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According to this week’s FluView report, seasonal influenza activity increased sharply in the United States. The number of states reporting widespread flu activity jumped from 23 to 36, the proportion of samples testing positive for influenza at clinical laboratories went from 14.0% to 22.4%, and the percentage of people seeking outpatient care for influenza-like illness (ILI) increased from 3.5% to 5.0% over last week’s report. ILI has been at or above the national baseline for five weeks so far this season. During recent seasons, ILI has remained at or above baseline for 13 weeks on average. H3N2 viruses continue to predominate.
While flu vaccination is still recommended for people who have not yet gotten vaccinated, antiviral drugs are an important second line of defense that can be used to treat flu illness. CDC recommends the use of antiviral drugs as early as possible to treat flu illness in people who are very sick with flu and those at high risk of serious flu complications.
Influenza-like Illness Surveillance: For the week ending December 23, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) was 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. 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: 21 states experienced high ILI activity (Alabama, Arizona, Arkansas, California, Georgia, Illinois, Indiana, Kansas, Kentucky, Louisiana, Mississippi, Missouri, Nebraska, Nevada, New Mexico, Oklahoma, Oregon, South Carolina, Tennessee, Texas, and West Virginia). New York City and five states (Colorado, Hawaii, New York, North Dakota, and Virginia) experienced moderate ILI activity. Eight states (Alaska, Florida, Massachusetts, Minnesota, New Jersey, Pennsylvania, South Dakota, and Wyoming) experienced low ILI activity. 14 states experienced minimal ILI activity (Delaware, Idaho, Iowa, Maine, Maryland, Michigan, Montana, New Hampshire, Ohio, Rhode Island, Utah, Vermont, Washington, and Wisconsin). Data was insufficient to calculate an ILI activity level from the District of Columbia, Puerto Rico and two states (Connecticut and North Carolina). 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 36 states (Alabama, Arizona, Arkansas, California, Colorado, Connecticut, Florida, Georgia, Idaho, Illinois, Indiana, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Minnesota, Mississippi, Missouri, Montana, Nebraska, New Mexico, New York, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, South Dakota, Texas, Virginia, Washington, West Virginia, Wisconsin, and Wyoming). Regional influenza activity was reported by Puerto Rico and 13 states (Alaska, Hawaii, Iowa, Maine, Michigan, Nevada, New Hampshire, New Jersey, North Carolina, Rhode Island, Tennessee, Utah, and Vermont). Local influenza activity was reported by one state (Delaware). The District of Columbia, the U.S. Virgin Islands and Guam did not report. 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, 2.485 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 8.7 hospitalizations per 100,000 people in the United States.
The highest hospitalization rates are among people 65 years and older (35.8 per 100,000), followed by adults aged 50-64 years (9.4 per 100,000), and children younger than 5 years (6.5 per 100,000). During most seasons, children younger than 5 years and adults 65 years and older have the highest hospitalization rates. 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) was 2% for the week ending December 9, 2017 (week 49). This percentage is below the epidemic threshold of 6.8% for week 49 in the National Center for Health Statistics (NCHS) Mortality Surveillance System.
Region and state-specific data are available at https://gis.cdc.gov/grasp/fluview/mortality.html.
Three influenza-associated pediatric death was reported to CDC during week 51.
One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 50 (the week ending December 16, 2017). Two deaths were associated with an influenza B virus and occurred during week 51 (the week ending December 23, 2017).
A total of 12 influenza-associated pediatric deaths for the 2017-2018 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 December 23 was 22.4%.
Regionally, the three week average percent of specimens testing positive for influenza in clinical laboratories ranged from 6.5% to 24.9%.
During the week ending December 23, of the 6,373 (22.4%) influenza-positive tests reported to CDC by clinical laboratories, 5,527 (86.7%) were influenza A viruses and 846 (13.3%) were influenza B viruses.
The most frequently identified influenza virus subtype reported by public health laboratories was influenza A(H3N2) virus.
During the week ending December 23, 740 (89.0%) of the 831 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 91 (11.0%) were influenza B viruses. Of the 707 influenza A viruses that were subtyped, 652 (92.2%) were H3N2 viruses and 55 (7.8%) were (H1N1)pdm09 viruses.
The majority of the influenza viruses collected from the United States during October 1 through December 23, 2017 were characterized antigenically and genetically as being similar to the cell-grown reference viruses representing the 2017–18 Northern Hemisphere influenza vaccine viruses.
Of the influenza viruses tested and collected during October 1-December 23, 2017, none were found to be resistant to antiviral medications (i.e. oseltamivir, zanamivir, or peramivir).