Position Paper on Influenza Vaccines
Extracts from a World Health Organization document. 1
Data on vaccination coverage show that even in industrialized countries, large proportions of the population at risk do not receive the influenza vaccine. WHO therefore encourages initiatives to raise awareness of influenza and influenza vaccination among health care workers and the public, and encourages setting of national targets for vaccination coverage.
The justification for vaccine use
During influenza epidemics, attack rates of 1–5% are most commonly observed, but the attack rate may reach 40–50% or more among elderly persons in institutions and in other high-risk groups. At least in western communities, bacterial complications such as pneumonia are frequently associated with influenza; the total annual excess mortality during influenza epidemics is estimated at 7.5–23 per 100,000. Influenza poses a considerable economic burden both on society and the individual in terms of consumption of health care resources and lost productivity.
Internationally licensed influenza vaccines have proven to be efficacious and safe. During influenza outbreaks, appropriate vaccination may significantly reduce respiratory illness and sick-leave among healthy adults. More importantly, vaccination may reduce severe disease and premature death in the elderly and in persons with underlying ailments or disease (for details on vaccine efficacy, see below).
Antiviral drugs such as the M2 inhibitors (acting against type A virus) and the more recently developed neuraminidase inhibitors (acting against both type A and type B viruses) have been shown to be effective for treatment (and for some agents, prophylaxis) and are now available in many industrialized countries. Resistant mutants to both classes of antiviral agents have been detected, and antimicrobial resistance surveillance is important to assess the magnitude of this problem. Also, costs, occasional side effects and the likely limited availability of such drugs during major outbreaks highlight the role of vaccination as the primary preventive measure against influenza.
Influenza virus vaccines
The three types of inactivated influenza vaccine show comparable efficacy, but differ in terms of reactogenicity. Thus in 15–20% of vaccines, the whole virus vaccines cause local reactions lasting for 1–2 days. Such reactions appear to be more common in young children than in adults. Transient systemic reactions such as fever, malaise and myalgias may occur in a minority of vaccine recipients within 6–12 hours of the vaccination. Split vaccines and subunit vaccines show reduced systemic reactogenicity both in children and in adults as compared to whole virus preparations. Consequently, subunit and split-virus vaccines are more attractive, particularly for use in children.
WHO position on influenza vaccines
The main purpose of influenza vaccination is to avoid severe influenza and its complications. This paper is concerned mainly with epidemic influenza and the public health impact of yearly influenza vaccination. Authoritative information on pandemics can be found in the WHO influenza pandemic plan. Recommendations for the use of inactivated influenza vaccines and other preventive measures are published in the weekly epidemiological record.
Most of the widely licensed influenza vaccines are manufactured according to the quality requirements defined by WHO and have proven efficacious in the elderly and other groups at risk. If influenza vaccination of children is required, for example as a consequence of predisposing conditions, the vaccine will not interfere with diphtheria–tetanus–pertussis (DTP) or other childhood vaccines, possibly due at the same time. To reduce adverse effects, only split vaccines or subunit vaccines should be given to children. Influenza vaccine should not be given to children aged under 6 months, and those aged 6–35 months should only receive half the adult vaccine dose.
Ideally, when major outbreaks are expected, all individuals should have the opportunity to be vaccinated against influenza. However, limited health budgets and, at least initially, shortage of the appropriate vaccine may force health authorities to restrict influenza vaccine to groups at particular risk. The following priority is recommended:
- 1. Residents of long-term care facilities for the elderly and the disabled – they are considered at high risk of influenza and its complications
- 2. Elderly non-institutionalized individuals suffering from chronic conditions such as pulmonary or cardiovascular illness, metabolic diseases including diabetes mellitus and renal dysfunction, and various types of immunosuppression including persons with AIDS and transplant recipients
- 3. All adults and children aged over 6 months suffering from any of the conditions mentioned above
- 4. Individuals who are above a nationally defined age limit irrespective of other risk factors. Although the appropriate age for general vaccination may be considerably lower in countries with poor living conditions, most countries define the limit age >65 years
- 5. Other groups defined on the basis of national data
- 6. Health care workers in regular, frequent contact with high-risk persons
- 7. Household contacts of high-risk persons.
Pregnant women who will be in their second or third trimester by the start of the influenza season and who are likely to be exposed are advised to consider vaccination in careful consultation with a competent healthcare provider. From a societal perspective, there are good arguments for influenza vaccination of children and healthy adults. Where adequate vaccine supplies are available, vaccination of the general public may be considered. However, the implementation of large-scale influenza vaccination programmes for these groups requires further evaluation of cost-effectiveness and cannot be generally recommended until firm data are presented. Nevertheless, persons who provide essential community services should be considered for vaccination. In some developed countries, companies find it economically justifiable to offer vaccination to their employees.
Although the WHO global influenza surveillance network has proven to be a reliable and successful system, it is important to increase worldwide coverage. Many countries are not included in the network, and in some large countries more than one centre is required. Surveillance is of particular importance in rural areas where potential animal hosts and humans live in close proximity, since it is in such areas that new viral recombinants are likely to originate.
The relatively low uptake of influenza vaccines in most industrialized countries implies that significant proportions of the groups at risk of complications from influenza are not vaccinated. WHO strongly emphasizes the importance of raising the public consciousness of influenza and its complications as well as of the beneficial effects of influenza vaccination.