Both trivalent (or quadrivalent) inactivated and live attenuated influenza vaccines are available. There are three types inactivated vaccines : whole-virus vaccines, split-virus vaccines, and subunit vaccines. In most countries, whole-virus vaccines have been replaced by less reactogenic split virus and subunit vaccines. Inactivated vaccines are the only influenza vaccines licensed for vaccination of children <2 years of age, for persons aged ≥50 years, and for pregnant women. Healthy non-pregnant individuals aged 249 years may receive live attenuated influenza vaccines. There are inactivated influenza vaccines licensed for ages 6 months and higher, although specific age groups vary by product. Inactivated influenza vaccine may be used to prevent influenza among individuals aged ≥ 6 months, including healthy persons and those with high risk medical conditions.
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. Therefore, the composition of influenza vaccines is reviewed annually, separately for the two hemispheres. During years when the composition of the seasonal influenza vaccines differs antigenically between the northern and southern hemispheres, vaccines designed for one hemisphere may offer only partial protection against the prevailing strains of the other.
Available seasonal influenza vaccines do not protect against so-called avian influenza.
Travellers with conditions that place them at high risk for complications of influenza should be vaccinated every year. In years when the vaccine strains differ between the northern and southern hemispheres, high-risk individuals travelling from one hemisphere to the other shortly before or during the other hemisphere’s influenza season should obtain the vaccination recommended 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.
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.
Contraindications and precautions
Vaccination is contraindicated in case of severe egg allergy, including anaphylactic reaction.