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Single Dose of Influenza A(H1N1) Vaccine Triggers 'Robust' Immune Response BOSTON -- September 11, 2009 -- Preliminary results from an influenza A(H1N1) vaccination trial ongoing in Australia has demonstrated that a single dose of the vaccine triggered a protective immune response in more than 90% of the adults treated. The findings are available in the September 10 issue of the New England Journal of Medicine. In the study, investigators randomised 240 healthy volunteers aged 18 to 64 years -- equally divided into 2 age groups, <50 years and >= 50 years -- to receive an initial dose of the vaccine followed by a second vaccination 21 days later. The first group received a 15-mcg dose, which is the standard dose used for a single strain in the trivalent seasonal influenza vaccine, and the second group received a 30-mcg dose. Data showed that 3 weeks after the first of the 2 scheduled vaccinations, 96.7% of participants who received the lower dose and 93.3% of those treated with the higher dose achieved the desired immune response. The study authors write that the “robust immune response” to the single dose was “unanticipated,” adding: “Much of the current global pandemic planning is predicated on previous experience that two doses of vaccine are required to elicit a protective immune response in populations that are immunologically naive to a new influenza strain.” Earlier this week, the University of Leicester and Leicester Hospitals also reported a strong immune response after just 1 dose, based on findings from their H1N1 vaccination pilot study. The National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, also is conducting clinical trials of 2009 H1N1 vaccines. The trials are testing a 15-mcg dose and 30-mcg dose and evaluating the immune response to 1 and 2 doses of these vaccines in more than 2,800 people. Today, the NIAID reported that preliminary analyses of their trial data align with the recently announced findings, and those to be announced imminently by other companies, in that both vaccines studied induced what is likely to be a protective immune response in most adults following a single dose in the same amount (15 mcg) used in seasonal flu vaccines. Specifically, in blood samples obtained 8 to 10 days after vaccination. Additional data from the NIAID trials are forthcoming; however, these strong early data are consonant with other reports that a single 15-mcg dose of unadjuvanted 2009 H1N1 influenza vaccine is well tolerated and induces a robust immune response in healthy adults between the ages of 18 and 64. For adults aged 65 and older, the immune response to 2009 H1N1 influenza vaccine is somewhat less robust, as is the case with seasonal influenza vaccines. In the Australian study, reported in NEJM, the most commonly reported adverse events in the study were injection-site tenderness (36.7%) and pain (94.6%). Systemic adverse events considered to be related to the vaccine were reported by 30.4% of participants and included headache, malaise, and myalgia. The majority of these events were mild to moderate in intensity. Three participants reported influenza-like illness. Of these, only 1 tested positive for 2009 H1N1. The full safety profile of the H1N1 vaccine is still unknown, so continued population-based surveillance is needed for all H1N1 vaccines, especially to assess for rates of Guillain-Barre syndrome and other rare adverse events. That said, these findings are likely to assist with pandemic planning, “especially in light of widespread concern about vaccine availability because of low manufacturing yields,” write the NEJM study authors. “The high level of immune protection afforded by a single 15-mcg dose should improve the coverage and logistics of mass H1N1 vaccination programs.” SOURCE: The New England Journal of Medicine and the National Institute of Allergy and Infectious Diseases E-mail this page to a friend or colleague! To print, use this version