Immune response to influenza vaccination in community-dwelling Chinese elderly persons
Introduction
Influenza and its associated complications leads to considerable morbidity and mortality in the elderly population [1], [2], [3], [4], [5]. Globally, the influenza virus infects about 10–20% of total population annually and is estimated to lead to more than 3 million cases of severe illness and with up to half a million deaths [5], [6], [7], [8]. A recent systematic review of influenza vaccination in temperate regions and with Caucasian ethnic groups reports that vaccination prevents 42% deaths from pneumonia or influenza, 46% of pneumonia, 45% of hospitalizations due to pneumonia or influenza and 60% of all cause mortality in the elderly living in nursing homes but only 26% of hospital admissions for influenza and pneumonia and 42% of all cause mortality among elderly living in the community [9]. The Advisory Committee on Immunization Practices (ACIP) recommended annual influenza vaccination for persons aged 50 years old and over as well as for residents of nursing homes in the US while the World Health Organization (WHO) recommended annual influenza vaccination to persons aged ≥65 years [2], [6].
However, in countries with a tropical or sub-tropical climate, the impact of influenza is less well appreciated and there is a perception that influenza may have less clinical impact in warmer climates [10]. The lack of good estimates of the clinical disease burden of influenza and of the immunogenicity and efficacy of influenza vaccine in the tropics adds to scepticism on the efficacy and utility of influenza vaccine in these regions. In contrast to temperate regions where influenza activity is concentrated within a few weeks in the winter of each year, the more diffuse seasonality of influenza in the tropics and subtopics obscures the impact of influenza. As a consequence of this perception that the clinical disease burden of influenza is less in the tropics, vaccine utilization is less. For example, in the year 2000, Hong Kong which has a per capita GDP comparable to that of Australia used less that 1/5th the doses of influenza vaccine per 1000 population [11].
In the Chinese population, data from a nationally representative Chinese cohort of 169,871 men and women 40 years of age and older in China shows that influenza and pneumonia have an age-standardized mortality rate of 43.9 per 100,000 person-years and are the fourth leading cause of death in China [12]. The Hong Kong SAR Government has provided free influenza immunization to institutional elderly in the past 8 years (i.e. since 1998) and more recently, to persons in high-risk groups such as health care workers, the elderly and the disabled [13], [14]. For elderly residents living in old aged homes, the vaccine uptake rate was reported to be over 87% [15].
In a recent quantitative review of antibody response to influenza vaccination in 31 studies, the antibody response is concluded to be considerably lower in the elderly (17–53%) than in younger adults (70–90%) [16]. Although there are many studies on the antibody immune response to the influenza vaccine in Caucasian populations in temperate regions [9], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], there is a paucity of information from the tropics and on non-Caucasian ethnic groups. Jianping and colleagues have reported the use of influenza vaccine in 1356 very fit Chinese from the Chinese Army. Influenza and common symptoms were reduced in all age groups but less effective in the elderly aged 60 years and over (i.e. 84.8% in children, 74.0% in adults and 68.6% in elderly people) [28]. Antibody response was not investigated in this study. Hospitalization data were not reported too. Moreover, nutritional, cognitive and functional profiles were not performed in this study. Previous Caucasian studies have reported that elderly with poor nutritional and functional status might have poor antibody responses after vaccination. Functional limitations were associated with both an increased risk of death and a decreased likelihood of vaccination [29], [30], [31]. We therefore included chronic diseases, nutritional, functional and cognitive function observations in our study.
The variable seasonal pattern of influenza in the tropics and sub-tropics has also led to uncertainty about the immunogenicity and efficacy of influenza vaccines in such regions [10], [32]. In a recent study in Hong Kong, we have shown that influenza-related mortality from respiratory and ischaemic heart disease in the elderly (≥65 years) was comparable to that of the USA [32]. In this study, we address the antibody response to influenza vaccine in community-dwelling elderly Chinese persons in Hong Kong. The secondary objective was to explore the possible beneficial effect of influenza vaccination in decreasing unplanned hospital admissions.
Section snippets
Design
This was a randomized, single-blind, placebo-controlled study. The study protocol was approved by the Institutional Review Board of the University of Hong Kong and Hospital Authority Hong Kong West Cluster and complied with the Declaration of Helsinski.
Subjects
Subjects were recruited from the geriatrics out-patient clinic at the Queen Mary Hospital and Sai Ying Pun general out-patient clinic of Hong Kong. Subjects were recruited from October 2003 to February 2004. The inclusion criteria were Chinese
Subjects’ recruitment and follow-up
One hundred and twenty-eight subjects were recruited from 23 October 2003 to February 2004. The overall drop-out rate was 3.9% at 6 months. One subject in vaccinated group defaulted follow-up at week 4. The reason was refusal of blood taking at week 4. At 6 months, another four subjects defaulted from the study. One subject died of an accident a few days before the month 6 visit. Three subjects refused blood taking.
Baseline characteristics of subjects
The mean age was 74.4 ± 7.4 years and 52.3% were males. Comparisons of baseline
Discussions
We found good immune HI antibody responses to influenza vaccination in community elderly Chinese living in the sub-tropical city of Hong Kong which were comparable with previous findings from temperate regions and on predominantly Caucasian ethnic groups [9], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. A higher HI antibody titre was found in vaccinated subjects for all the serological parameters examined (i.e. seroprotection rate, GMT or serological response rate).
Conclusions
We have demonstrated that influenza vaccination was safe and effective in eliciting protective HI antibody responses in Chinese living in a sub-tropical city, Hong Kong. Our finding also shows a reduction in unplanned hospital admissions in vaccinated community-dwelling elderly persons. Currently, the Hong Kong SAR Government's policy primarily provides free influenza vaccination for the institutionalized elderly and those with chronic illness. Ambulatory community-dwelling elderly persons are
Acknowledgements
Part of the work in the present study was performed by Miss Hui Sau Lan in partial fulfilment for the degree of Master Medical Sciences (Specialised Modules in Geriatrics Medicine) of the Faculty of Medicine at the University of Hong Kong. This study was partly supported by the Vice Chancellor's Development Fund, The University of Hong Kong.
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