Effectiveness of half-dose versus full-dose influenza vaccine in health care workers

A prospective, randomized, double-blind study comparing the effectiveness of half-dose versus full-dose trivalent inactivated influenza vaccine (TIV) in health care workers was conducted.

The study was conducted at a 760-bed tertiary care community hospital. Study participants (n = 444) were hospital staff or employees over 18 years of age who were enrolled during a two-week period in October and November 2004. Participants were assigned to either full-dose (n = 222) or half-dose (n = 222) TIV. The participants completed a standardized questionnaire monthly from November 2004 through March 2005. The primary outcome was the incidence of influenza between the two groups; the secondary outcomes were self-reported days of work missed as a result of influenza and the proportion of participants in each group self-reporting signs and symptoms of influenza. Laboratory confirmation of influenza diagnosis was sought in participants reporting a clinical diagnosis by their physicians. Differences in the groups were analyzed with Fisher's exact test, with a < 0.05 considered significant.

There was no difference between the full-dose and half-dose groups in the diagnosis of influenza (4% versus 7%, p = 0.198). Laboratory confirmation of an influenza diagnosis occurred in one participant who received full-dose TIV.

Half-dose TIV demonstrated a similar effectiveness to full-dose TIV, measured by the number of diagnoses of influenza and symptom survey of participating health care workers. Administration of half-dose TIV is an acceptable strategy for maximizing the number of vaccinations in health care workers, but additional studies are needed to confirm the comparable immunogenicity and effectiveness between the two doses.