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While influenza activity continued to decrease in the latest FluView report, it remains high across much of the United States. ILI dropped from 6.4% reported last week to 5.0%, and is now similar to ILI observed at the peak of last season. Current data indicate that the 2017-2018 flu season peaked at 7.4% in early February (during weeks 5 and 6) and is now on the decline, however 45 states plus Puerto Rico continue to report widespread flu activity and 32 states plus New York City and the District of Columbia continue to experience high influenza-like illness (ILI) activity. The overall hospitalization rate and all age-specific hospitalization rates, with the exception of children 5-17 years, are now higher than the end-of-season hospitalization rates for 2014-2015; a high severity, H3N2-predominant season. The hospitalization rate for children 5-17 is similar to that of 2014-2015. CDC also is reporting an additional 17 flu-related pediatric deaths during week 8, bringing the total number of flu-related pediatric deaths reported this season to 114. Flu activity is likely to remain elevated for several more weeks.
CDC continues to recommend influenza vaccination for all persons 6 months of age and older as flu viruses are likely to continue circulating for weeks. While H3N2 viruses remain predominant overall this season, the proportion of B viruses versus A viruses is now almost even. In recent weeks, B viruses have been increasing while H3N2 viruses have been decreasing. Early vaccine effectiveness (VE) estimates show that flu vaccine has reduced the risk of having to go to the doctor due to flu by 36% overall through February 3, 2018. VE against H3N2 viruses was 25%. VE against H1N1 67% and VE against B viruses was 42%. In addition, in the context of widespread influenza activity, CDC clinicians and the public are reminded of importance of prompt treatment with influenza antiviral medications in people who are severely ill and people who are at high risk of serious flu complications who develop flu symptoms. Below is a summary of the key flu indicators for the week ending February 24, 2018 (week 8):
Influenza-like Illness Surveillance: For the week ending February 24, the proportion of people seeing their health care provider for influenza-like illness (ILI) was 5.0%, which is a decrease from last week (6.4%), but still 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. ILI has been at or above the national baseline for 14 weeks so far this season. Over the past five seasons, ILI has remained at or above baseline for 16 weeks on average. Additional ILINet data, including national, regional, and select state-level data for the current and previous seasons, can be found at http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html.
Influenza-like Illness State Activity Indicator Map: New York City, the District of Columbia, and 32 states experienced high ILI activity (Alabama, Alaska, Arizona, Arkansas, Colorado, Georgia, Illinois, Indiana, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New Hampshire, New Jersey, New Mexico, New York, North Carolina, Oklahoma, Pennsylvania, Rhode Island, South Carolina, South Dakota, Texas, Vermont, Virginia, and Wyoming). Puerto Rico and nine states experienced moderate ILI activity (California, Connecticut, Delaware, Nevada, Ohio, Oregon, Utah, West Virginia, and Wisconsin). Six states experienced low ILI activity (Hawaii, Idaho, Iowa, North Dakota, Tennessee, and Washington). Three states experienced minimal ILI activity (Florida, Maine, and Montana).
Additional data, including data for previous seasons, can be found at https://gis.cdc.gov/grasp/fluview/main.html.
Geographic Spread of Influenza Viruses: Widespread influenza activity was reported by Puerto Rico and 45 states (Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Utah, Virginia, Washington, West Virginia, Wisconsin, and Wyoming). Regional influenza activity was reported by Guam and two states (Minnesota and Texas). Local influenza activity was reported by the District of Columbia and three states (Hawaii, Oregon, and Vermont). No flu activity was reported by the U.S. Virgin Islands. Geographic spread data show how many areas within a state or territory are seeing flu activity.
Additional data are available at: https://gis.cdc.gov/grasp/fluview/FluView8.html.
Flu-Associated Hospitalizations: Since October 1, 2017, 23,324 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. This translates to a cumulative overall rate of 81.7 hospitalizations per 100,000 people in the United States.
The highest hospitalization rate is among people 65 years and older (350.7 per 100,000), followed by adults aged 50-64 years (88.5 per 100,000), and younger children aged 0-4 years (57.8 per 100,000). During most seasons, adults 65 years and older have the highest hospitalization rates, followed by children 0-4 years.
These rates are higher than the final week 8 rates for the 2014-2015 season for those age groups, when the hospitalization rate for people 65 years and older was 275.5 per 100,000; 44.6 per 100,000 for people 50-64 years and 48.4 per 100,000 for children 0-4 years were. Current week 8 rates are higher than end-of-season hospitalization rates for all ages (cumulative), people 65 and older, people 50-64 years, people 18-49 years, and children 0-4 years. Current rates for children 5-17 years are now similar to the end-of-season rates for 2014-2015.
Hospitalization data are collected from 13 states and represent approximately 9% of the total U.S. population. The number of hospitalizations reported does not reflect the actual total number of influenza-associated hospitalizations in the United States. Additional data, including hospitalization rates during other influenza seasons, can be found at http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html.
The proportion of deaths attributed to pneumonia and influenza (P&I) decreased but is high again at 9.0% for the week ending February 10, 2018 (week 6). This percentage is above the epidemic threshold of 7.4% for week 6 in the National Center for Health Statistics (NCHS) Mortality Surveillance System.
NCHS data is delayed for two weeks to allow for the collection of enough data to produce stable P&I percentages.
Region and state-specific data are available at https://gis.cdc.gov/grasp/fluview/mortality.html.
17 influenza-associated pediatric deaths were reported to CDC during week 8.
Three deaths were associated with an influenza A(H3) virus and occurred during weeks 7 and 8 (the weeks ending February 17 and February 24, 2018, respectively). One death was associated with an influenza A(H1N1)pdm09 virus and occurred during week 7 (the week ending February 17, 2018). Four deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 52, 7 and 8 (the weeks ending December 30, 2017, February 17, and February 24, 2018, respectively). Nine deaths were associated with an influenza B virus and occurred during weeks 6, 7 and 8 (the weeks ending February 10, February 17, and February 24, 2018, respectively).
A total of 114 influenza-associated pediatric deaths for the 2017-2018 flu season have been reported to CDC.
Additional information on pediatric deaths is available on FluView Interactive at: https://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html.
Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending February 24 was 21.6%.
Regionally, the three-week average percent of specimens testing positive for influenza in clinical laboratories ranged from 13.7% to 31.9%.
During the week ending February 24, of the 9,191 (21.6%) influenza-positive tests reported to CDC by clinical laboratories, 4,742 (51.6%) were influenza A viruses and 4,449 (48.4%) were influenza B viruses.
While influenza A(H3) viruses continue to be predominant this season, during week 8 the overall proportion of influenza A viruses is declining and the proportion of influenza B viruses is increasing.
During the week ending February 24, 593 (54.2%) of the 1,095 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 502 (45.8%) were influenza B viruses. Of the 582 influenza A viruses that were subtyped, 467 (80.2%) were H3N2 viruses and 115 (19.8%) were (H1N1)pdm09 viruses.
The majority of the influenza viruses collected from the United States during October 1, 2017 through February 24, 2018 were characterized antigenically and genetically as being similar to the cell-grown reference viruses representing the 2017–18 Northern Hemisphere influenza vaccine viruses.
Since October 1, 2017, CDC has tested 520 influenza A(H1N1)pdm09, 1,117 influenza A(H3N2), and 482 influenza B viruses for resistance to antiviral medications (i.e. oseltamivir, zanamivir, or peramivir). While the majority of the tested viruses showed susceptibility to the antiviral drugs, five (1.0%) H1N1pdm09 viruses were resistant to both oseltamivir and peramivir, but was sensitive to zanamivir.