Impact of Seasonal Influenza
Until recently, much of the information on the impact of the annual outbreaks was related to mortality. This is limited typically to older individuals and those with underlying conditions, usually cardiopulmonary. We now realise that we have been underestimating these numbers. Now, using new epidemiologic methods, the true magnitude of influenza toll on the population of a developed country has been well quantified. Each year in the US, it is estimated that on average 36,000 people die of influenza¹ and 200,000 are hospitalised.² In addition, there are 200,000 hospitalisations. A large proportion of those who experience such severe outcomes are older individuals or those with underlying cardiopulmonary illnesses. With the American healthcare system, it is difficult to precisely estimate similarly the impact on physicians’ practices and on school and industrial absenteeism, but it is certainly high during an active influenza season. This occurs over a short period of time so that disruptions are possible. This has happened in a number of countries during major outbreaks, typically caused by A (H3N2) viruses that have drifted or changed antigenically.

Vaccines – New Developments
Inactivated influenza vaccines have been available for 60 years. They are still made in fertile eggs. It has been recognised over the years that vaccine was 70–90% efficacious in healthy adults, as long as the circulating virus had not changed from the one included in the vaccine.³ However, it has also been confirmed that in the frail elderly vaccines are not as effective as in younger individuals.^4,5 Even in this situation, there is some degree of protection against the more severe symptoms, such as pneumonia and death.⁶ Adjuvants may be necessary to produce good antibody response to A (H5N1) avian viruses,⁷ but they may also be useful in protecting the frail elderly who remain susceptible after receiving regular vaccinations.

Antivirals
While vaccines have been used increasingly to prevent influenza in vulnerable and more general populations, antivirals, the other major intervention known to be of value, have been almost completely neglected in most, although not all, countries. The older class of antivirals, M2 inhibitors or adamantanes, were useful in both prophylaxis and treatment of the type A influenza infections. Amantadine has been used for prophylaxis in special situations, and sometimes in treatment. The problem here is that, while illness duration was shortened, there was not convincing evidence that complications were prevented. The only situation in which the drug was used with some regularity was in control of outbreaks of influenza in nursing homes, particularly when a vaccine had not prevented outbreaks of A (H3N2) viruses.^8,9 Even after the development of neuraminidase inhibitors (NAIs), adamantanes continued to be used, principally because of the low cost. Currently, with the sharp increase in resistance to this class of drug throughout the world, there appears to be little rationale for its continued use, possibly only if there is a change in resistance patterns.¹⁰