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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 3.7% reported last week to 3.3%. Current data indicate that the 2017-2018 flu season peaked at 7.5% in early February (during week 5) and is now on the decline. However, 26 states plus Puerto Rico continue to report widespread flu activity and 12 states continue to experience high influenza-like illness (ILI) activity. The overall hospitalization rate and all age-specific hospitalization rates are higher than the end-of-season hospitalization rates for 2014-2015, a high severity, H3N2-predominant season. CDC also is reporting an additional 9 flu-related pediatric deaths during week 10, bringing the total number of flu-related pediatric deaths reported this season to 128. Flu activity is likely to remain elevated for several more weeks.
CDC routinely recommends influenza vaccination for all persons 6 months of age and older as long as flu viruses are circulating. While H3N2 viruses remain predominant overall this season, the proportion of influenza B versus influenza A viruses is now about even. Early vaccine effectiveness (VE) estimates through February 3, 2018 show that flu vaccine has reduced the risk of having to go to the doctor due to flu by 36% overall. VE against H3N2 viruses was 25%. VE against H1N1 67% and VE against B viruses was 42%. CDC recommends prompt treatment with influenza antiviral medications for 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 March 10, 2018 (week 10):
Influenza-like Illness Surveillance: For the week ending March 10, the proportion of people seeing their health care provider for influenza-like illness (ILI) was 3.3%, which is a decrease from last week (3.7%), 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 16 weeks so far this season. Over the past five seasons, ILI has remained at or above baseline for 16 weeks on average with 20 weeks being the longest.
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: 12 states experienced high ILI activity (Alaska, Arizona, Georgia, Kansas, Kentucky, Missouri, Nebraska, New Jersey, New Mexico, South Carolina, Virginia, and Wyoming). Thirteen states experienced moderate ILI activity (Arkansas, California, Hawaii, Indiana, Massachusetts, Michigan, Minnesota, New York, North Carolina, Pennsylvania, Texas, Vermont, and Wisconsin). New York City and 14 states experienced low ILI activity (Alabama, Colorado, Connecticut, Illinois, Iowa, Louisiana, Maryland, Mississippi, Oklahoma, Oregon, Rhode Island, South Dakota, Utah, and West Virginia). 11 states experienced minimal ILI activity (Delaware, Florida, Idaho, Maine, Montana, Nevada, New Hampshire, North Dakota, Ohio, Tennessee, and Washington). Data were insufficient to calculate an ILI activity level from the District of Columbia and Puerto Rico
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 26 states (Alaska, Arizona, California, Colorado, Connecticut, Delaware, Florida, Indiana, Kansas, Maine, Maryland, Massachusetts, Michigan, Montana, Nebraska, New Hampshire, New Jersey, New York, Ohio, Oklahoma, Rhode Island, South Carolina, Virginia, Washington, Wisconsin, and Wyoming). Regional influenza activity was reported by Guam and 18 states (Alabama, Arkansas, Georgia, Idaho, Illinois, Iowa, Kentucky, Louisiana, Minnesota, Mississippi, Missouri, New Mexico, North Carolina, North Dakota, Pennsylvania, South Dakota, Tennessee, and Utah). Local influenza activity was reported by the District of Columbia and five states (Hawaii, Nevada, Oregon, Texas, and West Virginia). Sporadic influenza activity was reported by one state (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, 25,676 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 89.9 hospitalizations per 100,000 people in the United States.
The highest hospitalization rate is among people 65 years and older (386.2 per 100,000), followed by adults aged 50-64 years (97.3 per 100,000), and younger children aged 0-4 years (64.9 per 100,000). During most seasons, adults 65 years and older have the highest hospitalization rates, followed by children 0-4 years.
The rates reported during week 10 are higher than the end-of-season hospitalization rates for all ages (cumulative) and all age-group specific rates for the 2014-2015 flu season.
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 remains high at 8.5% for the week ending February 24, 2018 (week 8). This percentage is above the epidemic threshold of 7.4% for week 8 in the National Center for Health Statistics (NCHS) Mortality Surveillance System.
NCHS data are 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.
Nine influenza-associated pediatric deaths were reported to CDC during week 10.
Two deaths were associated with an influenza A(H1N1)pdm09 virus and occurred during weeks 1 and 9 (the weeks ending January 6, 2018 and March 3, 2018, respectively). Two deaths were associated with an influenza A(H3) virus and occurred during weeks 6 and 9 (the weeks ending February 10, 2018 and March 3, 2018, respectively). Five deaths were associated with an influenza B virus and occurred during weeks 6 and 9 (the weeks ending February 10, 2018, March 3, 2018, respectively).
A total of 128 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 March 10 was 15.0%.
Regionally, the three-week average percent of specimens testing positive for influenza in clinical laboratories ranged from 13.4% to 25.5%.
During the week ending March 10, of the 4,223 (15.0%) influenza-positive tests reported to CDC by clinical laboratories, 1,963 (46.5%) were influenza A viruses and 2,260 (53.5%) were influenza B viruses.
While influenza A(H3) viruses continue to be predominant this season, during week 10, the number and proportion of influenza A and influenza B viruses reported were similar.
During the week ending March 10, 242 (43.1%) of the 561 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 319 (56.9%) were influenza B viruses. Of the 228 influenza A viruses that were subtyped, 173 (75.9%) were H3N2 viruses and 55 (24.1%) were (H1N1)pdm09 viruses. The majority of the influenza viruses collected from the United States during October 1, 2017 through March 10, 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 657 influenza A(H1N1)pdm09, 1,453 influenza A(H3N2), and 598 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, nine (1.4%) H1N1pdm09 viruses were resistant to both oseltamivir and peramivir, but were sensitive to zanamivir.