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On September 15, 2009, four influenza vaccine manufacturers received approval from the Food and Drug Administration for use of influenza A (H1N1) 2009 monovalent influenza vaccines in the prevention of influenza caused by the 2009 pandemic influenza A (H1N1) virus.* Both live, attenuated and inactivated influenza A (H1N1) 2009 monovalent vaccine formulations are available; each contains the strain A/California/7/2009(H1N1)pdm. None of the approved influenza A 2009 (H1N1) monovalent vaccines or seasonal influenza vaccines contains adjuvants.1-5 CDC's Advisory Committee on Immunization Practices has made recommendations previously for which persons should be the initial targets for immunization with influenza A (H1N1) 2009 monovalent vaccines and has issued guidelines on decisions for expansion of vaccination efforts to other population groups.6 Children aged 6 months–9 years receiving influenza A (H1N1) 2009 monovalent vaccines should receive 2 doses, with doses separated by approximately 4 weeks; persons aged ≥10 years should receive 1 dose.1-4
The approved age groups for use of inactivated influenza A (H1N1) monovalent influenza vaccines differ by manufacturer. Three manufacturers that produce inactivated vaccines approved for prevention of seasonal influenza6 also produce formulations of influenza A (H1N1) 2009 monovalent influenza vaccines. Vaccine produced by CSL Limited is approved for use in persons aged ≥18 years,1 vaccine produced by Novartis Vaccines and Diagnostics Limited is approved for persons aged ≥4 years,2 and vaccine produced by Sanofi Pasteur, Inc. is approved for persons aged ≥6 months.3 A live attenuated influenza vaccine (LAIV) manufactured by MedImmune LLC is approved for persons aged 2-49 years.1 The 2009 (H1N1) monovalent LAIV has the same age range for use as the seasonal LAIV and should not be used to vaccinate children aged <2 years, adults aged >49 years, pregnant women, persons with underlying medical conditions that confer a higher risk for influenza complications, or children aged <5 years old with one or more episodes of wheezing in the past year.5
Influenza A (H1N1) 2009 monovalent vaccine approvals were made on the basis of standards developed for vaccine strain changes for seasonal influenza vaccines, adherence to manufacturing processes, product quality testing, and lot release procedures developed for seasonal vaccines. The age groups, precautions, and contraindications approved for the influenza A (H1N1) 2009 monovalent vaccine are identical to those approved for seasonal vaccines. All influenza vaccines available in the United States for the 2009-10 influenza season are produced using embryonated hen's eggs and contain residual egg protein.
Preliminary data indicate that the immunogenicity and safety of these vaccines are similar to those of seasonal influenza vaccines. An immunogenicity study of an inactivated influenza A (H1N1) monovalent vaccine manufactured by CSL Limited (Parkville, Victoria, Australia) demonstrated that by day 21 after vaccination, antibody titers of 1:40 or more (hemagglutination-inhibition assay) were observed in 116 (97%) of 120 adults who received the 15 μg dose. Local discomfort (e.g., injection site tenderness or pain) was reported by 46% of subjects, and one or more systemic symptoms (e.g., headache, malaise, or myalgia) by 45% of subjects.7 This safety profile is consistent with results from studies of the seasonal influenza vaccine manufactured by CSL Limited.8 In studies of other seasonal inactivated influenza vaccines, rates of adverse events were not significantly different from placebo injections except for arm soreness and redness at the injection site.9 The National Institute of Allergy and Infectious Diseases (NIAID) reported preliminary results of a study among children aged 6 months–18 years. Among children aged 6-35 months, 3-9 years, and 10-17 years immunized with a 15 μg inactivated influenza A 2009 (H1N1) monovalent vaccine (Sanofi Pasteur, Inc., Swiftwater, PA), 25%, 36% and 76%, respectively, developed antibody titers of 1:40 or more (hemagglutination-inhibition assay) after a single dose of vaccine.† Immunogenicity and safety study results similar to those observed for seasonal vaccines also have been reported by the other manufacturers (MedImmune LLC, Gaithersburg, MD and Novartis Vaccines and Diagnostics, Limited, Liverpool, UK, unpublished data, 2009).
Influenza activity attributed to 2009 H1N1 viruses has increased during September 2009 and is expected to continue through the fall and winter influenza season. Surveillance data indicate that the 2009 H1N1 viruses have not undergone substantial antigenic change since they were first characterized in April 2009 and should be well-matched to the monovalent vaccine strain.10 Influenza A (H1N1) 2009 monovalent vaccines will be available in many areas by mid-October. Vaccines against seasonal influenza are available now, and immunization programs and providers should begin or continue administering seasonal influenza vaccines as recommended.5,6 Additional data from clinical trials will be available over the coming weeks, and immunization providers should periodically look for updates on use of influenza A (2009) H1N1 monovalent vaccines at http://www.cdc.gov/flu.
*Food and Drug Administration. FDA approves vaccines for 2009 H1N1 influenza virus. Available at http://www.fda.gov/newsevents/newsroom/pressannouncements/ucm182399.htm.
†National Institutes of Health. Early results: in children, 2009 H1N1 influenza vaccine works like seasonal flu vaccine. Available at http://www.nih.gov/news/health/sep2009/niaid-21.htm.
Update on Influenza A (H1N1) 2009 Monovalent Vaccines. JAMA. 2009;302(19):2084-2085. doi: