Immunity Debt Accumulated... Why Flu Hits Especially Hard This Year

High-Intensity Social Distancing and Mask Wearing
"Virus Immunity May Have Decreased"

A (39), a resident of Gyeonggi-do, spent almost every night sleepless at the end of last month. This was because their 7-year-old child tested positive for influenza A and became severely ill. A said, "At one point, my child's fever wouldn't go down from 38 degrees Celsius. I was so worried I thought I might die," adding, "I put a mask on the child when going out and strictly manage personal hygiene, but it seems this flu couldn't be avoided."

The flu is rampant this winter. According to health authorities, the level has exceeded the seasonal flu epidemic threshold by more than 10 times. There are also concerns that a 'twindemic'?the simultaneous outbreak of two infectious diseases?could occur alongside COVID-19. Why has the flu, which had been quiet for the past two years, suddenly exploded now?

Flu, Quiet for Several Years, Exceeds Epidemic Threshold by 10 Times This Year

This year's flu exceeded more than 10 times the seasonal epidemic threshold.

According to the Korea Disease Control and Prevention Agency on the 30th of last month, during the week from December 18 to 24 last year, the number of suspected flu patients nationwide was 55.4 per 1,000 outpatients, a 32.2% increase from 41.9 per 1,000 outpatients the previous week. This already exceeds the 2022?2023 seasonal flu epidemic threshold (4.9) by more than 10 times.

During 2020?2021, when COVID-19 shook the world, respiratory diseases were relatively low. This was true not only for the flu but also for colds and epidemic pediatric diseases.

This phenomenon occurred in almost every country, including the United States, the United Kingdom, and South Korea. At that time, the World Health Organization (WHO) cited increased social distancing and personal hygiene practices such as mask-wearing as reasons for the disappearance of the flu.

Immunity Debt Accumulated Over Two Years of COVID-19 May Have Fueled Flu Outbreak

Flu Vaccination / Photo by Yonhap News

So why is the flu suddenly spreading now? Some scientists point to 'immunity debt' as the cause.

The term 'immunity debt' was first introduced in a paper published in the international academic journal Nature in August 2021. It refers to the reduction in the global population's immunity to the influenza virus during the COVID-19 pandemic.

In fact, scientists have been concerned for years about a surge in flu infections due to social distancing. The influenza virus, commonly called the flu, circulates seasonally from November to April in the Northern Hemisphere and from May to September in the Southern Hemisphere. Because it returns annually, it is called a seasonal virus.

Before COVID-19, we were periodically exposed to this flu virus, unknowingly building immunity. However, in 2020, when COVID-19 spread, strict quarantine measures drastically reduced person-to-person contact, and repeated use of masks and hand sanitizers lowered the chances of the virus entering the respiratory tract. As a result, the flu did not spread sufficiently, causing a decline in human immunity.

Regarding this, John Tregoning, an immunologist at Imperial College London, explained to Nature, "Normally, everyone would have been exposed to some flu virus, and the body would have fought the virus and formed antibodies to build immunity. But over the past few years, the absence of the virus has led to a noticeable decrease in antibody levels in the body."

Absence of Virus Also Makes Vaccine Development Difficult

Ironically, the 'absence of the virus' may negatively affect the development of vaccines to prevent infectious diseases. The influenza virus, like the COVID-19 virus, is highly variable, producing new variants every winter that invade the human body.

Therefore, the WHO monitors circulating viruses in the Northern and Southern Hemispheres annually to predict which virus strains will dominate in the next season. Based on this prediction data, vaccine manufacturers produce vaccines tailored to each season. However, because flu outbreaks were subdued over the past two years, this monitoring process has become more challenging.

Of course, the most effective way to defend against the flu in a timely manner remains vaccination. Especially, the currently administered 'quadrivalent flu vaccine' in South Korea provides protection against the four types of flu viruses predicted by the WHO, making it highly effective.

This content was produced with the assistance of AI translation services.