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.

The Department of Health (DH) has implemented the Government Vaccination Programme (GVP), the Vaccination Subsidy Scheme (VSS) and the Residential Care Home Vaccination Programme (RVP) since 2009. Among these, GVP provides free Seasonal Influenza Vaccination (SIV) to eligible groups which are at higher risk. The Hospital Authority (HA) and DH administer SIVs at its medical facilities to eligible persons. Under VSS, the Government provides monetary subsidy per dose of SIV given by private doctors to eligible persons. To further enhance coverage, particularly among schoolchildren, the Government introduced several new / enhanced initiatives in the 2018-19 season : (1) subsidy for SIV under VSS was increased from $190 to $210 per dose for all eligible groups; (2) VSS was expanded to cover Hong Kong residents aged 50-64 years; (3) School Outreach Vaccination Pilot Programme was launched for primary schools; (4) DH has actively assisted primary schools not participating in the Pilot Programme and kindergartens/child care centres (KG/CCCs) to arrange SIV outreach, such as through the newly launched Enhanced VSS Outreach Vaccination (eVSS) scheme. As at 10 March 2019, total doses administered via various SIV programmes was about 1,175,000, a 46.9% increase compared with same period last year. SIV administered via GVP and VSS (including eVSS Outreach) has increased by 1.9% and 74.3% respectively. The number of doses received by children aged 6 months to under 12 years increased by 107.4% (coverage 45.6%). Doses received by children aged 6 months to under 6 years increased by 33.5% (coverage 34.2%); and by children 6 years to under 12 years increased by 211.2% (coverage 55.3%). The number of primary schools with SIV outreach has increased more than fivefold, while the number of KG/CCCs with SIV outreach has increased by 206.7%. As of 12 March 2019, the relative risk of KG/CCCs with SIV outreach influenza-like illness (ILI) outbreak compared with those without was 0.63 (95% CI 0.49-0.80; p=0.0002). For primary schools with SIV outreach, the relative risk of ILI outbreak was 0.48 (95% CI 0.35-0.65; p< 0.0001). These showed that significantly lower proportions of schools with outreach SIV encountered ILI outbreak. In view of high acceptance and effectiveness of SIV outreach programmes, the Government has decided to cover more primary schools / KG/CCCs starting from 2019-20.

Each year, influenza viruses circulate worldwide and are responsible for a considerable burden of medical visits, hospitalizations and deaths, especially at the extremes of age, among the very young and the very old. Older adults are more vulnerable to influenza illness and less likely to benefit from preventive measures such as influenza vaccination due to the increased prevalence of chronic medical conditions with age and the systemic degeneration of immune functioning, known as “immunosenescence”. Indeed, although influenza vaccines have been used since the 1940s and the World Health Organization (WHO) recommends vaccinating older adults, there is limited evidence on vaccine efficacy in this age group and numerous reports of lower immunogenicity and clinical vaccine effectiveness (VE) in older compared to younger and middle-aged adults. We consider two strategies that may overcome some of these challenges.

I will discuss the potential for vaccinating older adults every 6 months, instead of every year. This would increase the likelihood that vaccine components would match circulating influenza viruses, and may be particularly valuable in subtropical or tropical areas where there are multiple epidemics each year or prolonged periods of influenza activity. In a randomized controlled trial, we found that twice annual vaccination did provide higher year-round antibody titers in the first year, when vaccine strains were unchanged. The trial is ongoing. Then I will discuss the potential for using enhanced influenza vaccines in older adults. Vaccines which include a higher antigen content, or are adjuvanted, generate stronger immune responses, and may provide prolonged protection, and broader protection against mismatched viruses. A randomized trial is now being conducted to examine in more detail the immune responses to enhanced vaccination in older adults in Hong Kong, including the use of these vaccines in alternating/combination strategies.

Seasonal Influenza Vaccination (SIV) is the most effective measure to build up one’s defense against influenza and to prevent its complications. The vaccine efficacy ranges from 60-80% depending on the closeness of the vaccination strains and the circulating strains. The Government of the HKSAR implemented the Government Vaccination Programme (GVP) by providing free or subsidized SIV and pneumococcal vaccination to eligible persons every year since 2009. Hospital Authority (HA) has also been supporting the GVP to provide the vaccination to the eligible groups, including Health Care Workers (HCWs). 

SIV of HCWs constitutes a significant infection preventive measure. It protects both HCWs and patients by preventing onward transmission of influenza and to reduce the chance of outbreak occurrence in healthcare settings. However, the vaccination uptake rate of HCWs in HA remained low and unsatisfactory when compared to UK and the USA. In 2017/18, around 32% of HCWs in HA had received SIV; while 68.7% and 78.4% of HCWs in UK and the USA received it respectively. 

Therefore, for the GVP 2018/19, HA determined to work proactively on new initiatives to encourage vaccination among HCWs. With the support from top management, multidisciplinary approaches were adopted. Infection control teams collaborated with various departments to launch a series of promotional activities. These promulgation strategies aimed to raise the awareness of the importance of SIV; rectify the myths; enhance the accessibility and to recognize the effort and creativity of colleagues responsible for the promotion. We saw a flamboyant display of all sorts of promulgation materials. As a result of these campaigns, the uptake rate of HCWs in HA increased significantly. In this session I will share with you the promulgation plans and the results.