Flu Shots Have No Impact on Hospitalizations for Young Children

ROCHESTER, N.Y., Oct. 6 — Immunizations did not reduce emergency department visits or hospitalizations for children younger than five during two recent flu seasons, researchers here reported.

Looking at data from the 2003-2004 and the 2004-2005 flu seasons, there was no evidence that the immunization made any significant difference, although the vaccine was not a good match for circulating flu strains in those years, said Peter G. Szilagyi, M.D., M.P.H., of Strong Memorial Hospital, and colleagues.


The case-cohort study, reported in the October issue of Archives of Pediatrics and Adolescent Medicine, compared cases of acute respiratory illnesses in children six months to 59 months treated in hospitals — as inpatients or in the emergency department or outpatient clinic — with a control cluster sample of children treated at pediatric practices.

…The vaccine effectiveness ranged from 7% to 52% across all settings and three age groups (six-23 months, 24-59 months, six-59 months), they said.

Earlier studies had suggested that vaccination could reduce hospitalizations or emergency department visits, but those studies were limited by the use of nonspecific endpoints rather than laboratory-confirmed diagnoses or a narrow focus that concentrated on only a single healthcare setting — such as emergency departments — or a single flu season, Dr. Szilagyi wrote.


This study, he said, attempted to address those limitations, only to discover other factors that “contributed to the difficulty in demonstrating a positive [vaccine effectiveness].”


Chiefly, during the “2003-2004 season, 99% of the strains in these three communities were due to influenza A virus, but only 11% of influenza A specimens across the United States were similar to a strain included in the vaccine.”


And a year later, when the flu season was less severe and the vaccine was considered a better match, still “only 36% of the virus isolates were antigenically similar to vaccine strains,” they wrote.


Moreover, they acknowledged, the case-cohort design may have been a poor choice since it might be particularly susceptible to bias because parents who have had their children vaccinated might be more likely to seek medical care for their children than other parents.


In any case, they said, more studies are needed to adequately “assess the yearly impact of influenza vaccination programs for children.”

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