Health officials in China are monitoring a significant shift in the seasonal flu landscape as influenza positivity rates have seen a steady climb. According to recent monitoring data from the Chinese Center for Disease Control and Prevention (CDC), the positivity rate for influenza among patients with suspected symptoms has approached 45%, signaling that the country has entered a medium-level epidemic stage.
While influenza activity is common during the winter and spring months, this current surge is characterized by a transition in the dominant viral strains. While the previous season was primarily dominated by the A(H1N1) subtype, health experts are now seeing a resurgence of the A(H3N2) subtype. This shift is particularly noteworthy because the A(H3N2) variant is often associated with higher transmissibility and a faster rate of spread compared to H1N1.
For the global community and travelers, these trends underscore the volatile nature of respiratory viruses. The current situation in China reflects a broader pattern where different subtypes—including A(H1N1), A(H3N2), and Influenza B—can alternate in dominance within a single season or across successive years, challenging the existing immunity of the population.
Understanding the Shift to A(H3N2) and Population Immunity
The resurgence of the A(H3N2) subtype is not due to the emergence of a “new” or “novel” virus, but rather a cyclical return of a known strain. Wang Dayan, Director of the National Influenza Center at the Chinese CDC’s Virus Disease Control Office, clarified that H3N2 is a known subtype and that it is common for different strains to fluctuate in dominance throughout a flu season.
A critical factor in the current vulnerability of the population is the “immunity gap.” Dr. Li Zhengdong, Director of the Infectious Diseases Department at Beijing Anzhen Hospital, noted that A(H3N2) was prevalent in the winter of 2023, while A(H1N1) dominated the winter of 2024. Because it has been two years since H3N2 was the primary circulating strain, the collective immune barrier has lowered, and antibody levels against H3N2 are relatively low, making the population more susceptible to infection.
Medical professionals emphasize that while H3N2 may spread more rapidly than H1N1, the clinical approach to diagnosis and treatment remains largely the same. Hospitals are generally equipped with the necessary nucleic acid testing capabilities to differentiate between these subtypes quickly, ensuring that patients receive appropriate care.
Treatment Efficacy and Antiviral Resistance
One of the primary concerns during a surge in influenza cases is whether the circulating strains have developed resistance to standard antiviral medications. To address this, the National Influenza Center conducted resistance analyses on various influenza virus variants collected since March 31, 2025.
The results of these analyses provide a reassuring outlook for public health. The data indicates that all tested A(H3N2) subtypes and Influenza B variants remain sensitive to neuraminidase inhibitors, such as Oseltamivir and Peramivir. This means that the standard frontline treatments used to reduce the severity and duration of the flu are still effective against the currently circulating strains.
Because current medical interventions remain potent, health authorities have advised the public against excessive anxiety. The focus remains on early detection through testing and the timely administration of antiviral drugs for those at higher risk of complications.
Contextualizing the 45% Positivity Rate
While a 45% positivity rate may seem alarming, medical experts caution against misinterpreting this statistic. This figure does not mean that 45% of the general population is infected with the flu. Instead, it refers specifically to the percentage of people who already exhibit respiratory symptoms (such as fever and cough) and sought medical attention at a healthcare facility.
In many cases, patients presenting with “flu-like” symptoms are actually infected by other pathogens. Common alternatives include:
- Rhinoviruses
- Respiratory Syncytial Virus (RSV)
- Mycoplasma pneumoniae
This “co-circulation” of multiple viruses is a hallmark of the winter and spring respiratory season. The high positivity rate among symptomatic patients simply indicates that influenza is currently a primary driver of illness in those seeking care, rather than a reflection of the total population’s infection rate.
Key Takeaways for Public Health
| Feature | A(H1N1) | A(H3N2) |
|---|---|---|
| Recent Dominance | Dominant in Winter 2024 | Dominant in Winter 2023. Resurging now |
| Transmission Speed | Standard | Higher/Faster |
| Antiviral Sensitivity | Sensitive to inhibitors | Sensitive to inhibitors (per 2025 data) |
| Immunity Level | Relatively higher (recent) | Relatively lower (2-year gap) |
Regional Impact and Preventative Measures
The impact of this surge is not limited to China. Similar trends have been observed in neighboring regions. For instance, South Korea reported a significant increase in influenza-like illness (ILI), with the “influenza caution” alert being issued approximately two months earlier than in previous years. Data from the Korea Disease Control and Prevention Agency showed that during the 46th week (Nov 9–15), the number of influenza suspected patients per 1,000 outpatients rose to 66.3, a 30.8% increase from the previous week.
Given the increased transmissibility of the A(H3N2) strain and the lowered population immunity, health experts strongly recommend vaccination. While vaccines are updated annually to match predicted circulating strains, they remain the most effective tool for preventing severe illness and reducing the burden on healthcare systems.
Public health guidelines continue to emphasize basic preventative measures: frequent handwashing, the use of masks in crowded indoor spaces, and staying home when symptomatic to prevent further community spread.
Health authorities will continue to monitor viral mutations and resistance patterns through the National Influenza Center. The next phase of monitoring will focus on whether the A(H3N2) dominance persists through the spring or if Influenza B becomes more prevalent as the season progresses.
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