CDC Flu Update: Activity Likely to Remain Elevated for Several More Weeks

While influenza activity continued to decrease in the latest FluView report from the Centers for Disease Control and Prevention (CDC), it remains high across much of the United States. ILI dropped from 4.9% reported last week to 3.7%, and is now similar to ILI observed at the peak of the 2015-2016 season. 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, 34 states plus Puerto Rico continue to report widespread flu activity and 21 states plus New York City continue to experience high influenza-like illness (ILI) activity. The overall hospitalization rate and all age-specific hospitalization rates are now higher than the end-of-season hospitalization rates for 2014-2015, a high severity, H3N2-predominant season. CDC also is reporting an additional 5 flu-related pediatric deaths during week 9, bringing the total number of flu-related pediatric deaths reported this season to 119. 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 B viruses versus A viruses is now about even. In recent weeks, B viruses have been increasing while H3N2 viruses have been decreasing. 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%. In addition, in the context of widespread influenza activity, CDC clinicians and the public are reminded of the 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 March 3, 2018 (week 9):

Influenza-like Illness Surveillance: For the week ending March 3, the proportion of people seeing their health care provider for influenza-like illness (ILI) was 3.7%, which is a decrease from last week (4.9%), 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 15 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

Influenza-like Illness State Activity Indicator Map: New York City and 21 states experienced high ILI activity (Alaska, Arizona, Arkansas, Georgia, Indiana, Kansas, Kentucky, Massachusetts, Michigan, Missouri, Nebraska, New Jersey, New Mexico, New York, Pennsylvania, South Carolina, South Dakota, Texas, Vermont, Virginia, and Wyoming). 15 states experienced moderate ILI activity (Alabama, California, Colorado, Connecticut, Louisiana, Maryland, Minnesota, Mississippi, New Hampshire, North Carolina, Oklahoma, Oregon, Rhode Island, West Virginia, and Wisconsin). The District of Columbia, Puerto Rico and five states experienced low ILI activity (Hawaii, Illinois, Montana, Nevada, and Washington). Nine states experienced minimal ILI activity (Delaware, Florida, Idaho, Iowa, Maine, North Dakota, Ohio, Tennessee, and Utah). Additional data, including data for previous seasons, can be found at

Geographic Spread of Influenza Viruses: Widespread influenza activity was reported by Puerto Rico and 34 states (Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Idaho, Indiana, Iowa, Kansas, Maine, Maryland, Massachusetts, Michigan, Missouri, Montana, Nebraska, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Dakota, Virginia, Washington, Wisconsin, and Wyoming). Regional influenza activity was reported by Guam and 12 states (Alabama, Alaska, Illinois, Kentucky, Louisiana, Minnesota, Mississippi, Nevada, South Carolina, Tennessee, Texas, and Utah). Local influenza activity was reported by the District of Columbia and four states (Hawaii, Oregon, Vermont, and West Virginia). 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:

Flu-Associated Hospitalizations: Since October 1, 2017, 24,644 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 86.3 hospitalizations per 100,000 people in the United States.
The highest hospitalization rate is among people 65 years and older (370.6 per 100,000), followed by adults aged 50-64 years (93.6 per 100,000), and younger children aged 0-4 years (62.5 per 100,000). During most seasons, adults 65 years and older have the highest hospitalization rates, followed by children 0-4 years. Current week 9 rates 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 and

Mortality Surveillance:The proportion of deaths attributed to pneumonia and influenza (P&I) decreased but is high again at 8.8% for the week ending February 17, 2018 (week 7). This percentage is above the epidemic threshold of 7.4% for week 7 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

Pediatric Deaths:
Five influenza-associated pediatric deaths were reported to CDC during week 8.
Two deaths were associated with an influenza A(H1N1)pdm09 virus and occurred during week 6 (the week ending February 10, 2018). One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 8 (the week ending February 24, 2018). Two deaths were associated with an influenza B virus and occurred during weeks 8 and 9 (the weeks ending February 24, 2018 and March 3, 2018, respectively).
A total of 119 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:

Laboratory Data:
Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending March 3 was 17.7%.
Regionally, the three-week average percent of specimens testing positive for influenza in clinical laboratories ranged from 13.2% to 30.3%.
During the week ending March 3, of the 6,193 (17.7%) influenza-positive tests reported to CDC by clinical laboratories, 3,090 (49.9%) were influenza A viruses and 3,103 (50.1%) were influenza B viruses.
While influenza A(H3) viruses continue to be predominant this season, during week 9, the number and proportion of influenza A and influenza B viruses reported were similar.
During the week ending March 3, 312 (47.1%) of the 663 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 351 (52.9%) were influenza B viruses. Of the 301 influenza A viruses that were subtyped, 212 (70.4%) were H3N2 viruses and 89 (29.6%) were (H1N1)pdm09 viruses. The majority of the influenza viruses collected from the United States during October 1, 2017 through March 3, 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 566 influenza A(H1N1)pdm09, 1,244 influenza A(H3N2), and 568 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, eight (1.4%) H1N1pdm09 viruses were resistant to both oseltamivir and peramivir, but were sensitive to zanamivir.

Source: CDC