Top virologist warns of mutated influenza strain spread in Thailand

A leading virologist has warned the public today, January 8, that a mutated strain of influenza is spreading across Thailand, as reduced vaccine effectiveness contributes to rising case numbers.

Professor Yong Poovorawan, a member of the Royal Institute and director of the Centre of Excellence in Clinical Virology at Chulalongkorn University’s Faculty of Medicine, issued the warning on his Facebook page.

He noted that the current outbreak follows last year’s surge in influenza cases, which reached the highest level ever recorded by the Ministry of Public Health.

In early 2025, the A H1N1 2009 strain was responsible for a significant number of cases. However, by the end of the year, A H3N2 became the dominant strain. The new A H3N2 subtype, identified as the K type, is particularly concerning due to multiple mutations within the HA1 region of the virus, allowing it to better evade the immune system and reduce vaccine efficacy.

Although the number of patients has increased as a result, Professor Yong emphasised that the severity of illness has not worsened, and antiviral treatments remain effective.

Influenza B cases have also been reported, though only the Victoria lineage has been detected. The Yamagata strain has not been found in Thailand for over five years, leading experts to conclude that three-strain vaccines are currently sufficient, with no added benefit from using four-strain formulations.

Despite the drop in effectiveness, Professor Yong continues to recommend vaccination. He said the currently available vaccines are inactivated, meaning they use a killed version of the virus that cannot cause infection. They are also generally safe, cause minimal side effects, and are affordable.

Thailand is expected to receive the updated Southern Hemisphere flu vaccines in March, in time for the rainy season, when influenza cases are likely to rise again.

In similar news, Thailand’s Ministry of Public Health has issued a public health warning following a sharp rise in respiratory illnesses, with over 555,000 influenza cases and more than 16,000 RSV infections reported so far this year.