For avian zoonotic influenza, see Chapter 5.
Influenza viruses constantly evolve, with rapid changes in their characteristics. To be effective, influenza vaccines need to stimulate immunity that protects against the principal strains of virus circulating at the time. Every year, the composition of influenza vaccines is modified separately for the northern and southern hemispheres. Since the antigenic changes in circulating influenza viruses can occur abruptly and at different times of the year, there may be significant differences between prevailing influenza strains in the northern and southern hemispheres. The internationally available vaccines contain three inactivated viral strains, the composition of which is reviewed every 6 months to ensure protection against the strains prevailing in each influenza season. The composition of vaccines is adjusted for the hemisphere in which the vaccine will be used. Thus, a vaccine obtainable in one hemisphere may offer only partial protection against influenza infection in the other hemisphere, although in some years the viruses in the northern and southern vaccines may be antigenically identical. Available seasonal influenza vaccines do not protect against avian influenza.
Travellers with conditions that place them at high risk for complications of influenza should be vaccinated every year. In years in which the northern and southern hemisphere influenza vaccine strains differ, high-risk individuals travelling from one hemisphere to the other shortly before or during the other hemisphere’s influenza season should obtain vaccination for the opposite hemisphere two weeks before travel. Where this is not possible, the traveller should arrange vaccination as soon as possible after arriving at the travel destination.
Trivalent inactivated influenza vaccines are injected into the deltoid muscle (vaccinees aged >1 year) or the anterolateral aspect of the thigh (vaccinees aged 6–12 months). These vaccines should not be given to children under the age of 6 months; those aged 6–36 months should receive half the adult dose. Previously unvaccinated children aged less than 9 years should receive two injections, administered at least 1 month apart. A single dose of the vaccine is appropriate for schoolchildren aged 9 years and over and for healthy adults. Mild local reactions such as pain or swelling at the injection site are common; systemic reactions such as fever are less common.
Precautions and contraindications
Vaccination is contraindicated in case of severe egg allergy, including anaphylactic reaction.