Background. Due to the lack of effectiveness of live attenuated influenza vaccine (LAIV) against pandemic H1N1 virus in children during 2013–14, the US Influenza Vaccine Effectiveness (VE) Network expanded enrollment of children during 2014–15 to determine influenza VE against medically attended acute respiratory illnesses (ARI) by vaccine type.
Methods. In a prospective, test-negative case-control study, patients >6 months of age with ARI of ≤7 days' duration were enrolled at ambulatory care facilities in 5 sites nationally from November 10, 2014 to April 10, 2015. Specimens were collected and tested for influenza by real-time reverse-transcriptase polymerase chain reaction (RT-PCR). Influenza vaccination status was based on medical record (EMR) or state immunization registry documentation, and if not available, by self-report. VE (1-adjusted odds ratio) was estimated by vaccine type with unvaccinated as the referent group in logistic regression adjusted for age, sex, site, days from illness onset to enrollment, calendar time (2-week intervals), race/ethnicity and parent-rated general health.
Results. Among 9506 enrollees, 1976 (21%) were 2–8 years (younger) and 1352 (14%) were 9–17 years (older). In the younger group 428 (22%) were RT-PCR influenza positive, 653 (33%) had received ≥1 dose of 2014–15 inactivated influenza vaccine (IIV) and 332 (17%) had received ≥1 dose of LAIV. In the older group, 394 (29%) were RT-PCR influenza positive, 353 (26%) had received ≥1 dose of 2014–15 IIV and 173 (13%) had received ≥1 dose of LAIV. Among 822 influenza positives, 81% (664) were H3N2 (1 co-infected with B/Yamagata), 14% (115) were B Yamagata, 4% (31) were B Victoria. VE against H3N2 viruses among the younger group was −12% (−61, 22) for LAIV and 20% (−9, 41) for IIV; among the older group, VE was 3% (−58, 41) for LAIV and 14% (−21, 39) for IIV. VE against the B Yamagata strain in the younger group was 71% (12, 90) for LAIV and 49% (−2, 75) for IIV; in the older group, VE was 72% (3, 92) for LAIV and 78% (45, 91) for IIV.
Conclusion. VE estimates are consistent with a drifted H3N2 strain circulating in 2014–15. There is no evidence that VE against drifted H3N2 for LAIV is higher than for IIV in 2–8 year-olds, supporting the 2015 ACIP decision for no preference among vaccine types in this age group.