ATLANTA — Full immunization against influenza is about 75% effective in preventing hospitalizations in 6- to 23-month-old children, Dr. David Shay said at the winter meeting of the Advisory Committee on Immunization Practices of the Centers for Disease Control and Prevention.
The ACIP's recommendation for annual influenza immunization for all children aged 6–23 months beginning in the 2004–2005 influenza season was based on the burden of disease in that age group and the fact that hospitalization rates among this age group were similar to those among the elderly, for whom annual flu vaccination was already recommended. However, no previous study has assessed the effectiveness of the trivalent inactivated vaccine (TIV) in preventing laboratory-confirmed hospitalizations in this age group, said Dr. Shay of the CDC's National Center for Immunization and Respiratory Diseases, Influenza Division.
Now, a multistate case-control study conducted during the 2005–2006 and 2006–2007 flu seasons has confirmed that TIV indeed prevents influenza-related hospitalizations in 6- to 23-month-olds, but only if they receive “full” immunization. “Partial immunization was less effective, and not significantly protective, based on two seasons of data. It is critical to ensure that children aged 6–23 months are fully immunized if we seek to prevent influenza-associated hospitalizations among children,” Dr. Shay commented.
The data were analyzed using the 2007 definition of “full” immunization, which is more stringent than it had been during the study period: The child must have received two doses during the current season if they had never previously received TIV or if they had received only one dose in the previous season. A child who received just one dose in the current season would be considered “fully” immunized if he or she received two doses in a single prior season or had one dose in two or more prior seasons.
The study population comprised 93 of a total 191 eligible 6- to 23-month-old children who were hospitalized with laboratory-confirmed influenza (85% type A, 12% B, and 3% unknown), identified at eight U.S. state health department surveillance sites, and 334 age-matched controls. Cases and controls also were well matched by gender (56% of cases and 52% of controls were male) and by race (72% and 80% were white, respectively).
During the 2005–2006 season, only 9% of cases were fully immunized, compared with 20% of controls. Sixty-seven percent of cases were not immunized, compared with 55% of controls, while about a quarter of both groups was partially immunized. In 2006–2007, only 13% of cases had been immunized, compared with 32% of controls, while 65% of cases were not immunized vs. 38% of controls. Again, the rates of partial immunization were similar, 23% among cases and 30% among controls. (Cumulative percentages might exceed 100% because of rounding.)
Overall effectiveness of TIV in preventing hospitalization was 74% for full immunization, compared with just 39% for partial immunization. Adjustment for high-risk conditions, very low birth weight, and insurance status did not significantly change the result for full immunization (76%), but it dropped the effectiveness of partial immunization to just 27%, Dr. Shay reported.
After Dr. Shay's presentation, ACIP member Dr. Carol J. Baker urged meeting participants to inform their constituencies of the importance of giving two doses in the 6- to 23-month age group, even if it means vaccinating late into the season. “We have created a culture of stopping vaccination in late November, early December. … That mentality must change,” said Dr. Baker, professor of pediatrics, molecular virology, and microbiology at Baylor College of Medicine, Houston.