Early Estimates of Seasonal Influenza Vaccine Effectiveness

United States, January 2013

Lisa Jackson, MD, Michael L. Jackson, PhD, C. Hallie Phillips, MEd, Joyce Benoit; Edward A. Belongia, MD, Deanna Cole, Sarah Kopitzke, MS, Tamara A. Kronenwetter Koepel, Huong Q. McLean, PhD, Jennifer K. Meece, PhD, Sandra K. Strey, Maria E. Sundaram, MSPH, Mary Vandermause; Manjusha Gaglani, MBBS, Juhee Song, PhD, Lydia Clipper, Dean Kjar, MS, Anne Robertson, Kempapura Murthy, MPH, Melinda Dunnahoo, Stephanie Oliver, MS, Monica Weir, Hope Gonzales, Martha Zayed, Michael Reis, MD, Cathleen Rivera, MD, David Morgan, MD, Pedro Piedra, MD, Vasanthi Avadhanula, PhD; Arnold S. Monto, MD, Suzanne E. Ohmit, DrPH, Joshua G. Petrie, MPH, Emileigh Johnson, Rachel T. Cross, MPH, Casey Martens, Marcus Zervos, MD, Lois Lamerato, PhD; Mary Patricia Nowalk, PhD, Stephen R. Wisniewski, PhD, Richard K. Zimmerman, MD, Charles R. Rinaldo, Jr, MD, Arlene Bullotta, Joe Suyama, MD, Evelyn Reis, MD, Donald B. Middleton, MD, Rachel Hess, MD; Mark Thompson, PhD, Alicia M. Fry, MD, Swathi N. Thaker, PhD, Sarah Spencer, PhD, Po-Yung Cheng, PhD, David K. Shay, MD, LaShondra Berman, MS, Joseph Bresee, MD, Erin Burns, MA, Jerome Tokars, MD, Nancy Cox, PhD


Morbidity and Mortality Weekly Report. 2013;62(2):32-35. 

In This Article


In the United States, annual vaccination against seasonal influenza is recommended for all persons aged ≥6 months.[1] Each season since 2004–05, CDC has estimated the effectiveness of seasonal influenza vaccine to prevent influenza-associated, medically attended acute respiratory infection (ARI). This season, early data from 1,155 children and adults with ARI enrolled during December 3, 2012–January 2, 2013 were used to estimate the overall effectiveness of seasonal influenza vaccine for preventing laboratory-confirmed influenza virus infection associated with medically attended ARI. After adjustment for study site, but not for other factors, the estimated vaccine effectiveness (VE) was 62% (95% confidence intervals [CIs] = 51%–71%). This interim estimate indicates moderate effectiveness, and is similar to a summary VE estimate from a meta-analysis of randomized controlled clinical trial data;[2] final estimates likely will differ slightly. As of January 11, 2013, 24 states and New York City were reporting high levels of influenza-like illness, 16 states were reporting moderate levels, five states were reporting low levels, and one state was reporting minimal levels.[3] CDC and the Advisory Committee on Immunization Practices routinely recommend that annual influenza vaccination efforts continue as long as influenza viruses are circulating.[1] Persons aged ≥6 months who have not yet been vaccinated this season should be vaccinated. However, these early VE estimates underscore that some vaccinated persons will become infected with influenza; therefore, antiviral medications should be used as recommended for treatment in patients, regardless of vaccination status. In addition, these results highlight the importance of continued efforts to develop more effective vaccines.

To make these interim 2012–13 VE estimates, prospective enrollment of patients at any of the outpatient medical facilities affiliated with the five study sites of the U.S. Influenza Vaccine Effectiveness (Flu VE) Network* began after at least 2 consecutive weeks of laboratory-confirmed cases of influenza were identified through local surveillance.† Details of methods used by the U.S. Flu VE Network have been published previously.[4] Trained study staff members reviewed appointment schedules and lists of clinical symptoms and complaints to identify patients with ARI and approached potentially eligible patients (or parents/guardians) to complete a brief screening survey. Patients were eligible for enrollment if they 1) were aged ≥6 months on September 1, 2012, and thus were eligible for vaccination; 2) reported an ARI with onset ≤7 days earlier; and 3) had not been treated with influenza antiviral medication (e.g., oseltamivir). Consenting participants completed an enrollment interview. Because date and type of vaccination were not available for this early estimate, participants were considered vaccinated if they reported having received at least 1 dose of any 2012–13 influenza vaccine before enrollment. At one study site, vaccine receipt was confirmed by a real-time Internet-based vaccine registry (https://www.recin.org) that captures 95% of all influenza vaccinations in that population.[5]

Respiratory specimens were collected from each patient using nasal and oropharyngeal swabs, which were placed together in a single cryovial with viral transport medium. Only nasal swabs were collected for patients aged <2 years. Specimens were tested at U.S. Flu VE Network laboratories using CDC's real-time reverse transcription–polymerase chain reaction (rRT-PCR) protocol for detection and identification of influenza viruses using dual-labeled probe chemistry. VE was estimated as 100% X (1 – odds ratio) using logistic regression, adjusting for study site.[4] Stratified analyses were performed by influenza virus type.

Of the 1,155 children and adults with ARI enrolled during December 3, 2012–January 2, 2013, at the five study sites, 416 (36%) tested positive for influenza A or B virus by rRT-PCR; 236 (57%) of these were influenza A, and 180 (43%) were influenza B viruses ( Table 1 ); among 158 subtyped influenza A viruses, all (100%) were influenza A (H3N2) viruses. The 2012–13 seasonal influenza vaccination rate among influenza cases was 32%, compared with 56% among influenza-negative controls. The overall VE (adjusted for study site) for all ages against influenza A and B virus infection associated with medically attended ARI was 62% (CI = 51%–71%) ( Table 2 ). The vaccination rate was lower among influenza B cases (26%) than influenza A cases (37%). The stratified VE against influenza A was 55% (CI = 39%–67%) and against influenza B was 70% (CI = 56%–80%).

* The U.S. Flu VE Network includes Group Health Cooperative (Seattle, Washington), the Marshfield Clinic Research Foundation (Marshfield, Wisconsin), the University of Michigan School of Public Health (the University of Michigan School of Public Health, partnered with the University of Michigan Health System, Ann Arbor, and the Henry Ford Health System, Detroit), the University of Pittsburgh Schools of the Health Sciences (the University of Pittsburgh Schools of the Health Sciences, partnered with UPMC, Pittsburgh, Pennsylvania), and Scott and White Healthcare (Temple, Texas).
† Beginning enrollment dates were December 3, 2012, in Pittsburgh; December 9, 2012, in Temple; December 17, 2012, in Marshfield and Ann Arbor; December 26, 2012, in Seattle; and January 2, 2013, in Detroit.