CDC Flu Update: Activity Remains Elevated


According to this week’s FluView report, seasonal influenza activity remains elevated and is similar to overall activity last week. Nationally, influenza-like illness (ILI) activity remains at 5.0%, and influenza activity is widespread in 49 states and Puerto Rico. Cumulatively, influenza A(H1N1)pdm09 viruses remain predominant for this flu season, however, the number of influenza A(H3) viruses continues to increase and, for the most recent week, accounts for slightly more than half of the subtyped influenza A viruses reported to CDC. Severity indicators remain substantially lower than what was observed last season. However, this week another 15 flu-related pediatric deaths were reported to CDC, bringing the total to 56 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 23, 2019:

Influenza-like Illness Surveillance: For the week ending February 23 (week 8), the proportion of people seeing their health care provider for influenza-like illness (ILI) remained at 5.0%, which is above the national baseline of 2.2%.
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 30 states plus New York City last week to 33 states plus New York City. The 33 states are Alabama, Alaska, Arizona, Arkansas, Colorado, Georgia, Illinois, Indiana, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Mississippi, Missouri, Montana, Nebraska, Nevada, New Jersey, New Mexico, New York, North Carolina, North Dakota, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, Texas, Utah, Virginia, West Virginia, and Wyoming. The District of Columbia and eight states (California, Connecticut, Hawaii, Idaho, Iowa, Maine, South Dakota, and Vermont) experienced moderate ILI activity. Puerto Rico and eight states (Delaware, Florida, Michigan, Minnesota, Ohio, Tennessee, Washington, and Wisconsin) experienced low ILI activity. One state (New Hampshire) experienced minimal ILI activity. Data were insufficient to calculate an ILI activity level from the U.S. Virgin Islands. Additional data, including data for previous seasons, can be found at

Geographic Spread of Influenza Viruses: The number of jurisdictions reporting widespread influenza activity increased from 48 states and Puerto Rico to Puerto Rico and 49 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, West Virginia, Wisconsin and Wyoming). Local influenza activity was reported by the District of Columbia and one state (Hawaii). Sporadic influenza activity was reported by the U.S. Virgin Islands. 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, 9,274 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 32.1 hospitalizations per 100,000 people in the United States
The highest hospitalization rate is among adults aged 65 years and older (91.5 per 100,000) followed by children younger than 5 years (45.5 per 100,000), and adults aged 50-64 years (43.2 per 100,000). During most seasons, adults 65 years and older have the highest hospitalization rates, followed by young>
For comparison purposes:
The final, cumulative overall hospitalization rate for week 8 last season was 84.0 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 7.1% during the week ending February 16, 2019 (week 7). This percentage is below the epidemic threshold of 7.3% for week 7 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. P&I has been at or above threshold for two 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: 15 influenza-associated pediatric deaths were reported to CDC during week 8 (the week ending February 23, 2019).
Eight deaths were associated with influenza A(H1N1)pdm09 viruses and occurred during weeks 5, 6, 7 and 8 (the weeks ending February 2, February 9, February 16 and February 23, 2019, respectively). One death was associated with an influenza A(H3) virus and occurred during week 6 (the week ending February 9, 2019). Six deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 5, 6, 7 and 8.
A total of 56 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 23 was 26.4%, a slight increase from 26.2% 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 12.6% to 33.2%.
During the week ending February 23, of the 10,316 (26.4%) influenza-positive tests reported to CDC by clinical laboratories, 10,027 (97.2%) were influenza A viruses and 289 (2.8%) were influenza B viruses.
Nationally, during the week ending February 23, influenza A(H3) viruses were reported more frequently than influenza A(H1N1)pdm09 viruses. However, cumulatively, influenza A(H1N1)pdm09 viruses have predominated nationally for the 2018-2019 season.
In recent weeks H3N2 viruses have accounted for a growing proportion of influenza viruses detected nationally and, among viruses collected during the last three weeks, were predominant in Regions 2, 4, 6 and 7.
During the week ending February 23, 1,128 (98.0%) of the 1,151 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 23 (2.0%) were influenza B viruses. Of the 1,077 influenza A viruses that were subtyped, 583 (54.1%) were H3N2 viruses and 494 (45.9%) were (H1N1)pdm09 viruses.
The majority of the influenza viruses collected from the United States during September 30, 2018, through February 23, 2019, were characterized antigenically and genetically as being similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses.
o 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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