Vaccine Efficiency Estimates for 2014-2015 Influenza Season

An assessment of vaccine efficiency during the 2014-2015 influenza season has found that the available vaccines offered limited protection against influenza A/H3N2, but elevated protection against influenza B, according to research published in Clinical Infectious Diseases.¹

Richard K. Zimmerman, MD, MPH, of the Department of Family Medicine at the University of Pittsburgh, and colleagues from the US Flu VE (vaccine efficiency) Network provided vaccine efficiency estimates for patients with acute respiratory illness (influenza A or B) among various influenza vaccines, including the standard-dose trivalent inactivated influenza vaccine (IIV3), the standard-dose quadrivalent IIV (IIV4), and the live attenuated influenza vaccine (LAIV).

Dr Zimmerman and colleagues analyzed data from 9311 participants from US Flu VE Network sites across the US. Participants were ≥6 months old, and were diagnosed with an acute respiratory illness between November 2014 and April 2015.

Among the participants, 76% tested negative for influenza; of the remaining participants, 1840 had influenza A and 395 had influenza B. Two participants were co-infected with the A/H3N2 and B viruses, while 99% had A/H3N2 only and less than 1% had A/H1N1. Of all influenza B infections, 88% were B/Yamagata, 12% were B/Victoria, and 8 patients had no definable lineage. Characterization of the A/H3N2 and B viruses indicated that while more than 80% of A/H3N2 viruses belonged to a drifted genetic group, 86% of B/Victoria and 100% of B/Yamagata viruses were similar to their respective 2014/2015 vaccine strains.

Researchers determined vaccine effectiveness among A and B subtypes by participants’ age groups. Combined, the overall adjusted vaccine effectiveness was 19% (95% confidence interval [CI], 10%-27%) and was statistically significant in all age groups except for participants aged 18-49 years. In A/H3N2 and B/Yamagata, vaccine efficiency was 6% (95% CI, -5%-17%) and 55% (95% CI, 43%-65%), respectively; after including the small number of B/Victoria cases, adjusted vaccine efficiency was 54% (95% CI, 43%-64%).

 Among participants who were vaccinated (n=4360), 39.7% received standard-dose IIV3, 46.8% received standard-dose IIV4, and 11.9% received LAIV4—a statistically significant change from the US Flu VE Network’s 2013-2014 vaccinations, where 78% received standard-dose IIV3, 14% received IIV4, and 8% received LAIV4 (P <.001). 

Using 1817 A/H3N1-positive, 340 B/Yamagata-positive, and 7078 influenza test-negative participants, researchers were able to estimate vaccine efficiency by vaccine type. Sensitivity analyses were conducted to assess the effect of various vaccination reporting methods on vaccine efficiency; adjusted vaccine efficiency against A/H3N2 was 3% (95% CI, -11%-16%) for self-reported vaccination vs 12% (95% CI, 1%-22%) for EIR-documented vaccination. Adjusted vaccine efficiency for the B/Yamagata strain was 46% (95% CI, 25%-62%) vs 53% (95% CI, 39%-63%), respectively.

“For the past 4 years, the US Flu VE Network has provided effectiveness estimates for seasonal influenza vaccine for the purposes of monitoring and guiding influenza vaccine policy,” Dr Zimmerman and colleagues wrote. “These results highlight the importance of vaccinating through the influenza season because the vaccine may protect against a second wave of influenza.”

Dr Zimmerman and colleagues also noted that as the landscape of available influenza vaccines continues to change, researchers may need to conduct vaccine effectiveness estimates by vaccine type, in addition to estimates by age range.

“With the current [Advisory Committee on Immunization Practices] recommendation not to use LAIV during the 2016-2017 season, there may be little opportunity to evaluate LAIV effectiveness in the coming season in the US,” the researchers concluded. “However, offering choice of vaccine type (eg, either LAIV or IIV) has been shown to increase vaccine uptake, resulting in protection for a larger number of people.”

Disclosures: Dr Zimmerman has received research funding from Sanofi Pasteur, Pfizer, and Merck & Co. Drs Belongia and McLean have received research funding from MedImmune, LLC. Dr Gaglani has an institutional research contract with MedImmune and AstraZeneca.

Reference

1. Zimmerman RK, Nowalk MP, Chung J, et al, for the US Flu VE Investigators. 2014-2015 influenza vaccine effectiveness in the United States by vaccine type. Clin Infect Dis. 2016 Oct 4; doi: 10.1093/cid/ciw635. [Epub ahead of print]