Hong Kong is currently experiencing its winter influenza season. According to a joint analysis published in 2017, an average of 12,700 respiratory hospitalizations and 431 respiratory deaths are attributable to influenza each year in Hong Kong. The exact impact varies from year to year, depending on the circulating strains, with influenza A(H3N2) epidemics tending to have the highest impact. This winter Hong Kong is experiencing an epidemic of influenza B including strains from the B/Yamagata lineage.

Influenza vaccines are the most widely used vaccines in the world, and around 700,000 doses were administered last year in Hong Kong according to the Centre for Health Protection (CHP). However, some concerns have been raised about the effectiveness of influenza vaccines, following a recent report of low effectiveness of the vaccine against influenza A(H3N2) in Canada. The effectiveness of influenza vaccination varies by location, because of the different strains that are circulating in different parts of the world.

Monitoring influenza vaccine effectiveness

The University of Hong Kong (HKU) has been collaborating with Queen Mary Hospital and Princess Margaret Hospital to monitor influenza vaccine effectiveness each year for the past 9 years. Using well-established methods that are used in many other countries around the world to continuously monitor vaccine effectiveness, a research team comprising Professor Ben Cowling, Professor Malik Peiris and Dr Susan Chiu from HKU and Dr Mike Kwan and Dr CW Leung from Princess Margaret Hospital has enrolled children who are admitted to these hospitals with an acute respiratory illness including a fever and other respiratory symptoms, and determined which children had previously received influenza vaccination and which children were infected with influenza.

By comparing vaccination status in the children with influenza or without influenza, the research team makes an estimate of the effectiveness of influenza vaccination in preventing hospitalization. Very similar methodology was used in Canada this season to estimate low influenza vaccination effectiveness of 17% against influenza A(H3N2) and moderate effectiveness of 55% against influenza B.

Vaccine effective against locally circulating strains

Most recently, the research team conducted a hospital-based study in Hong Kong to estimate influenza VE for the winter of 2017/18. In this analysis, the team included data on 1,078 children who were admitted between 4 December 2017 and 31 January 2018 with febrile acute respiratory illness and who were tested for influenza. There were 339 children with laboratory-confirmed influenza, among which 80% had influenza B. Using the data collected on these children, the team estimated that influenza vaccination effectiveness was 66%. Most vaccinated children had received the quadrivalent vaccine which includes a B/Yamagata strain, rather than the trivalent vaccine that does not include a B/Yamagata strain.

“These are the first local estimates of the performance of the influenza vaccine in Hong Kong this winter, and reassure us that the vaccine is very effective against the locally circulating strains,” said Professor Ben Cowling, Head of Division of Epidemiology and Biostatistics at the HKU School of Public Health and researcher at the WHO Collaborating Centre of Infectious Disease Epidemiology and Control. 

Dr Susan Chiu, Clinical Associate Professor of HKU Department of Pediatrics and Adolescent Medicine, added: “These results indicate that influenza vaccination is effective and should be more widely used in children in Hong Kong. Looking forward, it is important for parents to get their children vaccinated in October and November each year, before the influenza season starts.”

Professor Malik Peiris, Chair Professor of Virology and Tam Wah-Ching Professor in Medical Science of HKU School of Public Health, further remarked: “The influenza vaccine tends to be more effective against influenza B and influenza A(H1N1) than against influenza A(H3N2), and that is consistent with these results.” MIMS

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