Update: Influenza Activity in the United States During the 2018-19 Season and Composition of the 2019-20 Influenza Vaccine

Abstract

Influenza activity* in the United States during the 2018-19 season (September 30, 2018-May 18, 2019) was of moderate severity (1). Nationally, influenza-like illness (ILI)^† activity began increasing in November, peaked during mid-February, and returned to below baseline in mid-April; the season lasted 21 weeks,^§ making it the longest season in 10 years. Illness attributed to influenza A viruses predominated, with very little influenza B activity. Two waves of influenza A were notable during this extended season: influenza A(H1N1)pdm09 viruses from October 2018 to mid-February 2019 and influenza A(H3N2) viruses from February through May 2019. Compared with the 2017-18 influenza season, rates of hospitalization this season were lower for adults, but were similar for children. Although influenza activity is currently below surveillance baselines, testing for seasonal influenza viruses and monitoring for novel influenza A virus infections should continue year-round. Receiving a seasonal influenza vaccine each year remains the best way to protect against seasonal influenza and its potentially severe consequences.

FIGURE 1

Number of respiratory specimens testing positive for influenza reported to CDC by public health laboratories, by influenza virus type, subtype, and surveillance week – United States, September 30, 2018–May 18, 2019 * N = 40,674. ^† 1,629 influenza A viruses not subtyped are excluded. ^§ As of June 14, 2019.

FIGURE 2

Percentage of outpatient visits for influenza-like illness (ILI) reported to CDC, by surveillance week — U.S. Outpatient Influenza-like Illness Surveillance Network, 2018–19 influenza season and selected previous influenza seasons * Defined as fever (temperature of ≥100°F [≥37.8°C], oral or equivalent) and cough or sore throat, without a known cause other than influenza. ^† As of June 14, 2019.