Article

Effectiveness of the 2003-2004 influenza vaccine among children 6 months to 8 years of age, with 1 vs 2 doses

Clinical Research Unit, Kaiser Permanente Colorado, 580 Mohawk Dr, Boulder, CO 80302, USA.
PEDIATRICS (Impact Factor: 5.3). 07/2005; 116(1):153-9. DOI: 10.1542/peds.2005-0049
Source: PubMed

ABSTRACT To evaluate the effectiveness of 1 and 2 doses of the 2003-2004 influenza vaccine in preventing medically attended influenza-like illness (ILI) among children 6 to 23 months and 6 months to 8 years of age. Design and
Outpatient and emergency department visits and immunization records were used to conduct a retrospective cohort study among children 6 months to 8 years of age. ILI and pneumonia and influenza (P&I) outcomes were defined on the basis of International Classification of Diseases, Ninth Revision, codes. Influenza vaccine effectiveness (VE) was calculated as (1 - hazard rate ratio) x 100.
A total of 29726 children were included in the analyses; 17.3% were 6 to 23 months of age. By November 19, 2003, the start of peak influenza activity, 7.5% and 9.9% of children 6 months to 8 years were fully or partially vaccinated against influenza, respectively. For fully vaccinated children 6 to 23 months of age, VE against ILI and P&I was 25% and 49%, respectively. No statistically significant reduction in ILI or P&I rates was observed for partially vaccinated children 6 to 23 months of age (-3% and 22%, respectively). For fully vaccinated children 6 months to 8 years of age, VE against ILI and P&I was 23% and 51%, respectively. For partial vaccination, VE was significant only for P&I (23%).
Despite a suboptimal match between the influenza vaccine and predominant circulating strains, influenza vaccination provided substantial protection for fully vaccinated children and possibly some protection for partially vaccinated children <9 years of age. These findings support vaccinating targeted children even when the vaccine match is suboptimal, and they highlight the need to vaccinate previously unvaccinated children with 2 doses for optimal protection.

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