Seasonal influenza viruses include influenza A(H1N1)pdm09, influenza A(H3N2) and influenza B viruses. In Hong Kong, there is a distinct seasonal pattern with two peaks. The winter influenza season usually occurs from January to March/April, and another summer influenza season occurs at variable time between June and August. From 2015-2019, the typical winter seasons lasted for 12-17 weeks while the summer seasons lasted for 5-8 weeks only.
The Centre for Health Protection of the Department of Health monitors seasonal influenza activity in the community through a series of surveillance systems. We have used the moving epidemic method to assess the impact and severity of seasonal influenza since 2019 as a pilot.
Influenza seasons were usually predominated by one type of virus but there was co-circulation of more than one type in some seasons, e.g. 2015/16 winter season with co-circulation of H1N1 and B viruses. In seasons predominated by H3N2 (e.g. 2014/15 winter season and 2017 summer season), elderly ≥65 years were mostly affected. On the other hand, children were relatively more affected in seasons predominated by H1N1 or B (e.g. 2015/16, 2017/18 and 2018/19 winter seasons).
Regarding reported outbreaks of influenza-like illness, about 50% occurred in elderly homes during H3N2-predominant seasons while 70-80% occurred in child care centre/kindergartens and primary schools during seasons with predominance of H1N1 or B viruses. For influenza-associated hospitalisation rate, it was usually highest among young children < 6 years, followed by elderly ≥65 years and then children aged 6-11 years. The rate among elderly was relatively higher in H3N2-predominant seasons.
During major influenza seasons in 2015-2019, 409-647 severe influenza cases (intensive care unit admissions/deaths) among adult patients were recorded with the majority affecting elderly ≥65 years (about 85% for H3N2 cases, 75% for B cases and 50% for H1N1 cases). Persons aged 50-64 years constituted up to about 35% of severe adult cases of H1N1, as compared to < 20% for H3N2 and B cases. For paediatric cases of influenza-associated complications/deaths, 18-27 cases (1-3 deaths) were recorded in major seasons with the highest incidence in young children < 6 years.
In summary, the disease burden of seasonal influenza is significant in terms of institutional outbreaks, hospitalisations and mortality, especially among elderly and young children. Promoting seasonal influenza vaccination is the most effective means for prevention of seasonal influenza.
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