HealthDay News — Most influenza viruses identified in the 2017 to 2018 season are influenza A, with A(H3N2) viruses predominating, according to research published in an issue of the US Centers for Disease Control and Prevention’s (CDC) Morbidity and Mortality Weekly Report.

Alicia P. Budd, MPH, from the CDC in Atlanta, and colleagues examined influenza activity in the United States from October 1, 2017, through February 3, 2018.1 The researchers observed an increase in influenza activity in early November 2017, followed by a sharp increase from December through February 3, 2018. The most commonly identified viruses were influenza A viruses, with A(H3N2) viruses predominating; influenza A(H1N1)pdm09 and influenza B viruses were also reported. A total of 17,101 laboratory-confirmed influenza-related hospitalizations were reported during the study period (cumulative incidence, 59.9 per 100,000 population).

Brendan Flannery, PhD, also from the CDC, and colleagues used data from 4562 children and adults enrolled in the US Influenza Vaccine Effectiveness Network during November 2, 2017, to February 3, 2018.2 The researchers found that the overall adjusted vaccine effectiveness (VE) against influenza A and influenza B virus infection correlated with medically-attended acute respiratory illness was 36%. A total of 69% of influenza infections were caused by A(H3N2) viruses. VE was estimated to be 25%, 67%, and 42% against illness caused by influenza A(H3N2) viruses, A(H1N1)pdm09 viruses, and influenza B viruses, respectively.

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“These early VE estimates underscore the need for ongoing influenza prevention and treatment measures,” Dr Flannery and colleagues wrote. “Even with current vaccine effectiveness estimates, vaccination will still prevent influenza illness, including thousands of hospitalizations and deaths.”


  1. Budd AP, Wentworth DE, Blanton L, et al. Update: influenza activity — United States, October 1, 2017-February 3, 2018. MMWR Morb Mortal Wkly Rep. 2018;67(6):169-179.
  2. Flannery B, Chung JR, Belongia EA, et al. Interim estimates of 2017-18 seasonal influenza vaccine effectiveness — United States, February 2018. MMWR Morb Mortal Wkly Rep. 2018;16(67):180-185.