Mass influenza vaccination in Ontario: is it worthwhile?

The clinical and societal importance of influenza is usually taken for granted. Nevertheless, looking at the epidemiology of the disease and comparing it with other conditions, our knowledge of influenza appears only limited and indirect. The incidence and distribution of laboratory-confirmed cases of influenza A and B are only available from small samples, whereas information on the population distribution of influenza is based on a syndromic definition of cases usually known as influenza-like illness (ILI). The actual proportion of influenza A and B cases among ILI cases is not well known, but the few available studies indicate a modest proportion of probably less than 10%, regardless of age group.(f.3-6) The same considerations apply to outcomes such as mortality, hospital admissions and other complications that are conventionally measured in terms of "excess" incidence when epidemic and nonepidemic periods are compared. Thus, our knowledge comprises estimates of the generic impact of ILI during epidemic periods, but we do not know what share of the problem is directly caused by influenza A and B viruses. This greatly limits the value of studies that estimate comparatively the population impact of the vaccine. Questions about the safety of vaccines can only partly be answered by clinical trials, because possible long-term effects and rare adverse effects can only be assessed by means of observational studies carried out for longer periods of time (so-called "postmarketing" studies). In the case of influenza, both clinical trials and other types of studies show that safety does not appear to be a particular problem with any of the available vaccines. Millions of doses of influenza vaccine are administered each year, and the current vaccines appear to be well tolerated. Serious or life-threatening adverse effects are rare.(f.9) However, the decision to extend the influenza vaccination program to a whole population may mean that safety issues will become of crucial importance. A study of young, healthy adults(f.7) in which the acceptability of influenza vaccination was assessed, by calculating individual preferences using a cost-utility study design, revealed that vaccines have such low population effectiveness and numerous local trivial adverse effects that the trade-off was unfavourable.(f.10) This finding may have been partly influenced by the timing of this exercise, which was carried out in an interepidemic period when memories of the latest ILI peak