HKUMed study reveals low immunity against H3N2 strain in Hong Kong and urges early vaccination.
Flu activity has surged in many parts of the Northern Hemisphere, driven primarily by a newly emerged H3N2 strain known as 'subclade K'. A research team led by the Department of Microbiology and the Department of Medicine, both under the School of Clinical Medicine at the LKS Faculty of Medicine of the University of Hong Kong (HKUMed), has found that most hospital patients in Hong Kong have little to undetectable levels of neutralising antibodies against this mutated strain. HKUMed researchers strongly advise the public to get vaccinated as soon as possible.
Neutralising antibody levels are closely linked to protection against infection. The research team was led by Professor Kelvin To Kai-wang, Clinical Professor and Chairperson of the Department of Microbiology, and Professor Ivan Hung Fan-ngai, Chair Professor and Head of the Division of Infectious Diseases in the Department of Medicine of the School of Clinical Medicine at HKUMed. The team collected serum specimens from public hospitals to monitor viral genetics and antibody responses and assess the overall immunity level against H3N2 in Hong Kong.
Analysis of 277 serum samples collected in November 2025 revealed that 52% of individuals had detectable neutralising antibodies against the previously circulating subclade J.2.2, with 27% reaching a titer of 40 or above, indicating a relatively high level of immunity. In stark contrast, only 18% of individuals had detectable antibodies against the newly emerged subclade K, with 0.7% reaching a titer of 40 or above.
'Our findings suggest that the local community's immune barrier against the subclade K is insufficient and lower than that against the previously circulating subclade J.2.2,' explained Professor Ivan Hung.
To reduce the risk of severe infection, the research team recommends the following measures:
- Get vaccinated against influenza. A study from England found that vaccine effectiveness against emergency department visits and hospital admissions is between 72% and 75% for children and adolescents, and 32% to 39% for adults.
- Seek early diagnosis using rapid antigen tests.
- Consult a doctor promptly. Antiviral medications are most effective when administered within 48 hours of symptom onset.
Professor Kelvin To emphasised, 'These data serve as a clear alert for Hong Kong. With influenza A cases rising and new variants emerging, we must reinforce the message that vaccination is the most effective way to prevent infection. For those who do become infected, seeking medical care promptly is essential to reduce the risk of severe complications.'
About the research team
The research was jointly led by Professor Kelvin To Kai-wang, Clinical Professor and Chairperson of the Department of Microbiology, and Professor Ivan Hung Fan-ngai, Chair Professor and Head of the Division of Infectious Diseases in the Department of Medicine, both under the School of Clinical Medicine at HKUMed.
