ATLANTA — Children less than 9 years of age who received just one dose of influenza vaccine the first time they were immunized against influenza still don't require a second dose the following season … at least for now.
That was the vote from the Advisory Committee on Immunization Practices of the Centers for Disease Control and Prevention at its summer meeting.
Current recommendations from ACIP and the American Academy of Pediatrics call for previously unvaccinated children aged 6 months to 9 years to receive two doses of inactivated influenza vaccine, administered at least 1 month apart. For the live attenuated influenza vaccine (FluMist), children aged 5–8 years who had not previously received either type of influenza vaccine should receive two doses, separated by 6–10 weeks. If a child received only one dose in the previous year, only one dose is required (MMWR 2006;55[early release]:1–41).
However, published and unpublished data suggesting that children may not be adequately protected without receiving two doses in one season—particularly in the response to influenza B strains—have prompted both the ACIP and the AAP to consider recommending that partially immunized children receive a second dose the following season. But after much discussion, ACIP members ultimately decided to wait until more of the data are published.
The AAP's Committee on Infectious Disease (COID) had been leaning toward recommending a second dose prior to the ACIP meeting. But, given ACIP's decision, the COID also may decide to wait, COID chair Dr. Keith R. Powell, vice president and Noah Miller Chair of Pediatrics at Children's Hospital Medical Center of Akron (Ohio), and professor and chair of pediatrics at Northeastern Ohio Universities, Rootstown, said in an interview.
Dr. Kathleen M. Neuzil, a vaccine researcher from the University of Washington, Seattle, summarized the recent data, which include very limited information on immunogenicity in children under the age of 2 years. Children aged 2–6 years with no detectable hemagglutination inhibition assay antibody levels have lower responses than do children with detectable levels, suggesting that “preexisting immunity or infection matters.” Moreover, historical data suggest antibody responses to influenza B vaccine or infection can be substantially lower, compared with responses following influenza A vaccine or infection, she said.
In a published study from Dr. Neu- zil's group, giving 6- to 23-month-old children one dose of influenza vaccine in the spring and another the following autumn was not inferior to giving both doses during flu season. But that study was conducted in the 2002–2003 and 2003–2004 seasons, when the three antigens in the vaccine didn't change (Pediatrics 2005;115:1039–47).
Several yet-unpublished studies conducted during the 2004–2005 season—when two of the antigens differed from the previous season's vaccine—have yielded different results.
In two of those studies, also done in 6- to 23-month-olds who received the first dose in either the spring or the fall, response to the second dose differed by antigen. In 2003–2004, one of the strains was A/Panama/2007/99 (H3N2). In 2004–2005, the H3N2 strain had “drifted” to A/Wyoming/03/2003. Although the children who had been “primed” with the 2003–2004 vaccine had a less robust response to the H3N2 component than did those who received two doses of the identical vaccine, about 70% still had protective antibody levels in one of the studies, while priming had no impact on the H3N2 response in the other study.
In contrast, response to the B strain, which was completely different between the two seasons (B/HongKong/1434/2002 in 2003–2004 vs. B/Jiagsu/10/2003 in 2004–2005), was dramatically lower among those who received just one compared with two identical vaccine doses, in both studies.
A prospective, open-label study comparing one dose with two doses in the fall in vaccine-naive 5- to 8-year-olds yielded similar results: Two doses in the same season were better than one, and preexisting antibody was the strongest predictor of antibody response after one dose. In this study, one-third of the children did not achieve “protective” responses to the B antigen, even after two doses. However, other data suggest that results may differ greatly depending upon how the antibody response to B is measured, Dr. Neuzil remarked.
In a fourth unpublished study of children aged 6–21 months, giving two vaccine doses in the same year was 82% effective in preventing influenzalike illness, compared with 62% with two vaccines given in different years.
ACIP member Dr. Ban Mishu Allos summed up the committee's view prior to its vote: “We have a lot of data that haven't been published yet. We need to look at them further. … Given all that, I'm not in favor of changing.”