Abstract
In the United States, two types of vaccines are recommended for the prevention of influenza: an intranasal live attenuated influenza vaccine (LAIV) for eligible individuals aged 2-49 years and unadjuvanted injectable trivalent inactivated vaccines (TIV) for eligible individuals aged ≥ 6 months. Several recent studies have compared the efficacy of the 2 vaccines in children and adults. In children 6 months to 18 years of age, each of the four comparative studies of LAIV and TIV demonstrated that LAIV was more protective. In individuals 17-49 years of age, most comparative studies have demonstrated that LAIV and TIV were similarly efficacious or that TIV was more efficacious. However, LAIV was shown to be more protective than TIV in new military recruits of all ages, and placebo-controlled studies in adults in 1997-1998 suggested that LAIV was more protective against the mismatched A/H3N2 strain. The relative efficacy of LAIV and TIV among young adults may vary depending on the specific population and the antigenic match between the vaccines and circulating strains. In adults 60 years of age and older, limited data suggest that the two vaccines are similarly effective. In children and adults, studies also suggest that the relative efficacy of LAIV versus TIV may increase when measured against more severe illness. Additional research comparing LAIV and TIV is needed in adults and would also be valuable in older children and adolescents. Studies should examine the role of pre-existing immunity as well as vaccine impact on influenza illness of varying severity.