Effectiveness and cost-benefit of influenza vaccination of healthy working adults - A randomized controlled trial

Context Although the cost-effectiveness and cost-benefit of influenza vaccination are well established for persons aged 65 years or older, the benefits for healthy adults younger than 65 years are less clear. Objective To evaluate the effectiveness and cost-benefit of influenza vaccine in preventing influenzalike illness (ILI) and reducing societal costs of ILI among healthy working adults. Design Double-blind, randomized, placebo-controlled trial conducted during 2 influenza seasons. Setting and Participants Healthy adults aged 18 to 64 years and employed full-time by a US manufacturing company (for 1997-1998 season, n=1184; for 1998-1999 season, n=1191). Interventions For each season, participants were randomly assigned to receive either trivalent inactivated influenza vaccine (n =595 in 1997-1998 and n = 587 in 1998-1999) or sterile saline injection (placebo; n = 589 in 1997-1998 and n = 604 in 1998-1999). Participants in 1997-1998 were rerandomized if they participated in 1998-1999. Main Outcome Measures Influenzalike illnesses and associated physician visits and work absenteeism reported in biweekly questionnaires by all participants, and serologically confirmed influenza illness among 23% of participants in each year (n = 275 in 1997-1998; n = 278 in 1998-1999); societal cost of ILI per vaccinated vs unvaccinated person. Results For 1997-1998 and 1998-1999, respectively, 95% (1130/1184) and 99% (1178/1191) of participants had complete follow-up, and 23% in each year had serologic testing. In 1997-1998, when the vaccine virus differed from the predominant circulating viruses, vaccine efficacy against serologically confirmed influenza illness was 50% (P=.33). In this season, vaccination did not reduce ILI, physician visits, or lost workdays; the net societal cost was $65.59 per person compared with no vaccination. In 1998-1999, the vaccine and predominant circulating viruses were well matched. Vaccine efficacy was 86% (P =.001), and vaccination reduced ILI, physician visits, and lost workdays by 34%, 42%, and 32%, respectively. However, vaccination resulted in a net societal cost of $11.17 per person compared with no vaccination. Conclusion Influenza vaccination of healthy working adults younger than 65 years can reduce the rates of ILI, lost workdays, and physician visits during years when the vaccine and circulating viruses are similar, but vaccination may not provide overall economic benefits in most years.