ATLANTA-- The wisdom of vaccinating all healthy adults and children against influenza has been debated--particularly this flu season, when the vaccine has been in short supply. Two new studies from the Centers for Disease Control and Prevention (CDC) shed light on this issue. One indicates that vaccinating children in day care may reduce household influenza transmission.[1] The other suggests that although vaccinating healthy adults may reduce flu-related morbidity, it does not provide an economic benefit to society.[2]

Children in day care are considered to be at high risk for influenza infections and influenza-like illnesses. Furthermore, there is evidence that children in day care are more likely than school-age children or adults to transmit influenza to their families and other household contacts. Could vaccination of children in day care reduce influenza morbidity among household contacts?

This question was studied by Eugene S. Hurwitz, MD, and colleagues from the CDC. At the start of the 1996-1997 influenza season, they randomized 127 children (age 24 to 60 months) to receive either the influenza or hepatitis A vaccine. Included in the study were all children attending day care at one of 10 centers for offspring of US Navy personnel in San Diego; 60 of them were given the influenza vaccine, and 67 received the hepatitis A vaccine. These children and their household contacts were followed until the end of the flu season.

Dr. Hurwitz and colleagues found that the incidence of febrile respiratory illness was 42% lower among the 162 unvaccinated household contacts of the children given the influenza vaccine than among the 166 similar contacts of the children who received the hepatitis A vaccine. Among school-aged household contacts (age 5 to 17 years), there was an 80% reduction in febrile respiratory illnesses associated with influenza vaccination. There was also a significant decrease in the number of school days missed, reported earaches, physician visits, physician-prescribed antibiotics, and number of workdays missed by adults to take care of sick children.

"These findings show that the benefits of influenza vaccination for children attending day care centers extend to their family members, particularly children 5 to 17 years of age," said Dr. Hurwitz, a medical epidemiologist in the Division of Viral and Rickettsial Diseases at the CDC's National Center for Infectious Diseases. In fact, Dr. Hurwitz noted, these benefits may be even greater in the civilian population than among the military families in this study, because a large proportion of the adults in military families receive the influenza vaccine each year. Future studies should be conducted in the civilian population to further evaluate the effects of vaccinating children in day care against influenza, he and his colleagues suggested.

In another randomized, prospective, controlled trial, Carolyn Buxton Bridges, MD, and colleagues evaluated the effectiveness and cost-benefit of vaccination in preventing influenza-like illness and reducing related societal costs during two influenza seasons. These researchers are also from the Division of Viral and Rickettsial Diseases at the National Center for Infectious Diseases.

The study subjects included were healthy working adults (age 18 to 64 years; 1,184 during the first year, 1,191 during the second) who were randomly assigned to receive either the influenza vaccine or sterile injection as placebo. All of the participants were employees of the Ford Motor Company in Dearborn, Michigan. As in the Hurwitz study, participants underwent vaccination at the start of the flu season and were followed until the season ended.

An important difference between the two studies is that the investigation by Dr. Bridges and colleagues lasted for two years. Because both influenza rates and vaccine efficacy differ from year to year, these researchers felt that a multiyear study would allow them to perform a proper economic and efficacy analysis.

At the start of the 1997-1998 flu season, 595 participants received the vaccine and 589, placebo. That year, there was a poor antigenic match between the vaccine strains and the predominant circulating viruses (this occurs about once every 10 years). As a result, vaccine efficacy against serologically confirmed influenza illness was 50%. In this season, vaccination did not reduce the incidence of influenza-like illness or the number of physician visits or lost workdays. Furthermore, the net cost to society was $65.59 per person vaccinated.

At the start of the next flu season, 587 participants received the vaccine and 604, placebo. That year, the vaccine virus and predominant circulating viruses were well matched and vaccine efficacy against confirmed illness was 86%. Not surprisingly, therefore, vaccination reduced the incidence of influenza-like illness by 35%, the number of physician visits by 42%, and the number of lost workdays by 32%; however, the net societal cost of vaccination was still $11.17 per person.

A limitation of the study by Dr. Bridges and colleagues is that the participants were not representative of the general US population. For example, more than 75% of the participants were male, and more than 90% of them were nonsmokers. Furthermore, the fact that more than three quarters of the participants had a household income above $70,000 may have skewed the economic analysis. However, the researchers noted that the use of lower labor cost estimates would have only diminished the likelihood that vaccination would produce a cost savings.

These two studies leave a number of questions unanswered. For example, would the results obtained by Hurwitz et al have been replicated if the study had been repeated the following year? And what is the cost of vaccinating all children in day care?

The results of the study by Bridges et al contradict those of an earlier study by Nichol et al,[3] which found that influenza vaccination resulted in a net savings of $46.85 per healthy adult vaccinated. The discordant results may reflect differences in study design; for example, Bridges et al determined illness rates and infection virologically, whereas Nichol et al collected these data via telephone interviews. Thus, additional work is needed to determine the true costs--or savings--associated with widespread influenza vaccination of healthy adults.

Finally, are the benefits of widespread vaccination worth the costs? "There is no doubt that influenza vaccination has positive health benefits in preventing influenza, and these benefits apply to healthy adults," wrote Arnold S. Monto, MD, in an accompanying editorial.[4] He cautioned, however, that "programs for vaccination in the workplace should not be carried out solely in the belief that they will provide economic benefit in all years."

--Debra Hughes

References
1. Hurwitz ES, Haber M, Chang A, et al. Effectiveness of influenza vaccination of day care children in reducing influenza-related morbidity among household contacts. JAMA. 2000;284:1677-1682.

2. Bridges CB, Thompson WW, Meltzer MI, et al. Effectiveness and cost-benefit of influenza vaccination of healthy working adults. A randomized controlled trial. JAMA. 2000;284:1655-1663.

3. Nichol KL, Lind A, Margolis KL, et al. The effectiveness of vaccination against influenza in healthy, working adults. N Engl J Med. 1995;333:889-893.

4. Monto AS. Preventing influenza in healthy adults. The evolving story. JAMA. 2000;284:1699-1701.