

TOKYO -- As an unusually strong wave of flu infections spreads across Japan this winter, experts have shared tips with the Mainichi Shimbun on how to avoid catching it, while examining factors behind the outbreak.

"Infections are sweeping across all age groups, from children to the elderly," said Kurashiki Central Hospital vice-director Tadashi Ishida, a doctor of respiratory medicine who heads the Influenza Committee of the Japanese Association for Infectious Diseases.

There are three main types of influenza: A, B and C. The infections currently making the rounds are at least 90% comprised of the AH1pdm09 substrain of H1N1, an A-type influenza virus that first appeared in 2009, causing a pandemic. While this variety is known as being highly infectious, Osaka University's Satoshi Kutsuna, a professor of epidemiology, offered several factors behind its spread.

One is that the start of the flu season came early. Normally, the season peaks around January or February, but this time, signs of spreading infections emerged as early as November 2024, and a major peak was seen with the busy travel season at the end of the year.

Kurashiki Central Hospital vice-director Tadashi Ishida is seen in a photo provided by the hospital.

Secondly, herd immunity has fallen. Kutsuna explained, "During the COVID-19 pandemic, infectious disease control measures were well implemented and there was less movement of people. Other infectious diseases transmitted by droplets also decreased, resulting in a significant increase in the number of people without influenza immunity."

Besides this lack exposure to the flu, Kutsuna pointed out that people started to congregate once more after infection control countermeasures were eased in line with the government's downgrading of COVID-19 to an infectious disease on par with seasonal influenza. He diagnoses this as a factor contributing to the sudden spread of infections.

Kutsuna pointed out that COVID-19 infections are spreading as well and that supplies of flu-fighting medicines are short. He advises those with symptoms to get checked at medical institutions and for the elderly, pregnant and others at high risk to get immunized, while calling for measures such as wearing masks in crowded spaces.

Ishida said that he feels the peak has passed. However, based on the fact that last year a wave of type-B infections followed in February after type-A had peaked, he said, "Don't neglect steps such as cough etiquette and hand washing."

(Japanese original by Mai Suganuma, Lifestyle, Science & Environment News Department)