Provided there is a good antigenic match between the vaccine strains and those circulating in the general population (which is achieved in most years, due to the WHO surveillance system), inactivated influenza vaccines prevent laboratory-confirmed illness in approximately 70%–90% of healthy adults. For the elderly not living in nursing homes, vaccination may reduce the number of hospitalizations by 25%–40% and overall mortality by 40%–75%. Among nursing home residents, influenza vaccination can reduce hospitalizations (all causes) by about 50%, the risk of pneumonia by about 60% and the risk of death (all causes) by up to 70%.

Most inactivated influenza vaccines are given via the intramuscular route in the deltoid muscle, except in infants where the recommended site is the antero-lateral aspect of the thigh. A single dose of inactivated vaccine annually is appropriate, except for previously unvaccinated preschool children with pre-existing medical conditions who should receive two doses at least one month apart. The live attenuated vaccine is delivered intra-nasally.

Unadjuvanted whole virus, split virus and subunit influenza vaccines show comparable efficacy, but differ in terms of reactogenicity. Thus in 15%–20% of vaccine recipients, the whole-virus vaccines cause local reactions lasting for one to two days. Such reactions appear to be more common in young children than in adults. Transient systemic reactions such as fever, malaise and myalgias may occur in a minority of vaccine recipients within 6–12 hours of the vaccination. Split vaccines and subunit vaccines show reduced systemic reactogenicity both in children and in adults as compared to whole virus preparations. Consequently, subunit and split virus vaccines are more widely used than whole virus vaccines.

For more details see also the Seasonal Influenza section of this website.