At just 6% vaccine effectiveness, the 2014-2015 influenza vaccine offered little protection against the predominant influenza A (H3N2) virus, according to an analysis of individuals seeking outpatient treatment for acute respiratory illness. However, the vaccine was effective against influenza B.
“The 2014-2015 season was characterized by the emergence of an A/H3N2 drifted virus that was associated with low [vaccine-effectiveness (VE)] in an early season estimate and an earlier-than-normal peak in influenza cases occurring in late December,” Richard K. Zimmerman, MD, MPH, professor in the department of family medicine and clinical epidemiology at the University of Pittsburgh School of Medicine, and colleagues wrote in Clinical Infectious Diseases. “Thus, the unpredictability of influenza epidemiology continues to necessitate effectiveness studies of available influenza vaccines in a range of population subgroups.”
Richard K. Zimmerman
Recent changes in influenza vaccine science have led to changes in its use, including an increased market share for quadrivalent influenza vaccine. Further, for the 2014-2015 season, the Advisory Committee on Immunization Practices stated a preference for live-attenuated influenza vaccine over inactivated influenza vaccine (IIV) in children — ACIP later reversed its position and stated that LAIV, offered as a nasal spray, should not be used in any setting. Because of these changes, as well as the disease’s unpredictability, researchers aimed to provide influenza VE estimates for influenza A and B, estimate VE in children aged 2 through 8 years by vaccine type, and identify trends and compare VE in trivalent vs. quadrivalent vaccine use.
They analyzed data gathered by the U.S. Flu VE Network on 9,311 patients aged 6 months or older seeking outpatient care for acute respiratory illness from November 2014 to April 2015. VE across age groups and vaccine types was examined using a test-negative design that compared the odds of vaccination among those with and without influenza, which was confirmed by reverse transcription PCR.
Regarding examined patients, 76% were negative for influenza, 19.8% were positive for influenza A, and 4.2% were positive for influenza B. Of those with influenza A, 99% were positive for A/H3N2, and of those with influenza B, 88% were B/Yamagata, 12% were B/Victoria and eight had no lineage identified.
Overall, the adjusted VE for influenza A and B was 19% (95% CI, 10-27) and was statistically significant in all age groups except patients aged 18 to 64 years.
VE against the predominant A/H3N2 strain was insignificant at 6% (95% CI, –5 to 17), whereas VE against influenza B/Yamagata was 55% (95% CI, 43-65).
Among children aged 2 through 8 years, VE against A/H3N2 was 15% (95% CI, –16 to 38) for IIV and –3% (95% CI, –50 to 29) for LAIV. VE against B/Yamagata was 40% (95% CI, –20 to 70) for IIV and 74% (95% CI, 25-91) for LAIV.
Quadrivalent vaccine was not shown to be more effective than trivalent vaccine, but researchers noted that B/Yamagata was included in both trivalent and quadrivalent vaccine types. “Given the uncertainty of predicting which B lineage will circulate in any given year,” they wrote, “a quadrivalent vaccine obviates the possibility of a mismatch between vaccine and circulating B lineages.”
The findings of this study should be interpreted within a broader context, Saad B. Omer, MBBS, MPH, PhD, and Inci Yildirim, MD, MSc, PhD, both professors at Emory University, wrote in a related editorial.
“While the 19% vaccine effectiveness … is disappointing, it is an exception, not a pattern,” they wrote. “In the past decade, for most influenza seasons the vaccine has been effective in protecting approximately half of the recipients. While this level of effectiveness is not optimal — particularly compared to the vaccine effectiveness for most other vaccines in routine use — the policy of annual influenza vaccination provides substantial public health benefit.”– by Sarah Kennedy
Disclosure: Zimmerman reports research funding from Merck, Pfizer and Sanofi Pasteur. Please see the full study for a list of all other researchers’ relevant financial disclosures.