In the United States, annual vaccination against seasonal influenza is recommended for all persons aged≥ 6 months (1). Each influenza season since 2004–05, CDC has estimated the effectiveness of seasonal influenza vaccine to prevent influenza-associated, medically attended, acute respiratory illness (ARI). This report uses data, as of February 4, 2017, from 3,144 children and adults enrolled in the US Influenza Vaccine Effectiveness Network (US Flu VE Network) during November 28, 2016–February 4, 2017, to estimate an interim adjusted effectiveness of seasonal influenza vaccine for preventing laboratory-confirmed influenza virus infection associated with medically attended ARI. During this period, overall vaccine effectiveness (VE)(adjusted for study site, age group, sex, race/ethnicity, self-rated general health, and days from illness onset to enrollment) against influenza A and influenza B virus infection associated with medically attended ARI was 48%(95% confidence interval [CI]= 37%–57%). Most influenza infections were caused by A (H3N2) viruses. VE was estimated to be 43%(CI= 29%–54%) against illness caused by influenza A (H3N2) virus and 73%(CI= 54%–84%) against influenza B virus. These interim VE estimates indicate that influenza vaccination reduced the risk for outpatient medical visits by almost half. Because influenza activity remains elevated (2), CDC and the Advisory Committee on Immunization Practices recommend that annual influenza vaccination efforts continue as long as influenza viruses are circulating (1). Vaccination with 2016–17 influenza vaccines will reduce the number of infections with most currently circulating influenza viruses. Persons aged≥ 6 months who have not yet been vaccinated this season should be vaccinated as soon as possible.

Methods used by the US Flu VE Network have been published previously (3). At five study sites, patients aged≥ 6 months seeking outpatient medical care for an ARI with cough, within 7 days of illness onset, were enrolled.* Study enrollment began after≥ 1 laboratory-confirmed cases of influenza were identified through local surveillance for≥ 2 consecutive weeks. Patients were eligible for enrollment if they 1) were aged≥ 6 months on September 1, 2016, and thus eligible for vaccination; 2) reported an ARI with cough and onset≤ 7 days earlier; and 3) had not been treated with influenza antiviral medication (eg, oseltamivir) during this illness. After obtaining informed consent from patients or parents/guardians for their children, participants or their proxies were interviewed to collect demographic data, general and current health status, symptoms, and 2016–17 influenza vaccination status. 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 US Flu VE Network laboratories using CDC’s real-time reverse transcription–polymerase chain reaction (rRT-PCR) protocol for detection and identification of influenza viruses. Participants (including children aged< 9 years who require 2 vaccine doses during their first vaccination season) were considered vaccinated if they received≥ 1 dose of any seasonal influenza vaccine≥ 14 days before illness onset, according to medical records and registries (at Wisconsin site), medical records and self-report (at Texas and Washington sites), or self-report only (Michigan and Pennsylvania sites). VE was estimated as 100% x (1–odds ratio).† Estimates were adjusted for study site, age group, sex, race/ethnicity, self-rated general health …