Changes in the Seasonality of Influenza in Thailand

Abstract

Highlight

• Influenza is a common respiratory infectious disease. High-risk groups—such as the elderly, young children, and individuals with underlying health conditions—are more likely to experience severe complications and death.
• Data from the past 12 years (2014–2025) show that influenza outbreaks in Thailand usually occur in two seasonal waves: early and late in the year, with the more severe outbreaks often occurring at the end of the year. However, in some years such as 2014, 2019, and 2025, the early-year outbreaks were intense. This pattern appears to recur every 5–6 years. Additionally, the timing of peak transmission has shifted, as seen in 2024 when cases began rising in May and peaked in July—earlier than in previous years.
• Changes in the seasonality of influenza outbreaks have affected the effectiveness of disease control efforts, particularly regarding vaccination services. Although annual vaccination campaigns are in place, earlier-than-expected outbreaks may not allow sufficient time for immunity development among the population. This can reduce vaccine effectiveness and increase the burden on the healthcare system.