The effectiveness of influenza vaccine in elderly and debilitated patients has been demonstrated, but there have been no studies about the effectiveness of the vaccine in health care workers. This randomized trial conducted by Wilde and colleagues was designed to address this question. Physicians, nurses and respiratory therapists were recruited to participate. All were healthy and younger than 50 years of age. A history of influenza vaccine allergy, pregnancy or chronic medical conditions that would place the person at risk for influenza complications were exclusion criteria. Participants were allowed to enroll for one, two or three seasons. Those included in the study were randomized to either the influenza group or the control group.

A baseline blood test was followed in one month with a test of serologic response. A third blood test was performed one month after the influenza season ended to test for infection with influenza A or B. All participants were interviewed three days after vaccination for adverse reactions. Weekly interviews were also conducted for the duration of the study, specifically to determine if there had been any illness or work absence during the previous week.

A positive vaccine response was defined as a fourfold increase in hemagglutination-inhibiting antibodies. Respiratory illness was defined as at least two symptoms for at least two days. The study included 264 participants; of these, 49 participated for two years and 24 for three years. No work absences were attributed to adverse effects of the vaccine.

A positive vaccine response for influenza A occurred in 41 to 78 percent of subjects and for influenza B in 33 to 52 percent. Those who received the vaccine had an influenza A infection rate of 1.1 per 100 person-winters, compared with a rate of 8.9 in those who did not receive the vaccine. Rates for influenza B were 0.6 in those who received the vaccine and 5.0 in those who did not. The effectiveness of the vaccine was 88 percent for influenza A and 89 percent for influenza B. Immunization in a previous year did not confer any protection in subsequent years. The subjects who had not received vaccine were significantly more likely to have a febrile respiratory illness than those who were vaccinated (58 versus 14 percent, respectively). Similarly, the unvaccinated group had a longer duration of febrile respiratory illness and more and longer work absences.

The authors conclude that the influenza vaccine is effective in health care workers and seems to be associated with a decrease in the number of work days missed as well as the occurrence of respiratory illness.