Interim Estimates of 2017–18 Seasonal Influenza Vaccine Effectiveness — United States, February 2018

In the United States, annual vaccination against seasonal influenza is recommended for all persons aged ≥6 months (1). During each influenza season since 2004–05, CDC has estimated the effectiveness of seasonal influenza vaccine to prevent laboratory-confirmed influenza associated with medically attended acute respiratory illness (ARI). This report uses data from 4,562 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network (U.S. Flu VE Network) during November 2, 2017–February 3, 2018. During this period, overall adjusted vaccine effectiveness (VE) against influenza A and influenza B virus infection associated with medically attended ARI was 36% (95% confidence interval [CI] = 27%–44%). Most (69%) influenza infections were caused by A(H3N2) viruses. VE was estimated to be 25% (CI = 13% to 36%) against illness caused by influenza A(H3N2) virus, 67% (CI = 54%–76%) against A(H1N1)pdm09 viruses, and 42% (CI = 25%–56%) against influenza B viruses. These early VE estimates underscore the need for ongoing influenza prevention and treatment measures. CDC continues to recommend influenza vaccination because the vaccine can still prevent some infections with currently circulating influenza viruses, which are expected to continue circulating for several weeks. Even with current vaccine effectiveness estimates, vaccination will still prevent influenza illness, including thousands of hospitalizations and deaths. Persons aged ≥6 months who have not yet been vaccinated this season should be vaccinated.

Methods used by the U.S. Flu VE Network have been published previously (2). 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 local surveillance identified increasing weekly influenza activity or one or more laboratory-confirmed cases of influenza per week for 2 consecutive weeks. Patients were eligible for enrollment if they 1) were aged ≥6 months on September 1, 2017, and thus were eligible for vaccination; 2) reported an ARI with cough with onset ≤7 days earlier; and 3) had not been treated with influenza antiviral medication (e.g., oseltamivir) during this illness. After obtaining informed consent from patients or from parents or guardians for their children, participants or their proxies were interviewed to collect demographic data, information on general and current health status and symptoms, and 2017–18 influenza vaccination status. Nasal and oropharyngeal swabs (or nasal swabs alone for children aged <2 years) were collected to obtain respiratory specimens; nasal and oropharyngeal swabs were placed together in a single cryovial with viral transport medium. 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. 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 the Wisconsin site); medical records and self-report (at the Washington site); or self-report only (at the Michigan, Pennsylvania, and Texas sites). VE against all influenza virus types combined and against viruses by type/subtype was estimated as 100% x (1 – odds ratio).^† Estimates were adjusted for study site, age group, sex, race/ethnicity, self-rated general health, number of days from illness onset to enrollment, and week of illness (3-week intervals) using logistic regression. Interim VE estimates for the 2017–18 season were based on patients enrolled through February 3, 2018.

Among the 4,562 children and adults with ARI enrolled at the five study sites from November 2, 2017, through February 3, 2018, a total of 1,712 (38%) tested positive for influenza virus by rRT-PCR, including 1,392 (81%) influenza A viruses and 323 (19%) influenza B viruses (Table 1). Among 1,340 subtyped influenza A viruses, 1,143 (85%) were A(H3N2) viruses and 208 (16%) were A(H1N1)pdm09 viruses. Most (98%) influenza B viruses belonged to the B/Yamagata lineage. The proportion of patients with influenza differed by study site, sex, age group, race/ethnicity, self-rated health status, and interval from illness onset to enrollment (Table 1). The percentage of patients who were vaccinated ranged from 45% to 59% among study sites and differed by sex, age group, race/ethnicity, and self-rated health status.

Among ARI patient participants, 43% of those with influenza had received the 2017–18 seasonal influenza vaccine, compared with 53% of influenza-negative participants (Table 2). After adjusting for study site, age group, sex, race/ethnicity, self-rated general health, number of days from illness onset to enrollment, and week of illness onset (3-week intervals), VE against medically attended ARI caused by all influenza virus types combined was 36% (CI = 27%–44%). VE for all ages was 25% (CI = 13% to 36%) against medically attended ARI caused by A(H3N2) virus infection, 67% (CI = 54%–76%) against influenza A(H1N1)pdm09 virus infection, and 42% (CI = 25%–56%) against influenza B virus infection. VE point estimates against medically attended influenza for all virus types varied by age group; statistically significant protection against medically attended influenza was found among children aged 6 months through 8 years (VE = 59%; CI = 44%–69%) and adults aged 18–49 years (VE = 33%; CI = 16%–47%), whereas no statistically significant protection was observed in other age groups.

As of February 3, 2018, a total of 257 influenza A(H3N2) viruses from U.S. Flu VE Network participants had been characterized by CDC; 240 (93%) belonged to either genetic group 3C.2a (226 viruses) or the related subgroup 3C.2a1 (14), whereas 17 (7%) belonged to group 3C.3a. Genetic group 3C.2a includes the A/Hong Kong/4801/2014 reference virus representing the A(H3N2) component of the 2017–18 Northern Hemisphere influenza vaccines (3).

