A randomized, double-blind, placebo-controlled trial among 4,561 healthy working adults aged 18–64 years assessed multiple endpoints (i.e., targeted outcome measures), including reductions in self-reported respiratory tract illness without laboratory confirmation, absenteeism, health care visits, use of antibiotics, and use of over-the-counter medications for illness symptoms during peak and total influenza outbreak periods (Nichol et al., 1999). The study was conducted during the 1997-1998 influenza season, when the influenza vaccine and circulating A (H3N2) viruses were poorly matched. Vaccination was associated with reductions in severe febrile illnesses of 19%, and febrile upper respiratory tract illnesses of 24%.
Vaccination was also associated with fewer days of illness, fewer days of work lost, fewer days with health care provider visits, and reduced use of prescription antibiotics and over-the-counter medications. Among a subset of 3,637 healthy adults aged 18–49 years, LAIV recipients (n = 2,411) had 26% fewer febrile upper-respiratory illness episodes; 27% fewer lost work days as a result of febrile upper respiratory illness; and 18%–37% fewer days of health care provider visits caused by febrile illness, compared with placebo recipients (n = 1,226). Days of antibiotic use were reduced by 41%–45% in this age subset.
A randomized, double-blind, placebo-controlled influenza virus challenge study among 92 healthy adults (LAIV, n = 29; placebo, n = 31; inactivated influenza vaccine, n = 32) aged 18–41 years assessed the efficacy of both LAIV and inactivated vaccine (Treanor et al., 1999). The overall efficacy of LAIV and inactivated influenza vaccine in preventing laboratory-documented influenza from all three influenza strains combined was 85% and 71%, respectively. These results were obtained after study participants, all of whom were susceptible to recently circulating influenza viruses before vaccination, were experimentally exposed to viruses. The difference in efficacy between the two vaccines was not statistically significant.