Associate Professor, Doctor Pham Quang Thai, Deputy Head of the Department of Infectious Disease Control at the National Institute of Hygiene and Epidemiology, stated that while the flu circulates year-round without a fixed peak, outbreaks often coincide with periods of increased social activity such as Lunar New Year, festivals, or travel seasons. The end of the year also sees many large events organized, including conferences, year-end summaries, and weddings.

Crowded activities create a favorable environment for the flu virus to spread. *Shutterstock*

Associate Professor Pham Quang Thai also noted that when people gather in close proximity in enclosed spaces, the flu virus easily finds hosts to spread. Even a few cases in groups like schools or crowded events can lead to a rapid surge in infections. This is because the flu transmits quickly through the respiratory tract; saliva or nasopharyngeal secretions containing the flu virus spread when patients cough or sneeze into the air. The flu virus enters the body through the nasopharyngeal route. Transmission intensifies with direct, close contact, especially in cold, humid weather.

Currently, many localities, particularly Hanoi, are seeing a sharp increase in influenza A cases. From early October to early November, National Children's Hospital admitted over 3,700 seasonal flu cases, with nearly 500 children requiring hospitalization. At Thanh Nhan Hospital, the number of patients seeking examination for influenza A has increased significantly; 50% of admissions to the infectious diseases department are due to flu, mostly among those with underlying conditions who developed pneumonia complications upon admission. The outbreak at Con Cuong Boarding Ethnic Secondary School in Nghe An is an example of community spread: nearly 400 students were absent in november, with 163 confirmed influenza A cases.

Associate Professor Pham Quang Thai at a flu seminar. *NVCC*

Associate Professor Pham Quang Thai said that for the 2025-2026 flu season, international surveillance data indicates the predominance of the influenza A/H3N2 strain in many regions, including Northern Europe, South Asia, and Southeast Asia. The A/H3N2 virus has also been identified as the currently circulating main strain in Vietnam. The associate professor added that while no significant change in virulence has been observed, seasons dominated by H3N2 often involve high numbers of cases, hospitalizations, and deaths, particularly among the elderly and young children. The doctor explained that two main factors contribute to H3N2's strong outbreaks this year: First, an "immunity gap" due to the community's limited exposure to H3N2 in recent years and low vaccination rates. Second, the virus's adaptability, which allows H3N2 to attach and replicate effectively in the upper respiratory tract, making droplet transmission very easy.

To actively prevent seasonal flu, the Department of Preventive Medicine recommends that people maintain personal hygiene, cover their mouths when sneezing, wash hands regularly with soap and clean water, and clean their nose and throat daily with saline solution. Keeping the body warm, eating nutritious food to boost immunity, and limiting unnecessary contact with flu patients or suspected cases also help prevent the disease. If symptoms like cough, fever, runny nose, headache, or fatigue appear, seek medical attention promptly for timely examination and treatment.

Associate Professor Pham Quang Thai further stated that the current quadrivalent flu vaccine still offers good protection against H3N2, as it contains four strains: H1N1, H3N2, and two influenza B strains. The associate professor cited studies published by the U.S. National Institutes of Health, noting that during the 2022-2023 flu season in the U.S. – a period of strong H3N2 circulation – the vaccine reduced the risk of hospitalization by 27-46% and the risk of multi-organ failure or mechanical ventilation by 65-66%. Meta-analyses show that quadrivalent vaccines reduce the risk of H3N2 hospitalization by approximately 45%, slightly more effective than trivalent vaccines.

The expert recommends that people get flu vaccinations before the holiday season and year-end gatherings, as the body needs about two weeks to develop full immunity. Vaccination not only protects oneself but also reduces the risk of bringing the pathogen home and infecting high-risk family members (young children, pregnant women, the elderly, those with underlying conditions). Additionally, due to the virus's continuous mutation and antibody decline over time, annual booster shots are necessary to maintain sustained protection.

Kim Anh