CDC Surveillance Information for the 2017-2018 Influenza Season in the United States

The 2017 to 2018 influenza season in the United States (October 1, 2017-May 19, 2018) was a high-severity season with high levels of emergency department visits, according to a recently published Morbidity and Mortality Weekly Report. Influenza A (H3N2) viruses were the predominant influenza strain through February 2018, with influenza B viruses predominating starting from March 2018.

The Centers for Disease Control and Prevention (CDC) compiles and analyzes data on influenza activity and viruses in the United States. In 2017, the CDC began using a new methodology to classify influenza severity, using 3 indicators: the percentage of visits to outpatient clinics for influenza-like illness (ILI) from the US Outpatient Influenza-like Illness Surveillance Network (ILINet), the rates of influenza-associated hospitalizations through the Influenza Hospitalizations Surveillance Network (FluSurv-Net), and the percentage of deaths resulting from pneumonia or influenza from the CDC’s National Center for Health Statistics.

This report summarizes US influenza activity from October 1, 2017, to May 19, 2018. According to this method, the severity of the 2017 to 2018 season was classified as high severity overall, as well as high severity for each age group.

During October 1, 2017, to May 19, 2018, public health laboratories tested 98,446 specimens, finding 54.6% positive for influenza virus (71.2% for influenza A; 28.8% for influenza B).

Among positive influenza A specimens, 84.9% were influenza A(H3N2) and 15.1% were influenza A(H1N1)pdm09. Among positive influenza B specimens, 88.8% were B/Yamagata lineage and 11.2% were B/Victoria lineage.

Interim effectiveness of the 2017 to 2018 inactivated influenza vaccines published in February 2018 were: 36% overall, 25% against illness caused by influenza A(H3N2), 67% against illness caused by influenza A(H1N1)pdm09, and 42% against illness caused by influenza B viruses.

In addition, influenza viruses were tested for resistance to influenza neuraminidase inhibitor antiviral medications recommended for use against seasonal influenza (oseltamivir, peramivir, and zanamivir). Of the tested viruses for influenza A(H3N2) and influenza B, no resistance was detected for any of the 3 medications. Of the A(H1N1)pdm09 tested viruses, none were zanamivir-resistant, but 1.0% were resistant to both oseltamivir and peramivir.

Nationally, the weekly percentage of outpatient visits for ILI to healthcare providers participating in ILINet was at or above the national baseline level of 2.2% for 19 consecutive weeks during the 2017 to 2018 season. The percentage of outpatient ILI visits exceeded 7.0% for 3 consecutive weeks, peaking at 7.5%.

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A total of 30,453 laboratory-confirmed influenza-related hospitalizations through FluSurv-NET were reported. The hospitalization rate was highest among persons aged ≥65 years, who accounted for approximately 58% of reported influenza-associated hospitalizations.

Among all influenza-associated hospitalizations, 72.3% were for influenza A virus infections (83.8% H3N2; 16.2% [H1N1]pdm09), 27.0% for influenza B virus infections, 0.4% for influenza A virus and influenza B virus coinfections, and 0.3% for an influenza virus for which no type testing was done.

Furthermore, there were 171 pediatric deaths reported this season, approximately half in otherwise healthy children. Less than one fourth (22%) of vaccine-eligible children who died from influenza this season had received influenza vaccine before illness onset.

The recommended composition of the 2018 to 2019 influenza trivalent vaccine contains an A/Michigan/45/2015 A(H1N1)pdm09-like virus, an A/Singapore/INFIMH-16-0012/2016 A(H3N2)-like virus, and a B/Colorado/06/2017-like (B/Victoria lineage) virus; the quadrivalent vaccine recommendation includes the trivalent vaccine viruses, as well as a B/Phuket/3073/2013-like (B/Yamagata lineage) virus.

Overall, the study authors conclude that, “testing for seasonal influenza viruses and monitoring for novel influenza A virus infections should continue year-round.”


Garten R, Blanton L, Elal AIA, et al. Update: influenza activity in the United States during the 2017-18 season and composition of the 2018-19 influenza vaccine. Morb Mortal Wkly Rep. 2018;67(22):634-642.