CDC Flu Update: Activity Continues to Increase Nationwide

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According to this week’s FluView report, seasonal influenza activity is elevated nationally and continues to increase. The number of states experiencing high influenza activity went from two during the last reporting week to nine plus New York City. CDC also reported another four flu-associated pediatric deaths. Nationally, the proportion of people seeing their health care provider for influenza-like-illness (ILI) has been at or above the baseline for five consecutive weeks. By this measure, the past five flu seasons have lasted between 11 and 20 weeks with an average of 16 weeks. H1N1 viruses have been the most commonly identified flu viruses nationally. However, in the most recent four weeks, H3N2 viruses have predominated in the southeastern region of the United States.

An annual flu vaccine is the best way to protect against influenza and its potentially serious complications. There are many benefits to vaccination, including reducing the risk of flu illness, doctor’s visits, hospitalization, and even death in children. Flu vaccination also has been shown to reduce severity of illness among people who get vaccinated but still get sick. For anyone 6 months or older who has not yet been vaccinated this season, CDC recommends that they get vaccinated now. There also are flu antiviral drugs that can be used to treat flu illness. Below is a summary of the key flu indicators for the week ending December 22, 2018:

Influenza-like Illness Surveillance: For the week ending December 22 (week 51), the proportion of people seeing their health care provider for influenza-like illness (ILI) was 3.3%, which is above the national baseline of 2.2%. Over the past five flu seasons, the peak percent of visits due to ILI has ranged between 3.6% (2015-2016) and 7.5% (2017-2018). Nine of 10 regions (Regions 1, 2, 3, 4, 5, 6, 7, 8, and 9) reported a proportion of outpatient visits for ILI at or above their region-specific baseline level. Additional ILINet data, including national, regional, and select state-level data for the current and previous seasons, can be found at

Influenza-like Illness State Activity Indicator Map: New York City and nine states (Alabama, Colorado, Georgia, Kentucky, Louisiana, Maryland, New Jersey, New Mexico, and South Carolina) experienced high ILI activity. Puerto Rico and seven states (Arkansas, Arizona, Indiana, Mississippi, Oklahoma, Utah, and Virginia) experienced moderate ILI activity. 11 states (California, Illinois, Massachusetts, Minnesota, Missouri, North Carolina, Nevada, New York, Pennsylvania, Texas, and Wisconsin) experienced low ILI activity. The District of Columbia and 22 states experienced minimal ILI activity. Data were insufficient to calculate an ILI activity level for one state (Alaska). Additional data, including data for previous seasons, can be found at

Geographic Spread of Influenza Viruses: Widespread influenza activity was reported by Guam and 11 states (Arizona, California, Connecticut, Delaware, Florida, Georgia, Massachusetts, Nebraska, New Mexico, New York, and North Carolina). Regional influenza activity was reported by Puerto Rico and 19 states (Alabama, Colorado, Idaho, Illinois, Indiana, Kentucky, Louisiana, Montana, Nevada, New Hampshire, New Jersey, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Texas, Utah, and Vermont).. Local influenza activity was reported by 15 states (Arkansas, Iowa, Kansas, Michigan, Minnesota, Mississippi, Missouri, North Dakota, Oregon, South Dakota, Virginia, Washington, West Virginia, Wisconsin and Wyoming). Sporadic influenza activity was reported by the District of Columbia, the U.S. Virgin Islands and three states (Alaska, Hawaii, and Maine). Two states (Maryland and Tennessee) did not report. Geographic spread data show how many areas within a state or territory are seeing flu activity. Additional data are available at:


Flu-Associated Hospitalizations: Since October 1, 2018, 1,047 laboratory-confirmed influenza-associated hospitalizations have now been reported through the Influenza Hospitalization Network (FluSurv-NET), a population-based surveillance network for laboratory-confirmed influenza-associated hospitalizations covering approximately 9% of the U.S. This translates to a cumulative overall rate of 3.6 hospitalizations per 100,000 people in the United States.
The highest hospitalization rate is among children younger than 5 years (10.0 per 100,000) followed by adults aged 65 years and older (8.0 per 100,000), and adults aged 50-64 years (4.1 per 100,000). During most seasons, adults 65 years and older have the highest hospitalization rates followed by young children.
Additional data, including hospitalization rates during previous influenza seasons, can be found at and

Mortality Surveillance: The proportion of deaths attributed to pneumonia and influenza (P&I) was 6.2% during the week ending December 15, 2018 (week 50). This percentage is below the epidemic threshold of 6.8% for week 50 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. Additional P&I mortality data for current and past seasons and by geography (national, HHS region, or state) are available at
Pediatric Deaths: Four influenza-associated pediatric deaths were reported to CDC during week 51 (the week ending December 22, 2018). One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 49 (the week ending December 8, 2018).Three deaths were associated with an influenza A(H1N1)pdm09 virus and occurred during weeks 50 and 51 (the weeks ending December 15 and December 22, 2018, respectively). A total of eleven influenza-associated pediatric deaths have been reported for the 2018-2019 season.
Additional information on influenza-associated pediatric deaths reported during past seasons, including basic demographics, underlying conditions, bacterial co-infections, and place of death is available on FluView Interactive at: More detailed information about pediatric deaths reported during the current season will be available later in the season.

Laboratory Data:
Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending December 22 was 15.6%.
Regionally, the three-week average percent of specimens testing positive for influenza in clinical laboratories ranged from 4.1% to 19.9%.
During the week ending December 22, of the 3,651 (15.6%) influenza-positive tests reported to CDC by clinical laboratories, 3,529 (96.7%) were influenza A viruses and 122 (3.3%) were influenza B viruses.
The most frequently identified influenza A virus subtype reported by public health laboratories was influenza A(H1N1)pdm09 virus.
During the week ending December 22, 428 (96.2%) of the 445 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 17 (3.8%) were influenza B viruses. Of the 397 influenza A viruses that were subtyped, 43 (10.8%) were H3N2 viruses and 354 (89.2%) were (H1N1)pdm09 viruses.
The majority of the influenza viruses collected from the United States during September 30, 2018 through December 22, 2018 were characterized antigenically and genetically as being similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses.

Source: CDC