WASHINGTON — Two doses of influenza vaccine were up to 55% effective against influenza-like illness and 85% effective against pneumonia or flu in children, Mandy Allison, M.D., said at the National Immunization Conference sponsored by the Centers for Disease Control and Prevention.
The aim of Dr. Allison and her colleagues at Children's Hospital, Denver, and the University of Colorado Health Sciences Center, was to gather more data on the flu vaccine's effectiveness in children, especially since the CDC's Advisory Committee on Immunization Practices advised in 2004 that the shot should be included as a routine immunization for children aged 6–23 months.
Dr. Allison said that one study in the Journal of the American Medical Association calculated the vaccine's effectiveness at 66% in any given year, but a recent systematic review published in the Lancet found very little data on vaccine efficacy in children under 2 years old (2005;365:773–80).
A study conducted by Kaiser and the CDC found that the flu vaccine was only 25% effective against influenza-like illness, and 49% effective against pneumonia and flu, which is defined as a subset of influenza-like illness (MMWR 2004;53:707–10), she said.
Her group analyzed billing and immunization registry data from 5,913 healthy 6- to 21-month-old children from five Denver area pediatric practices.
ICD-9 codes for office visits between Nov. 1 and Dec. 31 were reviewed to determine the first influenza-like illness; the same nine codes were used in the Denver study as in the Kaiser/CDC study, Dr. Allison said.
Children were dubbed either partially vaccinated—one shot during the current season and 14 days before the first influenza-like illness—or fully vaccinated, which was defined as two shots more than 14 days before the first influenza-like illness.
During Colorado's flu season, which peaked early, 36% of the children were unvaccinated, 24% were partially vaccinated, and 40% were fully vaccinated.
Only 6% were fully vaccinated by Nov. 1, and 36% by Jan. 1, Dr. Allison said.
Twenty-eight percent of children had an influenza-like illness, and 5% pneumonia or flu, during the season.
The researchers also calculated hazard ratios that accounted for age, gender, and immunization status.
They determined that fully vaccinated children were less likely to have influenza-like illness (a ratio of 0.45), when compared with unvaccinated children, which was not surprising.
But partially vaccinated children were more likely to have influenza-like illness, compared with unvaccinated children, Dr. Allison said. She said the researchers weren't sure why one dose seemed to increase the chance of illness, but said there might be something different about those children or families.
She also wasn't certain why the Denver study showed much higher efficacy than the Kaiser/CDC study but noted that it might be that there was a significantly higher vaccination rate in the Denver population.
All the children in the Denver practices came from more affluent socioeconomic groups, which may have made a difference. That also limited the study's conclusions, though, she added.