CDC Flu Update: Activity is Widespread and Increasing


According to this week’s FluView report, seasonal influenza activity increased again this week, reaching a new high for this season. Influenza activity, predominantly driven by influenza A(H1N1)pdm09 virus infections this season, is widespread in 48 states and Puerto Rico and influenza like illness (ILI) activity increased to 5.1%. Flu activity has been similar to what has been seen during other H1N1-predominant seasons, but indicators that track hospitalizations and deaths remain well below what was observed last season. However, this week another seven flu-related pediatric deaths were reported to CDC, bringing the total to 41 flu-related deaths in children reported to CDC for the 2018-2019 flu season.

CDC expects flu activity to remain elevated for a number of weeks. 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 February 16, 2019:

Influenza-like Illness Surveillance: For the week ending February 16 (week 7), the proportion of people seeing their health care provider for influenza-like illness (ILI) increased from 4.8% to 5.1%, which is above the national baseline of 2.2%. This is the highest ILI has been this flu season.
All 10 regions reported a proportion of outpatient visits for ILI at or above their region-specific baseline levels.
For comparison purposes, 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).
Additional ILINet data, including national, regional, and select state-level data for the current and previous seasons, can be found at
Influenza-like Illness (ILI) State Activity Indicator Map: The number of states experiencing high ILI activity increased from 26 states plus New York City last week to 30 states plus New York City. The 30 states are Alabama, Alaska, Arkansas, Colorado, Georgia, Indiana, Kansas, Kentucky, Louisiana, Maine, Massachusetts, Mississippi, Missouri, Nebraska, New Jersey, New Mexico, New York, North Carolina, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, and Wyoming. The District of Columbia and 11 states (Arizona, California, Connecticut, Delaware, Hawaii, Idaho, Illinois, Iowa, Maryland, Montana, and Oregon) experienced moderate ILI activity. Six states (Florida, Michigan, Nevada, North Dakota, South Dakota, and Wisconsin) experienced low ILI activity. The U.S. Virgin Islands and three states (Minnesota, New Hampshire, and Ohio) experienced minimal ILI activity. Data were insufficient to calculate an ILI activity level from Puerto Rico. Additional data, including data for previous seasons, can be found at

Geographic Spread of Influenza Viruses: The number of jurisdictions reporting widespread influenza activity remained the same and included Puerto Rico and 48 states (Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, Wisconsin and Wyoming). Regional influenza activity was reported by one state (West Virginia). Local influenza activity was reported by the District of Columbia. Sporadic influenza activity was reported by the U.S. Virgin Islands and one state (Hawaii). Guam did not report. Geographic spread data reflect how many areas within a state or territory are seeing flu activity. Additional data are available at:

Flu-Associated Hospitalizations: Since October 1, 2018, 7,922 laboratory-confirmed influenza-associated hospitalizations have 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 27.4 hospitalizations per 100,000 people in the United States.
The highest hospitalization rate is among adults aged 65 years and older (75.6 per 100,000) followed by children younger than 5 years (40.2 per 100,000), and adults aged 50-64 years (37.7 per 100,000). During most seasons, adults 65 years and older have the highest hospitalization rates, followed by young children.
For comparison purposes:
The final, cumulative overall hospitalization rate for week 7 last season was 78.8 per 100,000.
Over the past 5 seasons, cumulative end-of-season hospitalization rates have ranged from 31.4 per 100,000 (2015-2016) to 102.9 per 100,000 (2017-2018).
Additional data, including hospitalization rates during previous influenza seasons, can be found at and
FluSurv-Net data is used to generate national estimates of the total numbers of flu cases, medical visits, and hospitalizations. This season, CDC is reporting preliminary cumulative in-season estimates, which are available at

Mortality Surveillance: The proportion of deaths attributed to pneumonia and influenza (P&I) was 7.0% during the week ending February 9, 2019 (week 6). This percentage is below the epidemic threshold of 7.3% for week 3 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. P&I has been at or above threshold for three weeks this season.
For comparison purposes, over the last five seasons, P&I has been at or above epidemic threshold for a range of four weeks (2015-2016) to 16 weeks (2017-2018).
Additional P&I mortality data for current and past seasons and by geography (national, HHS region, or state) are available at
Pediatric Deaths: Seven influenza-associated pediatric deaths were reported to CDC during week 7 (the week ending February 16, 2019).
Four deaths were associated with influenza A(H1N1)pdm09 viruses and occurred during weeks 2, 6, and 7 (the weeks ending January 12, February 9 and February 16, 2019, respectively). Two deaths were associated with influenza A viruses for which no subtyping was performed and occurred during weeks 5 and 6 (the weeks ending February 2 and February 9, 2019, respectively). One death was associated with an influenza B virus and occurred during week 52 (the week ending December 29, 2018).
A total of 41 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 February 16 was 26.7%, an increase from 25.4% the prior week. This is the highest proportion of specimens testing positive for flu viruses in clinical laboratories this season.
For comparison purposes, since laboratory data from clinical and public health laboratories was disaggregated three seasons ago, the peak percent of respiratory specimens testing positive for flu at clinical laboratories has ranged from 23.6% to 27.4%.
Regionally, the three-week average percent of specimens testing positive for influenza in clinical laboratories ranged from 12.9% to 32.5%.
During the week ending February 16, of the 10,210 (26.7%) influenza-positive tests reported to CDC by clinical laboratories, 9,936 (97.3%) were influenza A viruses and 274 (2.7%) were influenza B viruses.
The most frequently identified influenza virus type reported by public health laboratories was influenza A(H1N1)pdm09 virus.
While influenza A(H1N1)pdm09 have predominated nationally, influenza A(H3N2) viruses have predominated in the southeastern United States (Region 4). During the most recent three weeks, influenza A(H3) viruses were reported more frequently than influenza A(H1N1)pdm09 viruses in Regions 6 and 7 and influenza A(H1N1)pdm09 and influenza A(H3) viruses were reported in approximately equal numbers in Region 2.
During the week ending February 16, 1,058 (98.4%) of the 1,075 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 17 (1.6%) were influenza B viruses. Of the 996 influenza A viruses that were subtyped, 467 (46.9%) were H3N2 viruses and 529 (53.1%) were (H1N1)pdm09 viruses.
The majority of the influenza viruses collected from the United States during September 30, 2018, through February 16, 2019, were characterized antigenically and genetically as being similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses.
The vast majority (>99%) of influenza viruses tested showed susceptibility to oseltamivir, zanamivir and peramivir. This week, no new viruses with reduced susceptibility to antiviral drugs were reported. So far this season, two (0.2%) influenza A(H1N1)pdm09 viruses displayed highly reduced inhibition by oseltamivir and peramivir. An additional two (0.2%) influenza A(H1N1)pdm09 viruses showed reduced inhibition by oseltamivir. All influenza viruses tested showed susceptibility to zanamivir.

Source: CDC

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