Flu in China: Ongoing Spread and Pediatric Insights

China is experiencing a significant flu season, with widespread cases affecting various regions. Pediatric infections are drawing attention as hospitals report increased admissions and severe symptoms in young children. The ongoing spread raises concerns about dominant flu strains, potential complications, and public health responses.

Common Influenza Subtypes Observed

Recent surveillance data indicate that influenza A(H3N2) and influenza B Victoria lineage are the predominant circulating strains, with sporadic detections of influenza A(H1N1)pdm09. H3N2’s dominance is notable, as it has historically led to more severe outbreaks, particularly among young children and the elderly. Studies in The Lancet Infectious Diseases show that H3N2 infections often result in higher hospitalization rates due to their ability to evolve rapidly and evade immunity. Meanwhile, the resurgence of influenza B Victoria lineage, which had been relatively dormant in recent seasons, suggests a shift in viral dynamics that could affect vaccine effectiveness.

Genetic sequencing of recent H3N2 isolates from Chinese clinical samples has revealed antigenic drift mutations in the hemagglutinin (HA) protein, potentially reducing vaccine efficacy. A study in Nature Microbiology highlighted mutations such as T135K and L183P, which may enhance viral transmissibility. Reports from the Chinese Center for Disease Control and Prevention (China CDC) note an uptick in cases despite vaccination efforts, underscoring the challenge of predicting and formulating effective vaccines.

Influenza B Victoria lineage, while generally less virulent than H3N2, has been linked to severe pediatric cases this season. Unlike influenza A, which reassorts frequently, influenza B evolves more slowly but can still cause significant outbreaks. A meta-analysis in Clinical Infectious Diseases found that influenza B infections disproportionately affect children and adolescents, often leading to complications such as pneumonia and myocarditis. The current dominance of the Victoria lineage over the Yamagata lineage, which has not been detected globally in recent years, suggests a narrowing of influenza B diversity. This could impact long-term immunity, as reduced exposure to Yamagata strains may leave populations more vulnerable if they re-emerge.

Seasonal Transmission in Different Regions

Influenza activity in China follows distinct seasonal patterns across geographical regions due to climate, population density, and mobility differences. In the northern provinces, flu cases typically peak between December and February when colder temperatures and indoor crowding facilitate transmission. An epidemiological analysis in The Journal of Infection found that low humidity and decreased ultraviolet radiation in northern China contribute to prolonged viral survival in aerosols, enhancing spread. Historical surveillance data from China CDC consistently report higher influenza positivity rates in northern cities like Beijing, Harbin, and Shenyang during winter.

In contrast, southern China experiences a more prolonged and sometimes bimodal flu season, with peaks in winter and summer. Studies in Emerging Infectious Diseases show that the subtropical climate in cities like Guangzhou, Shenzhen, and Hong Kong allows influenza to circulate year-round, with increased transmission during periods of high humidity and fluctuating temperatures. This pattern is influenced by dense urban populations and high levels of domestic and international travel, which facilitate continuous viral introduction and reintroduction. The co-circulation of influenza with other respiratory viruses, such as respiratory syncytial virus (RSV) and adenoviruses, further complicates seasonal trends and healthcare burdens.

Meteorological factors play a significant role in shaping these regional differences. A multi-year study in Nature Communications found that temperature and absolute humidity strongly correlate with seasonal flu activity. In the north, sharp declines in humidity during winter favor viral stability, while in the south, persistent moisture supports year-round viral persistence. These findings highlight the need for localized public health strategies, with vaccination campaigns and awareness efforts timed to regional transmission patterns.

Clinical Observations in Pediatric Cases

Hospitals across China report a sharp rise in pediatric influenza admissions, with many children presenting unusually severe symptoms. Physicians note high, persistent fevers exceeding 39°C (102.2°F) that often last over five days despite standard treatment. Febrile seizures, particularly in infants and toddlers, are leading to increased intensive care unit (ICU) admissions. Pediatricians in Beijing and Shanghai have observed an uptick in influenza-associated encephalopathy, a rare but serious complication causing altered mental status and seizures, with H3N2 infections more frequently linked to neurological involvement.

Respiratory complications are also more common this season. A growing number of pediatric patients are developing severe pneumonia, with CT scans revealing diffuse lung opacities indicative of viral pneumonitis. Some children experience rapid progression to acute respiratory distress syndrome (ARDS), requiring mechanical ventilation. Secondary bacterial infections, particularly from Streptococcus pneumoniae and Staphylococcus aureus, are complicating recovery and prolonging hospitalization. A case series from a Guangzhou children’s hospital documented multiple instances of necrotizing pneumonia following influenza infection, reinforcing concerns over bacterial superinfections.

Gastrointestinal symptoms, including nausea, vomiting, and diarrhea, are also more frequent this season, leading to dehydration requiring intravenous fluids. While influenza is primarily a respiratory virus, certain strains show increased tropism for the gastrointestinal tract, particularly in younger children. Severe dehydration is prolonging hospital stays, with some cases requiring electrolyte correction and intensive supportive care. These findings highlight the need for vigilant fluid monitoring in pediatric influenza cases.

Co-Infections with Other Pathogens

Clinicians in China report a significant number of influenza cases complicated by co-infections with other respiratory pathogens, leading to more severe disease progression. Hospitals in Beijing and Guangzhou frequently detect RSV and Mycoplasma pneumoniae alongside influenza, particularly in children under five. Mixed infections often result in prolonged hospital stays and increased need for intensive care. Physicians note that children with dual infections experience more pronounced respiratory distress, with higher rates of wheezing and oxygen desaturation.

Laboratory testing from sentinel surveillance sites has also identified a rise in influenza and adenovirus co-infections, which exacerbate airway inflammation and increase the likelihood of post-viral bacterial pneumonia. A multicenter study across several provinces found that nearly 18% of hospitalized pediatric flu patients tested positive for at least one additional viral or bacterial pathogen. Adenovirus infections can cause persistent fever and severe bronchitis, further complicating influenza recovery. Some children initially diagnosed with influenza later developed adenovirus-induced hemophagocytic lymphohistiocytosis (HLH), a life-threatening inflammatory condition requiring immunosuppressive therapy.

Genetic Mutations in Circulating Strains

The genetic evolution of influenza viruses in China this season raises concerns over potential changes in virulence and transmissibility. Sequencing data from China CDC indicate antigenic drift mutations in the hemagglutinin (HA) and neuraminidase (NA) proteins of circulating H3N2 strains, reducing the effectiveness of existing immunity and contributing to vaccine mismatches. Virological analyses in Nature Microbiology identified HA mutations such as T135K and L183P, which alter receptor-binding properties, potentially enhancing viral spread. Additionally, changes in the NA protein, including E119V, have been linked to reduced susceptibility to neuraminidase inhibitors, raising concerns about antiviral resistance.

Influenza B Victoria lineage has also shown genetic changes that may influence disease severity. Researchers at the University of Hong Kong identified amino acid substitutions in the HA gene that could impact antigenicity, though the full clinical implications remain under investigation. Unlike influenza A, which undergoes frequent reassortment, influenza B evolves more gradually. The current dominance of the Victoria lineage over Yamagata suggests a narrowing of genetic diversity, which could affect long-term immunity. Continuous genomic surveillance remains essential to track these evolutionary trends and assess their impact on public health measures.