Update: Influenza Activity — United States and Worldwide, May 20–October 13, 2018

Eric J. Chow, MD; C. Todd Davis, PhD; Anwar Isa Abd Elal; Noreen Alabi, MPH; Eduardo Azziz-Baumgartner, MD; John Barnes, PhD; Lenee Blanton, MPH; Lynnette Brammer, MPH; Alicia P. Budd, MPH; Erin Burns, MA; William W. Davis, DrPH; Vivien G. Dugan, PhD; Alicia M. Fry, MD; Rebecca Garten, PhD; Lisa A. Grohskopf, MD; Larisa Gubareva, PhD; Yunho Jang, PhD; Joyce Jones, MS; Krista Kniss, MPH; Stephen Lindstrom, PhD; Desiree Mustaquim, MPH; Rachael Porter, MPH; Melissa Rolfes, PhD; Wendy Sessions, MPH; Calli Taylor, MPH; David E. Wentworth, PhD; Xiyan Xu, MD; Natosha Zanders, MS; Jacqueline Katz, PhD; Daniel Jernigan, MD

Disclosures

Morbidity and Mortality Weekly Report. 2018;67(42):1178-1185. 

In This Article

United States

The U.S. influenza surveillance system is a collaboration between CDC and federal, state, local, and territorial partners and uses eight data sources to collect influenza information, six of which operate year-round. During May 20–October 13, U.S. clinical laboratories tested 197,295 respiratory specimens for influenza, and 2,763 (1.4%) were positive (Figure 1), including 1,801 (65.2%) that were positive for influenza A viruses and 962 (34.8%) that were positive for influenza B viruses. Public health laboratories in the United States tested 5,863 respiratory specimens for influenza viruses; among these, 587 were positive for seasonal influenza viruses (Figure 2), including 442 (75.34%) positive for influenza A viruses and 145 (24.7%) for influenza B viruses. Influenza B viruses were more commonly detected than influenza A viruses from May until mid-June, whereas influenza A predominated from late June onward. A total of 400 (90.5%) of the seasonal influenza A viral specimens were subtyped by public health laboratories; among these, 233 (58.3%) were influenza A(H1N1)pdm09, and 167 (41.8%) were influenza A(H3N2). Of the 118 (81.4%) influenza B viruses for which lineage was determined, 94 (79.7%) belonged to the B/Yamagata lineage and 24 (20.3%) to the B/Victoria lineage. CDC received reports of a small number of influenza outbreaks during the summer, including domestic origin outbreaks along with influenza virus infection identified in returning international travelers.

Figure 1.

Number*and percentage of respiratory specimens testing positive for influenza reported by clinical laboratories, by influenza virus type and surveillance week — United States, October 1, 2017–October 13, 2018
*A total of 238,440 (16.4%) of 1,452,986 tested were positive during October 1, 2017–October 13, 2018.
As of October 19, 2018.

Figure 2.

Number* of respiratory specimens testing positive for influenza reported by public health laboratories, by influenza virus type, subtype/lineage, and surveillance week — United States, October 1, 2017–October 13, 2018
*N = 54,920.
As of October 19, 2018.

During May 20–October 13, data obtained from the U.S. Outpatient Influenza-Like Illness Surveillance Network (ILINet) indicated that the weekly percentage of outpatient visits to health care providers for influenza-like illness (ILI)** remained below the national baseline†† of 2.2%, ranging from 0.6% to 1.4%. All regions remained below their region-specific ILI baselines. During the first 2 weeks of October, ILI activity levels§§ for all reporting jurisdictions were minimal and, although a small number of jurisdictions have reported the geographic spread of influenza activity¶¶ as local, approximately 60% of all reporting jurisdictions reported sporadic activity. Data from CDC's National Center for Health Statistics Mortality Surveillance System indicated that the percentage of deaths attributed to pneumonia and influenza remained below the epidemic threshold*** during this period. Of the 183 influenza-associated pediatric deaths reported to CDC that occurred during the 2017–18 influenza season, five occurred during May 20–September 29. The first influenza-associated pediatric death occurring during the 2018–19 season was reported to CDC in mid-October.

The U.S. 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/overview.htm.
**Defined as a fever (temperature ≥100°F [≥37.8°C]), oral or equivalent, and cough and/or sore throat, without a known cause other than influenza.
††The national and regional baselines are the mean percentage of visits for influenza-like illness (ILI) during noninfluenza weeks for the previous three seasons plus two standard deviations. Noninfluenza weeks are defined as periods of ≥2 consecutive weeks in which each week accounted for <2% of the season's total number of specimens that tested positive for influenza in public health laboratories. National and regional percentages of patient visits for ILI are weighted based on state population. Use of the national baseline for regional data are not appropriate.
§§Activity levels are based on the percentage of outpatient visits in a jurisdiction attributed to ILI and are compared with the average percentage of ILI visits that occur during weeks with little or no influenza virus circulation. Activity levels range from minimal, corresponding to ILI activity from outpatient clinics at or below the average, to high, corresponding to ILI activity from outpatient clinics much higher than the average. Because the clinical definition of ILI is nonspecific, not all ILI is caused by influenza; however, when combined with laboratory data, the information on ILI activity provides a clearer picture of influenza activity in the United States.
¶¶Levels of activity are 1) no activity; 2) sporadic: isolated laboratory-confirmed influenza cases or a laboratory-confirmed outbreak in one institution, with no increase in activity; 3) local: increased ILI, or two or more institutional outbreaks (ILI or laboratory-confirmed influenza) in one region of the state, with recent laboratory evidence of influenza in that region; virus activity no greater than sporadic in other regions; 4) regional: increased ILI activity or institutional outbreaks (ILI or laboratory-confirmed influenza) in two or more outbreaks, but less than half of the regions in the state with recent laboratory evidence of influenza in those regions; and 5) widespread: increased ILI activity or institutional outbreaks (ILI or laboratory-confirmed influenza) in at least half of the regions in the state, with recent laboratory evidence of influenza in the state.
***The seasonal baseline proportion of pneumonia and influenza (P&I) deaths is projected using a robust regression procedure, in which a periodic regression model is applied to the observed percentage of deaths from P&I that were reported by the National Center for Health Statistics Mortality Surveillance System during the preceding 5 years. The epidemic threshold is set at 1.645 standard deviations above the seasonal baseline.

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