Full immunization in children aged 6–59 months and partial immunization in children aged 24–59 months provide significant protection from influenza, even when the match between vaccine and circulating strains is suboptimal, Carrie M. Shuler, D.V.M., an epidemiologist with the Georgia Division of Public Health, and her colleagues reported.
However, children aged 6–23 months with partial vaccination had no benefit in terms of influenza protection, the authors wrote (Pediatrics 2007;119:587–95).
The researchers evaluated the effectiveness of the trivalent inactivated influenza vaccine at a private pediatric practice in Atlanta during the 2003–2004 influenza season.
During that season, only one-quarter of circulating influenza viruses nationally and in Georgia were similar antigenically to the vaccine strain.
Case patients (290) were identified as having laboratory-confirmed influenza between Nov. 1, 2003, and Jan. 31, 2004. Case patients were randomly matched with two control children based on age.
Children who had received two doses of the vaccine at least 1 month apart and at least 14 days before the date of symptom onset (the anchor date for the case child and matched control children) were considered fully vaccinated.
Children who were vaccinated during a previous season needed only one dose during the 2003–2004 season that was administered at least 14 days before the anchor date to be considered fully vaccinated. In all, 322 children were considered fully vaccinated.
A child was considered partially vaccinated if he had not been vaccinated in a previous season and had received two doses of vaccine since September 2003, with an anchor date less than 14 days after the second dose. A child also was considered partially vaccinated if she had not been vaccinated in a previous season and had received only one dose since September 2003 and was vaccinated at least 14 days prior to the anchor date. In all, 103 children were considered partially vaccinated.
Children who received no doses during the 2003–2004 season on or before the anchor date and children who had received one dose since September 2003 that was administered less than 14 days prior to the anchor date were considered unvaccinated. In all, 445 children were considered unvaccinated.
Fully vaccinated children aged 6–23 months had a significant reduction (52%) in influenza, compared with unvaccinated children.
Likewise, fully vaccinated children aged 24–59 months had a significant reduction (45%) in influenza, compared with unvaccinated children.
Partially vaccinated children aged 24–59 months also had a significant reduction (65%) in influenza, compared with unvaccinated children. However, partially vaccinated children aged 6–23 months did not have a significant reduction in influenza, compared with unvaccinated children who were not vaccinated the previous season (adjusted odds ratio 1.70).