Why is the global flu epidemic evolving abnormally?

Several countries around the world are experiencing what experts say is an unusual flu season, with unusually high rates of positive tests and hospitalizations. Many medical facilities are overwhelmed by severe cases. This is a stark contrast to the flu seasons of recent years.

In the US, for example, the flu hospitalization rate has surpassed the peak of the Delta wave during the Covid-19 pandemic, with 14.4 people per 100,000 hospitalized between January 23 and February 1. Notably, California recorded 10 deaths in children, including 3 in San Diego County alone, which experts have called "a worrying tragedy".

Japan has recorded a record number of infections in the past 25 years. In just one week, the country detected 317,000 positive cases at 5,000 medical facilities, an average of 64.39 patients per facility, double the warning threshold. The total number of infections since the beginning of the epidemic has exceeded 6 million people.

This situation has led to a double crisis as hospitals and pharmacies in Japan are facing a shortage of treatment drugs. Major pharmaceutical companies such as Sawai Pharmaceutical and Chugai Pharmaceutical have been forced to temporarily stop supplying Tamiflu, making it impossible for patients to self-treat at home, leading to overload at medical facilities.

Vietnam has also recorded a surge in seasonal flu cases. A notable feature of this outbreak is the appearance of many cases with signs of white lung on X-rays, especially in elderly patients or those with many underlying diseases. Notably, some young people have also experienced serious flu complications due to subjectivity.

The main cause of this condition is a combination of factors.

Dr. Joseph Khabbaza, a critical care physician at the Cleveland Clinic, said the 2025 flu season in the United States and Canada will be longer than usual, with two peaks. Typically, the virus peaks around January and peaks later this month or in February.

However, in this outbreak, cases spiked in early December, then eased slightly, then rose again after the holidays, when children returned to school. This could have led to a larger and more intense spread of the disease, according to Dr. Khabbaza.

"This year's flu season is strange because of this double peak phenomenon," he said.

Second, the simultaneous circulation of multiple influenza strains, including highly pathogenic strains such as H3N2, has increased the spread and severity of the disease. In the United States, H3N2 and H1N1 strains coexisted at equal levels, a rare occurrence in previous flu seasons. H3N2, which is known to cause severe complications and is especially dangerous for children and the elderly, has led to a sharp increase in hospitalizations.

Next, low flu vaccination rates are alarming. In the US, only about 46% of people get vaccinated against seasonal flu, a number that has steadily declined in the years since the Covid-19 pandemic. Similarly, in Canada, vaccination rates are only 42%, which is not enough to create community immunity. This leaves many people, especially high-risk groups, vulnerable to serious complications of the flu. Experts say that the majority of severe cases occur in people who have not been vaccinated.

In addition, the decline in community immunity after a long period of lockdown and social distancing during the Covid-19 pandemic is also an important cause. Less exposure to pathogens during this period means that many people's immune systems are no longer strong enough to deal with new strains of influenza virus.

In Japan in particular, with the elderly accounting for 29% of the population, the risk of serious illness and death is also increased. In addition, the simultaneous appearance of many other infectious diseases such as Covid-19, mycoplasma pneumonia and gastroenteritis has created a "disaster recipe".

Countries are stepping up influenza vaccination as a top priority. Surveillance and research on new influenza strains are being strengthened to predict and respond promptly to dangerous variants. Wearing masks, washing hands regularly and avoiding large gatherings continue to be recommended by authorities.