Benefits of Increasing the Dose of Influenza Vaccine in Residents of Long-Term Care Facilities: A Randomized Placebo-Controlled Trial

Increased vaccine doses and mid-season boosting may increase the proportion of residents with protective immunity from influenza in long-term care facilities. In a multi-center study (19971998), 815 residents from 14 long-term care facilities were assigned at random to receive 15 or 30 mu g of inactivated influenza vaccine, followed by a 15 mu g booster vaccine or a placebo vaccine at Day 84. Seroresponses were re-analyzed by hemagglutination-inhibition (>= 4-fold titer increases, protective titer >= 40, geometric mean titers. forty percent of the participants had pre-vaccination titers >= 40. At Day 25 after vaccination, this increased to 66.3% after a 15 mu g dose versus 73.3% after a dose of 30 mu g (P=0.049). Participants receiving a 30 mu g dose followed by a 15 mu g booster showed more >= 4-fold titer increases at Day 109 (43.6% vs. 35.4%, P=0.003) and protective titers >= 40 (74.2% vs. 64.6%, P=0.041), compared to those receiving only a 15 mu g dose. Differences were most apparent in participants with low pre-vaccination titers. Booster vaccination after an initial 15 mu g dose of the vaccine did not increase the protective rate (61.9% vs. 63.9% after placebo). The number of participants needed to vaccinate to protect one additional resident by a dose of 15 mu g was 4, by a dose of 30 mu g 3, and 15 when using a 30 mu g dose instead of 15 mu g. Doubling the dose of influenza vaccine increased protection-related responses among residents of long-term care facilities, especially in those with low pre-vaccination titers. J. Med. Virol. 81:908-914, 2009. (C) 2009 Wiley-Liss, Inc.