Efficacy of flu shots in children under 2 questioned

Feb 25, 2005 (CIDRAP News) – An analysis of 24 studies yielded no clear evidence that influenza vaccines prevent flu in children younger than 2 years old, though they work reasonably well in older children, according to a new report in The Lancet.

"Immunization of very young children is not lent support by our findings," says the report by T. Jefferson of Cochrane Vaccines Field in Alessandria, Italy, and colleagues from there and the University of Oxford in England.

The report comes less than a year after the Centers for Disease Control and Prevention (CDC) recommended that children 6 to 23 months old routinely receive flu shots on grounds that they have an increased risk of flu and its complications.

The CDC says the Lancet analysis left out some important studies showing that flu shots are beneficial for young children. The agency will continue to recommend flu shots for 6- to 23-month-olds, a spokeswoman said.

Jefferson and colleagues combed the worldwide literature up to June 2004 and found 15 randomized controlled trials, eight cohort studies, and one case-control study comparing the effects of flu immunization with placebo or no immunization in children. The researchers looked at the vaccines' efficacy, defined as prevention of laboratory-confirmed flu; effectiveness in preventing flu-like illness; and effectiveness in reducing hospitalizations, complications, school absences, and secondary transmission.

The authors did separate analyses of studies dealing with live attenuated flu vaccines, which are not used in small children, and those involving inactivated vaccines. The analysis did not include studies assessing children's immune-system responses to vaccines (serologic studies).

In children under age 2, vaccination yielded 24% efficacy against flu, which was not significantly better than placebo, but this finding was based on one study involving about 800 children, the report says. No data were available on the effectiveness of vaccination for preventing flu-like illness in the under-2 group.

Among children 2 years and older, live attenuated vaccines were 79% efficacious in preventing flu, while inactivated vaccines were 65% efficacious, according to the analysis. The effectiveness of vaccines against flu-like illness was much lower: 38% for live vaccines, 28% for inactivated vaccines.

"Differences between efficacy and effectiveness of vaccines are not surprising because influenza vaccines are specifically targeted at influenza viruses and are not designed to prevent other causes of influenza-like illness," the report states.

The researchers found that immunization reduced long school absences, but they saw little evidence that it reduced hospital admissions and stays, secondary cases, lower-respiratory-tract disease, or acute otitis media.

"Although a growing body of evidence shows the effect of influenza on admissions and deaths of children, we recorded no convincing evidence that vaccines can reduce mortality, admissions, serious complications, and community transmission of influenza," the article concludes.

Kristine Sheedy, a spokeswoman for the CDC's National Immunization Program in Atlanta, told CIDRAP News that the analysis left out some important evidence on the effects of flu immunization in small children.

The authors "included many important studies, but they did exclude other important studies," she said. She mentioned a CDC study published in Morbidity and Mortality Weekly Report in August 2004 and a 2001 study by K. M. Neuzil and colleagues, published in the Pediatric Infectious Disease Journal.

A careful review of the Lancet report itself as well as the excluded studies "indicates that influenza vaccination is effective in this population" (6- to 23-month-olds), Sheedy said.

She added that serologic studies of small children have shown a strong immune response to flu vaccine. Also, additional studies soon to be reported by the CDC will provide more support for the effectiveness of flu immunization in preventing hospitalization and outpatient visits in children, she said.

"I hope people will keep in mind the burden of disease in children," Sheedy said. "When you have a safe and effective vaccine for influenza, why wouldn't you want to protect the children at greatest risk?"

Jefferson T, Smith S, Demicheli V, et al. Assessment of the efficacy and effectiveness of influenza vaccines in healthy children: systematic review. Lancet 2005;365(Feb 26):773-80 [Abstract]

See also:

Neuzil KM, Dupont WD, Wright PF, et al. Efficacy of inactivated and cold-adapted vaccines against influenza A infection, 1985 to 1990: the pediatric experience. Pediatr Infect Dis J 2001;20(8):733-40 [Abstract]

CDC. Assessment of the effectiveness of the 2003-04 influenza vaccine among children and adults—Colorado, 2003. MMWR 2004 Aug 13;53(31):707-10 [Full text]

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