Influenza vaccine effectiveness (VE) can vary by antigenic match between vaccine and circulating viruses. During 2014-15, most circulating A/H3N2 viruses in the U.S were antigenically drifted from the vaccine. We genetically characterized a larger number of US A/H3N2 viruses than previous years and provided VE estimates for genetic groups of A/H3N2 viruses using the test-negative design.
Among five US Flu VE Network study sites, outpatients aged >6 months with cough ≤7 days duration were enrolled November 10, 2014—April 10, 2015. Respiratory specimens were tested for influenza by real-time rtPCR. Vaccination was defined as receipt of ≥1 dose of vaccine at least 14 days before illness onset. VE was estimated as (1 – adjusted odds ratio) x 100% for vaccination in influenza cases vs. test negative controls from logistic regression models adjusting for age and potential confounding factors (study site, calendar time, race, general health status). Genetic groups were identified by MiSeq or pyrosequencing of the hemagglutinin gene. In general, viruses were classified as vaccine-like (genetic groups 3C.3 or 3C.3b) and vaccine low reactors or drifted (3C.2a and 3C.3a).
Overall, 9,707 patients were enrolled and 2,288 (24%) were influenza positive, 1,868 (81%) were A/H3N2. Among 1397 (75%) sequenced A/H3N2 viruses, most (1134 [81%]) were 3C.2a, 159 (11%) were 3C.3b, and <5% were in each other group. Among 398 (18%) influenza B cases, 341 (86%) were B/Yamagata (vaccine lineage). Adjusted VE against any A/H3N2 virus infection was 13% (95% confidence interval [CI]: 2 to 23) but varied by genetic group. Adjusted VE against infection with the A/H3N2 vaccine low reactor 3C.2a viruses was 9% (CI: -5 to 21) and 43% (CI: 15 to 62) against the A/H3N2 vaccine-like 3C.3b viruses. Adjusted VE against B/Yamagata virus infection was 55% (CI: 43 to 65).
During 2014-15, the influenza vaccine had no measured effectiveness against widely circulating A/H3N2 3C.2a viruses while VE against the less common vaccine-like A/H3N2 3C.3b and vaccine lineage influenza B viruses was moderate. Increased sequencing efforts for A/H3N2 viruses improved the interpretation of VE and the data evaluated for vaccine strain selection.
A. M. Fry,
R. Garten, None
J. Clippard, None
V. Mishin, None
S. Spencer, None
S. Thaker, None
M. P. Nowalk, Pfizer Inc.: Grant Investigator , Research grant
M. L. Jackson, None
L. A. Jackson, None
A. Monto, None
R. K. Zimmerman, Merck & Co.: Grant Investigator , Research grant
Pfizer: Grant Investigator , Research grant
Sanofi Pasteur: Grant Investigator , Research grant
J. G. Petrie, None
E. Belongia, Medimmune: Investigator , Research support
H. Q. Mclean, MedImmune: Investigator , Research support
M. Gaglani, MedImmune: Investigator , Research support
P. Piedra, None
J. R. Barnes, None
X. Xiyan, None
B. Flannery, None