Acknowledgments

Jennifer K. Meece, Jennifer P. King, Madalyn Palmquist, Lynn Ivacic, Carla Rottscheit, Sarah Kopitzke, Jacklyn Salzwedel, Deanna Cole, Trish Aldrich, Jennifer Anderson, Elizabeth Armagost, Cory Arnold, Marya Theresa Balinghasay, Kaleigh Bettinger Terry Foss, Dyan Friemoth, Wayne Frome, Keith Gilge, Sherri Guzinski, Tara Johnson, Julie Karl, Diane Kohnhorst, Tamara Kronenwetter Koepel, Karen McGreevey, Nidhi Mehta, Vicki Moon, Lisa Ott, Maisie Pettinger, Rebecca Pilsner, DeeAnn Polacek, Martha Presson, Emily Redmond, Megan Sauer, Eleanor Stockheimer, Patrick Stockwell, Sandy Strey, Julie Zierer, Tom Dalcher, Gregg Greenwald, Marshfield Clinic Research Institute, Marshfield, Wisconsin; Joshua G. Petrie, Lois E. Lamerato, Ryan E. Malosh, E.J. McSpadden, Hannah Segaloff, Caroline K. Cheng, Rachel Truscon, Emileigh Johnson, Anne Kaniclides, Elizabeth Alleman, Sarah Bauer, Michelle Groesbeck, Emerson Bouldin, Christoph Baker, Kimberly Berke, Mackenzie Smith, Niharika Rajesh, Kristyn Brundidge, Neha Hafeez, Jayla Jackson, Ian Anastasia, Gabriel Kadoo, University of Michigan, Ann Arbor, and Henry Ford Health System, Detroit, Michigan; G.K. Balasubramani, Todd M. Bear, Heather Eng, Samantha Ford, Edward Garofolo, Robert Hickey, Philip Iozzi, Monika Johnson, Donald B. Middleton, Krissy K. Moehling, Jonathan M. Raviotta, Evelyn C. Reis, Bret Rosenblum, Sean Saul, Theresa Sax, Michael Susick, Joe Suyama, Leonard F. Urbanski, John V. Williams, University of Pittsburgh Schools of the Health Sciences and University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania; Michael Smith, Chandni Raiyani, Lydia Clipper, Teresa Ponder, Todd Crumbaker, Mary Kylberg, Martha Zayed, Melissa Zdroik, Kimberley Walker, Marcus Volz, Arundhati Rao, Robert Fader, Lea Mallett, Hania Wehbe-Janek, Madhava Beeram, Michael Reis, Jennifer Thomas, Jaime Walkowiak, Jeremy Ray, Renee Day, Deborah Price, Jennifer Fox, Robert Probe, Baylor Scott & White Health, Texas A&M University Health Science Center College of Medicine, Temple, Texas; Erika Kiniry, Stacie Wellwood, C. Hallie Phillips, Suzie Park, Lawrence Madziwa, Matt Nguyen, Kaiser Permanente Washington Health Research Institute, Seattle, Washington; Erin Burns, Rebecca Garten, Thomas Stark, Shoshona Le, Juliana DaSilva, Influenza Division, National Center for Immunization and Respiratory Diseases, CDC.

Corresponding author: Brendan Flannery, bif4@cdc.gov, 404-718-4276.

¹Influenza Division, National Center for Immunization and Respiratory Diseases, CDC; ²Marshfield Clinic Research Institute, Marshfield, Wisconsin; ³Baylor Scott & White Health, Texas A&M University Health Science Center College of Medicine, Temple, Texas; ⁴University of Pittsburgh Schools of the Health Sciences and University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania; ⁵Kaiser Permanente Washington Health Research Institute, Seattle, Washington; ⁶University of Michigan, Ann Arbor, Michigan.

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* Defined as having received ≥1 dose of influenza vaccine ≥14 days before illness onset. A total of 102 participants who received the vaccine ≤13 days before illness onset were excluded from the study sample.
^† The chi-square statistic was used to assess differences between the numbers of persons with influenza-negative and influenza-positive test results, in the distribution of enrolled patient and illness characteristics, and in differences between groups in the percentage vaccinated.
^§ Enrollees were categorized into one of four mutually exclusive racial/ethnic populations: white, black, other race, and Hispanic. Persons identifying as Hispanic might have been of any race. Persons identifying as white, black, or other race were non-Hispanic. Race/ethnicity data were missing for 15 enrollees.
^¶ Fourteen patients had coinfection with influenza A and influenza B, making the sum 1,726, or 14 greater than the total number of influenza-positive patients.

Abbreviation: CI = confidence interval.
* Vaccine effectiveness was estimated as 100% x (1 – odds ratio [ratio of odds of being vaccinated among outpatients with influenza-positive test results to the odds of being vaccinated among outpatients with influenza-negative test results]); odds ratios were estimated using logistic regression.
^† Statistically significant at the p<0.05 level.