In children under the age of five, the influenza vaccine was not associated with decreased hospitalizations or physician visits in two recent flu seasons, according to an article released on October 6, 2008 in Archives of Pediatrics and Adolescent Medicine, one of the JAMA/Archives journals.
Influenza is a seasonal disease that can affect all age groups but is usually most severe in the very old and very young. To help prevent disease in young children, the United States and several other countries have taken an initiative to expand the childhood vaccination requirements. Presently, in the US, health officials recommend annual vaccinations for children between 6 and 10 years old, according to the article. However, the authors point out the vaccine efficacy should be verfiied in a recommendation of this magnitude: “An inherent assumption of expanded vaccination recommendations is that the vaccine is efficacious in preventing clinical influenza disease.”
To investigate the true health impact of childhood influenza vaccinations on young children, Peter G. Szilagyi, M.D., M.P.H., of the University of Rochester School of Medicine and Dentistry and Strong Memorial Hospital, Rochester, N.Y., and colleagues examined 414 children aged 5 and younger who developed influenza in the 2003-2004 or 2004-2005 flu seasons. Of these, 245 had been seen in hospitals or emergency departments, and 169 had been examined in outpatient groups. Their vaccination status was determined and compared to the vaccination status of over 5,000 children from the same counties who had not shown signs of influenza.
Before normalizing for other factors, the influenza group appeared to have lower vaccination rates than those without influenza. The authors then took into account many risk factors that could be indirectly linked to vaccination status. “However, significant influenza vaccine effectiveness could not be demonstrated for any season, age or setting after adjusting for county, sex, insurance, chronic conditions recommended for influenza vaccination and timing of influenza vaccination (vaccine effectiveness estimates ranged from 7 percent to 52 percent across settings and seasons for fully vaccinated 6- to 59-month olds),” write the authors. This indicates that the vaccine may have had very low efficacy, leading to almost no vaccine-linked prevention in this age group.
There are many reasons that a vaccine for influenza could be suboptimal. For one, in a virus that mutates as often as influenza, there is some chance that the strain that the vaccine was developed against could be too disparate, leading to poor immunity against the strain of exposure. For instance, in the 2003-2004 flu season, 99% of the circulating strains were caused by influenza A, but only 11% of these strains were similar to the strains in the vaccine. The authors compare this to the 2003-2004 season: “The 2004-2005 season was less severe and the vaccine was a better match to circulating strains than in 2003-2004, but still only 36 percent of virus isolates were antigenically similar to vaccine strains.”
The authors emphasize that further studies should be performed to confirm the potential reasons for this relative failure. This should include studies regarding years when the vaccine was actually effective. “Further studies of influenza vaccine effectiveness are needed using a variety of study designs (that adjust for confounders) to assess the yearly impact of influenza vaccination programs for children, particularly as higher rates of vaccination are achieved in the study population,” they say.
Influenza Vaccine Effectiveness Among Children 6 to 59 Months of Age During 2 Influenza Seasons: A Case-Cohort Study
Peter G. Szilagyi; Gerry Fairbrother; Marie R. Griffin; Richard W. Hornung; Stephanie Donauer; Ardythe Morrow; Mekibib Altaye; Yuwei Zhu; Sandra Ambrose; Kathryn M. Edwards; Katherine A. Poehling; Geraldine Lofthus; Michol Holloway; Lyn Finelli; Marika Iwane; Mary Allen Staat; for the New Vaccine Surveillance Network
Arch Pediatr Adolesc Med. 2008;162(10):943-951.
Written by Anna Sophia McKenney