Effectiveness and Cost-Benefit of Influenza Vaccination of Healthy Working Adults

abstract & commentary

Synopsis: The study found that influenza vaccination in healthy working adults younger than 65 years of age can reduce the rates of influenza-like illness, lost work days, and physician visits during years when the vaccine and circulating viruses were similar, but vaccination was not cost effective.

Source: Bridges CB, et al. JAMA 2000;284:1655-1663.

The cost effectiveness of influenza vaccination in reducing influenza illness, hospitalization, and death is well established in persons aged 65 years or older.1-3 However, the benefit of annual influenza vaccination in healthy young adults is less clear. The purpose of this study was to determine the clinical efficacy and cost benefits of influenza vaccination in healthy young adults.

Bridges and colleagues conducted a double-blind, randomized, placebo-controlled trial of influenza vaccine among healthy working adults during the 1997-1998 and 1998-1999 influenza seasons. The primary outcome measures were clinically defined respiratory illness based on ICD-9 codes, associated physician visits, lost workdays during the influenza season, and the cost benefits.

Patients between the ages of 18 and 64 years were randomly assigned to receive either trivalent inactivated influenza vaccine or placebo. Study participants were sent follow-up surveys via e-mail twice monthly and information was obtained on respiratory illness and related physician visits, medications, hospitalization, and lost work days. During November through April of each study year, throat swabs, nasopharyngeal swabs, or both were collected from participants who notified the study nurse of an influenza-like illness (ILI) and who had been ill for four days or less. Specimens were sent for viral culture. Influenza isolates were sent to the Centers for Disease Control and Prevention (CDC) and antigenically characterized. A total of 1184 participants were randomized in the 1997-1998 and 1191 in the 1998-1999 seasons. Complete follow-up was available for 95% (1130/1184) and 99% (1178/1191) of participants in each period, with 23% in each year having serologic testing. In 1997-1998, when the vaccine virus was different from the predominant circulating viruses, vaccine efficacy against serologically confirmed influenza illness was 50% (P = 0.33). The 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 = 0.001), 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.

Bridges et al thus conclude that in years in which there is a good match between vaccine and the circulating virus, vaccination against influenza can have substantial health benefits by reducing rates of ILI, physician visits, and work absenteeism. However, it does not provide societal economic benefits for healthy young adults.

Comment by David Ost, MD

The strategy for influenza vaccination in the United States has emphasized prevention of influenza in persons most likely to experience complications: those aged 65 years or older and younger individuals with cardiac, pulmonary, and other chronic conditions. Studies have shown that vaccination is cost effective in the elderly population.1 Although vaccination of healthy adults is known to be effective in preventing clinical influenza, cost-effectiveness has not been demonstrated conclusively in this population.

In a double-blind, placebo-controlled trial of vaccination against influenza done by Nichol and colleagues, vaccination resulted in substantial health-related and economic benefits with an estimated cost savings of $46.85 per person vaccinated for healthy working adults.4 This current study differs from the Nichol et al study in several important aspects. It was conducted during two consecutive influenza seasons, it defined the influenza period based on virologic surveillance at the study site, and it used diagnostic testing to confirm influenza infection rates in a subset of participants unlike the previous study where influenza was defined by clinical features only. This study illustrates that the clinical efficacy and the cost benefit of vaccination depends on the match between vaccine virus and the circulating virus, thus the need to take a multiyear approach in evaluating influenza vaccine programs. The findings of Bridges et al regarding cost effectiveness is especially important this year, because there is limited availability of influenza vaccine in the United States. It provides important clinical and cost-benefit data to help in the development of strategies for preventing influenza in healthy working adults. In conclusion, influenza vaccine is effective in preventing serologically proven influenza infection in young, healthy adults and may reduce cumulative days of illness and absence. However, programs for vaccination in the workplace may not provide economic benefit in all years.

References

1. Gross PA, Hermogenes AW. Ann Intern Med 1995;123: 518-527.

2. Govaert TM, Thijs CT. JAMA 1994;272:1661-1665.

3. Demicheli V, Jefferson J. Vaccine 2000;18:957-1030.

4. Nichol KL, et al. N Engl J Med 1995;333:889-893.