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Let's Thoroughly Prevent Influenza This Winter Season with Vaccination

Navitas Clinic Shinjuku

Tamae Hamaki, M.D.

(Director of Navitas Clinic Shinjuku, the Medical Corporation Association Tetsuikai)

Recently, a high school girl I know visited our clinic for her second influenza vaccine of the season. With the flu still widespread, her father, concerned about her upcoming exams, consulted with a familiar physician who recommended a second shot for added safety. I agree with this approach.

In Japan, it's recommended that children under 12 receive two doses of the flu vaccine, while those 13 and older get one. This girl had her first dose back in October. Why the need for a second one? It's largely due to a decline in immunity against various infections, a consequence of the four-year-long COVID-19 pandemic.

A striking example is the recent outbreak of infectious diseases in China. At the end of November, reports emerged of a cluster of pneumonia cases, mainly in Beijing and other northern areas. Initially feared as a mysterious respiratory illness, it turned out to be a known pathogen. Beijing's report of legally notifiable infectious diseases saw a sharp rise in November, primarily due to seasonal influenza (Reference 1). Since May this year, China has seen a surge in Mycoplasma, followed by influenza, RSV, and adenovirus infections in children, likely due to the easing of COVID-19 restrictions – an expected aftermath of the pandemic (Reference 2).

Similar trends have been observed in other countries that relaxed COVID-19 measures earlier. In the United States in 2022, a 'tripledemic' of COVID-19, influenza, and RSV infections swept across, leading to hospital overcrowding with severe cases in children and the elderly, extensively covered by the U.S. media. The flu season started unusually early around October, with hospitalizations in November hitting the highest since 2010. Pediatric flu-related deaths reached pre-COVID levels, having been virtually nonexistent in 2020 and 2021 (Reference 3). The CDC reports that this year again, the U.S. is experiencing a widespread outbreak of influenza, COVID-19, and RSV (Reference 4). This increase in various infections seems to be a common post-pandemic phenomenon.

I worry that Japan might see a similar situation. During the pandemic years of 2020-2022, there was no significant flu outbreak in Japan. The lack of exposure might have prevented the natural activation of immunity that minor or asymptomatic infections usually provide. Additionally, many skipped flu vaccinations. Thus, this year, the population might be more vulnerable to influenza. Japan has been experiencing a flu outbreak since the summer, showing no signs of abating – a clear sign of reduced immunity. This is medically referred to as a decline in 'herd immunity.'

My particular concern is for this winter. It's the first since COVID-19 has been downgraded to a Class 5 infectious disease. The unusual flu activity so far is alarming. In Tokyo, A-type flu cases became noticeable from mid-August, and as of September 21, a flu warning remains in effect. Despite a decrease after a mid-October peak, cases started rising again in mid-November (Reference 5).

Sapporo is experiencing a similar trend. Monitoring virus levels in sewage to track infectious diseases, the city has observed a rise in influenza virus concentration since November – higher than summer levels, indicating a potential explosion in cases (Reference 6). Normally, Japan's flu season is in the winter, starting around late December, peaking in January-February, and winding down after April. This year's resurgence of an outbreak that seemed to be subsiding in October is concerning. The summer outbreak might not have led to sufficient herd immunity, setting the stage for a situation similar to last winter in the U.S. or this winter in China.

So, what should we do? It's not too late for vaccination. The flu vaccine reduces the severity and mortality, especially in those over 65, with reports suggesting an 80% reduction in deaths. This year, even adults might consider a second dose. Normally, past infections or vaccinations provide sufficient immunity for adults, but the lack of flu outbreaks over the past four years likely means a significant drop in immunity levels. A second dose induces stronger immunity. I particularly recommend it for students and others facing major life events. Fortunately, this year's flu vaccine production is sufficient. I want everyone to consult with your close physician.


*This article is a translation of Japanese MRIC published on Dec 6, 2023


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