Influenza activity in the United States continued to decrease, but remained above the national baseline according to the latest FluView report. ILI dropped from 2.8% reported last week to 2.5%. Current data indicate that the 2017-2018 flu season peaked at 7.5% in early February (during week 5). However, 16 states plus Puerto Rico continue to report widespread flu activity and 4 states continue to experience high influenza-like illness (ILI) levels. 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 4 flu-related pediatric deaths during week 12, bringing the total number of flu-related pediatric deaths this season to 137. Flu activity is likely to remain elevated for a number of 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, during week 12, influenza B viruses were more frequently reported than influenza A viruses. Early vaccine effectiveness (VE) estimates through February 3, 2018 show that flu vaccine 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 24, 2018 (week 12):
Influenza-like Illness Surveillance: For the week ending March 24, the proportion of people seeing their health care provider for influenza-like illness (ILI) was 2.5%, which is a decrease from last week (2.8%), but remains above the national baseline of 2.2%. Nine of 10 regions (Regions 1, 2, 3, 4, 5, 7, 8, 9, and 10) 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 18 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: Four states experienced high ILI activity (Alaska, South Carolina, Virginia, and Wyoming). Eight states experienced moderate ILI activity (Arizona, Indiana, Kentucky, Missouri, Nebraska, New Mexico, Pennsylvania, and South Dakota). New York City, Puerto Rico, the District of Columbia and 14 states experienced low ILI activity (California, Connecticut, Georgia, Hawaii, Illinois, Kansas, Massachusetts, Minnesota, New Jersey, North Carolina, Oregon, Rhode Island, Vermont, and Washington). 24 states experienced minimal ILI activity (Alabama, Arkansas, Colorado, Delaware, Florida, Idaho, Iowa, Louisiana, Maine, Maryland, Michigan, Mississippi, Montana, Nevada, New Hampshire, New York, North Dakota, Ohio, Oklahoma, Tennessee, Texas, Utah, West Virginia, and Wisconsin). 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 16 states (Alaska, California, Connecticut, Delaware, Indiana, Maine, Maryland, Massachusetts, Nebraska, New Hampshire, New York, Ohio, Oklahoma, Rhode Island, Virginia, and Wisconsin). Regional influenza activity was reported by 22 states (Arizona, Colorado, Florida, Georgia, Idaho, Illinois, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Montana, New Jersey, New Mexico, North Dakota, Pennsylvania, South Carolina, South Dakota, Utah, Washington, and Wyoming). Local influenza activity was reported by the District of Columbia, Guam and eight states (Arkansas, Louisiana, Nevada, North Carolina, Oregon, Tennessee, Texas, and West Virginia). Sporadic influenza activity was reported by four states (Alabama, Hawaii, Mississippi, and Vermont). No influenza 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, 27,438 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 96.1 hospitalizations per 100,000 people in the United States.
The highest hospitalization rate is among people 65 years and older (412.6 per 100,000), followed by adults aged 50-64 years (104.2 per 100,000), and younger children aged 0-4 years (68.7 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 12 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 remained elevated at 7.7% for the week ending March 10, 2018 (week 10). The epidemic threshold for week 10 in the National Center for Health Statistics (NCHS) Mortality Surveillance System is 7.4%.
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.
Four influenza-associated pediatric deaths were reported to CDC during week 12. One death was associated with an influenza A(H3) virus and occurred during week 51 (the week ending December 23, 2017). Two deaths were associated with an influenza A virus for which subtyping was not performed and occurred during weeks 8 and 11 (the week ending February 24 and March 17, 2018, respectively). One death was associated with an influenza B virus and occurred during week 6 (the week ending February 10, 2018, respectively). A total of 137 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 24 was 14.7%. Regionally, the three-week average percent of specimens testing positive for influenza in clinical laboratories ranged from 9.2% to 21.3%.
During the week ending March 24, of the 3,575 (14.7%) influenza-positive tests reported to CDC by clinical laboratories, 1,509 (42.2%) were influenza A viruses and 2,066 (57.8%) were influenza B viruses. Influenza A(H3) viruses have predominated this season. However, the proportion of influenza B viruses is increasing and during week 12, influenza B viruses were more frequently reported than influenza A viruses.
During the week ending March 24, 131 (35.6%) of the 368 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 237 (64.4%) were influenza B viruses. Of the 124 influenza A viruses that were subtyped, 90 (72.6%) were H3N2 viruses and 34 (27.4%) were (H1N1)pdm09 viruses.
The majority of the influenza viruses collected from the United States during October 1, 2017 through March 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 770 influenza A(H1N1)pdm09, 1,671 influenza A(H3N2), and 696 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 (1.2%) H1N1pdm09 viruses were resistant to both oseltamivir and peramivir, but were sensitive to zanamivir.