The impact of influenza on the health and health care utilisation of elderly people

Abstract

Despite intensive research, influenza viruses still present one of the major causes of respiratory disease throughout the world. The elderly population and those individuals considered “at risk” due to presenting co-morbidity are especially vulnerable to influenza infection; this is evident from increased rates of morbidity and mortality in these populations during winter epidemic periods. Successful vaccination campaigns have targeted these groups over the last few years, providing protection to an increasing number of susceptible individuals. However, during periods of influenza virus activity there are still seasonal pressures put on both primary and secondary health care services. In the event of a serious influenza epidemic or pandemic, these burdens could jeopardise health care provision to at risk populations. In this report we discuss current issues surrounding the impact of influenza on the health care utilisation of elderly people.

Introduction

Influenza is one of the commonest and most important respiratory illnesses affecting persons of all ages and occurring in short epidemics during most winters in temperate countries. The epidemic characteristic is less apparent in tropical countries [1], [2]. Though the diagnostic term ‘influenza’ is often used by doctors as a syndromic diagnosis, influenza virus is the most important causative micro-organism and this article is focused primarily on influenza virus infection. Influenza viruses are constantly mutating (antigenic drift) and thus immunity acquired from infection or from vaccination lasts for a comparatively short time [3]. Annual revaccination with a virus strain known to be close to recently circulating viruses is the mainstay of influenza protection.

The elderly, and persons of all ages with co-morbidity (including those with asthma, chronic obstructive pulmonary disease, ischaemic heart disease, chronic renal disease, diabetes and those whose illness or treatment renders them immune compromised) are known to be at greater risk from influenza [4], [5], [6], [7], [8], [9]. These studies have disclosed greatly increased risk of death from influenza in patients with chronic diseases but increased risk is probably not simply that associated with increasing age: the older people become the more likely they are to suffer from chronic diseases and to receive medication which makes them immune compromised [2]. The initial immune responses of the elderly to influenza vaccination are comparable to those in younger individuals. However, Kang et al. [10] have shown the immune systems of the elderly fail to maintain or expand upon these initial responses, which renders them more susceptible to influenza infection despite being vaccinated. They suggested that current vaccination strategies could be modified, e.g. to include a booster vaccination several months following initial vaccination, in order to improve long-term protection. This may provide greater protection especially from influenza epidemics occurring late in the winter season. The elderly are at greater risk from influenza because they generally suffer a more serious illness and, are therefore, more likely to be hospitalised: if hospitalised they often need to remain in for longer periods than younger patients [11], [12]. The specific effects of aging on the immune system are reviewed in a separate paper in this supplement [13].

Influenza spreads rapidly, mainly by airborne respiratory droplets [14], [15]. It has been suggested that children are particularly important in promoting spread in the early stages of an epidemic [16], [17], [18], [19] but whether this property is specific to children or applies to any group of persons living or working in close proximity is debateable. The pathogenesis of influenza virus infection starts with entry of influenza virus into the cells lining the trachea and bronchial airways where replication of virus takes place with the consequent destruction of the tracheo-bronchial epithelium [2]. It has been shown that symptoms are related to the virus load in respiratory secretions and usually in immune competent persons the replicative process peaks about 48–72 h after initial virus entry [20]. For this reason, if influenza is to be treated by antiviral drugs it is important to introduce treatment in the first 2 days of illness. Most studies of virus shedding have been conducted in the otherwise healthy and in young persons [20], [21], [22], [23]. In children, especially when experiencing their first influenza attack, and in the immune compromised, virus shedding following infection is known to last longer than in older and otherwise healthy persons [21], [23], [24]. We have found no studies of virus shedding in the frail elderly who are particularly vulnerable to the more serious consequences of influenza. Furthermore, these persons sometimes experience fever and constitutional symptoms well beyond the third day of infection, the cause of which is not clear. In clinical trials of influenza treatment with zanamivir, the clinical benefit was greater in older persons than in younger age groups [25]. There is an implication here that virus replication is continuing for a longer period in older and frailer persons.

Influenza can be complicated by secondary bacterial infection, in older people causing acute bronchitis and broncho-pneumonia. Respiratory compromise, whether caused by the primary influenza infection or resulting from complications, may precipitate cardiac decompensation and the frail elderly are particularly vulnerable to these complications. Furthermore, there is some evidence from sero-conversion studies after vaccination that older persons do not mount as rigorous an immune response as younger people and by implication are less well protected by vaccination [26].

Many studies have demonstrated increased mortality associated with influenza outbreaks [27]. Increased mortality is evident mainly in cardiac and respiratory certified deaths [28], [29], [30]. Whilst respiratory deaths can easily be understood, the pathogenesis of cardiac related deaths is less clear. Some persons die as a result of cardiac decompensation after an attack of influenza occurring because of the increased metabolism associated with fever, but a few may die because of direct cardiac damage [2]. Glezen et al. [31] has reported a delay of 2 weeks between peak incidence of influenza-like illness (ILI) and death. However, age specific data from England suggest that deaths during influenza epidemics (which are almost exclusively among the elderly) occur at more or less the same time as the incidence of new cases presenting to general practitioners suggesting that death is occurring in the acute phase of the illness and caused directly by influenza and not as a secondary complication [30]. Further research is needed into the underlying cause of death from influenza.