This review indicates that effective influenza prevention interventions are available for healthy adults and children. Protection with influenza vaccine varied from moderate to high, with RRRs from 0% to 93%. We also found that the neuraminidase inhibitors oseltamivir and zanamivir were effective in people over 12 years and over 5 years of age respectively. These drugs have the advantage of having fewer side effects than rimantadine and amantadine and are effective against influenza A and B.49
In the studies included in this review, no serious adverse events were reported as a result of either the influenza vaccine or antiviral therapy.
The apparent variation in efficacy of the influenza vaccine over time and between trials is likely due to a number of factors, including vaccine immunogenicity and the degree of match between the vaccine strain chosen before the influenza season and the circulating virus strains. Influenza A virus changes over time as it propagates in humans, through an accumulation of point mutations and by genetic reassortment between viral strains.50
The World Health Organization recommends the composition of inactivated vaccines each year based on the occurrence of strains causing outbreaks that are reported from 110 surveillance centres in 83 countries and the availability of strains for production of vaccine. The “match” between predicted and circulating strains was 88% in a 10-year period.51
Previous exposure to vaccine or natural infection may increase vaccine efficacy.
Influenza viruses cause illness in up to 20% of the general population each winter in nonpandemic years. Severe illness requiring hospital admission is most likely to occur in people with pre-existing lung or cardiac disease or chronic medical conditions and in people over 65 years of age. Immunization programs for these target groups, known to decrease rates of hospital admission and death,3
are in place across Canada, although uptake is incomplete.52
Influenza also causes a significant health burden in the general population in terms of hospital admissions, outpatient visits, sick leave and antimicrobial use.53
In particular, children under 2 years of age may have hospital admission rates of up to 112 per 100 000 population.6,54
In an influenza pandemic, attack rates could exceed 30% of the general population.55
Recommendations for influenza vaccination have become more inclusive in recent years. The National Advisory Committee on Immunization suggests that “any individual who wishes to protect him/herself from influenza should be encouraged to receive the vaccine.” 56
The American Academy of Pediatrics57
and the Advisory Committee on Immunization Practices of the US Centers for Disease Control and Prevention58
recently revised its recommendations to include vaccination of children aged 6–23 months, and contacts of infants aged 0–23 months. Whereas Canada recommends immunization of people over 65 years of age, the US Centers for Disease Control and Prevention recommends the inclusion of people over 50 years, because of the increased incidence of high-risk conditions in that age group.53
To date, Ontario is the only jurisdiction in Canada to introduce a universal influenza immunization program.59,60
The goal of our review was to determine the efficacy of the influenza vaccine and neuraminidase inhibitors, not to determine the efficacy of a universal immunization program. With such a program, considerations that must be weighed against the potential benefits (preventing illness and death in high-risk groups, and decreasing economic loss associated with absenteeism at work, visits to health care providers and antibiotic use) include the economic costs of vaccine and program delivery, vaccine procurement for large populations, the need to immunize large populations in a short period each year and public acceptability. There is some evidence that universal influenza immunization of school children is associated with a reduction in excess winter deaths in the general population.61
Now in its fourth year, the universal influenza program in Ontario may help to further clarify the efficacy of such a program.
Although there is evidence to support the efficacy of neuraminidase inhibitor prophylaxis, the treatment is expensive (at least $50 per day) and was used within 36–48 hours of diagnosis of the index case in the studies reviewed here. Appropriate use during the winter respiratory illness season, when many viruses may be circulating, would require access to rapid microbiologic diagnosis to evaluate suspected exposures, or an active viral surveillance program in the community to determine whether influenza is epidemiologically the most likely cause of respiratory illness in the index case. Health Canada's Fluwatch Program (www.hc-sc.gc.ca/pphb-dgspsp/fluwatch/index.html
) provides biweekly summaries of disease activity across Canada, and local laboratories may also provide timely information.
Limitations of our systematic review include the restriction of reviewed publications to French or English and the inability to provide an overall estimate of vaccine efficacy, such as might be obtained through a meta-analysis. However, we concluded that the vaccines and outcome measures of influenza were sufficiently different across trials to prevent pooling of individual trial results.
Evaluation of new influenza vaccines is necessary. A live-attenuated, nasally administered vaccine is now licensed for use in the United States,62
and a nasally administered inactivated product is being developed.63
Furthermore, determining the efficacy of universal vaccination and treatment programs will require ongoing scrutiny.
β See related article page 1169