Influenza Activity — United States and Worldwide, May 19–September 28, 2019, and Composition of the 2020 Southern Hemisphere Influenza Vaccine

Scott Epperson, DVM; C. Todd Davis, PhD; Lynnette Brammer, MPH; Anwar Isa Abd Elal; Noreen Ajayi, MPH; John Barnes, PhD; Alicia P. Budd, MPH; Erin Burns, MA; Peter Daly, MPH; Vivien G. Dugan, PhD; Alicia M. Fry, MD; Yunho Jang, PhD; Sara Jo Johnson, MPH; Krista Kniss, MPH; Rebecca Kondor, PhD; Lisa A. Grohskopf, MD; Larisa Gubareva, PhD; Angiezel Merced-Morales, MPH; Wendy Sessions, MPH; James Stevens, PhD; David E. Wentworth, PhD; Xiyan Xu, MD; Daniel Jernigan, MD


Morbidity and Mortality Weekly Report. 2019;68(40):880-884. 

In This Article

Surveillance Update: United States and Worldwide

The U.S. Influenza Surveillance System is a collaboration between CDC and federal, state, local, and territorial partners and uses eight data sources, six of which operate year-round, to collect clinical and laboratory information on influenza. During May 19–September 28, 2019 (surveillance weeks 21–39), public health laboratories in the United States tested 7,637 respiratory specimens for influenza viruses; 1,737 (22.7%) were positive (Figure 1), including 1,213 (69.8%) for influenza A viruses and 524 (30.2%) for influenza B viruses. Among the 1,154 seasonal influenza A-positive specimens that were subtyped, 324 (28.1%) were influenza A(H1N1)pdm09, and 830 (71.9%) were influenza A(H3N2). Among the 440 influenza B viruses for which lineage was determined, 413 (93.9%) belonged to the B/Victoria lineage and 27 (6.1%) to the B/Yamagata lineage.

Figure 1.

Number of respiratory specimens testing positive for influenza* reported by public health laboratories, by influenza virus type, subtype/lineage, and surveillance week — United States, September 30, 2018–September 28, 2019
* N = 45,619.
As of October 4, 2019.

During May 19–September 28, 2019, the weekly percentage of outpatient visits to health care providers for influenza-like illness (ILI) from the U.S. Outpatient Influenza-Like Illness Surveillance Network (ILINet) was below the national baseline, and all regions were below their region-specific baselines. One human infection with a novel influenza A virus§ was reported, an influenza A(H1N1) variant virus. This virus had hemagglutinin (HA) and neuraminidase gene segments derived from the seasonal human influenza A(H1N1)pdm09 virus that were likely introduced into swine by a recent reverse zoonosis and were closely related to influenza A(H1N1) viruses now circulating in the U.S. swine population. The percentage of deaths attributed to pneumonia and influenza from CDC's National Center for Health Statistics Mortality Surveillance System was below the epidemic threshold during this period. Five influenza-associated pediatric deaths occurring during this period were reported to CDC. Additional information on influenza surveillance methods is available at https://www.cdc.gov/flu/weekly/overview.htm, and a full description of U.S. influenza activity over the summer months is available in the influenza surveillance report, FluView (https://www.cdc.gov/flu/weekly/).

The timing of influenza activity and the predominant circulating virus in the Southern Hemisphere during May 19–September 28, 2019 varied by region. Influenza A(H3N2) viruses were predominant in most regions; however, influenza A(H1N1)pdm09 and influenza B/Victoria viruses predominated in several countries. Additional information on global influenza virus circulation is available at https://www.who.int/influenza/surveillance_monitoring/updates/en/.

The CDC influenza surveillance system collects five categories of information from eight data sources: 1) viral surveillance (U.S. World Health Organization collaborating laboratories, the National Respiratory and Enteric Virus Surveillance System, and novel influenza A virus case reporting); 2) outpatient illness surveillance (U.S. Outpatient Influenza-Like Illness Surveillance Network); 3) mortality (the National Center for Health Statistics Mortality Surveillance System and influenza-associated pediatric mortality reports); 4) hospitalizations (FluSurv-NET, which includes the Emerging Infections Program and surveillance in three additional states); and 5) summary of the geographic spread of influenza (state and territorial epidemiologist reports). https://www.cdc.gov/flu/weekly/fluactivitysurv.htm.
§Influenza viruses that circulate in swine are called swine influenza viruses when isolated from swine but are called variant influenza viruses when isolated from humans. Seasonal influenza viruses that circulate worldwide in the human population have important antigenic and genetic differences form influenza viruses circulating in swine. https://www.cdc.gov/flu/swineflu/variant/preventspreadfactsheet.htm.
In temperate climates, the onset and peak of influenza activity might vary substantially from one influenza season to the next, but generally begins to increase in the late fall. In the Northern Hemisphere's temperate regions, annual epidemics of influenza typically occur during October–February, but the peak of influenza activity can occur as late as April or May. In temperate regions of the Southern Hemisphere, influenza activity typically peaks during May–August. Although temperate regions of the world experience a seasonal peak in influenza activity, influenza viruses can be isolated year-round. The timing of seasonal peaks in influenza activity in tropical and subtropical countries varies by region. Multiple peaks of activity during the same year have been observed in some areas, and influenza infection can occur year-round.