CDC Flu Update: Influenza Activity Spikes

Influenza activity increased sharply again in this week’s FluView report from the Centers for Disease Control and Prevention (CDC). The number of jurisdictions experiencing high activity went from 21 states to 26 states and New York City and the number of states reporting widespread activity went from 36 to 46. Influenza-like illness (ILI) went from 4.9% to 5.8%. ‎These indicators are similar to what was seen at the peak of the 2014-2015 season, which was the most severe season in recent years. Typically, severity indicators (e.g., hospitalization rates) lag behind activity indicators (ILI and geographic spread). CDC continues to recommend influenza vaccination as flu viruses are likely to continue circulating for weeks. In addition, in the context of widespread influenza activity, CDC is reminding clinicians and the public about the importance of antiviral medications for treatment of influenza in people who are severely ill and people who are at high risk of serious flu complications.

Influenza-like Illness Surveillance: For the week ending Dec. 30, the proportion of people seeing their health care provider for influenza-like illness (ILI) was 5.8%, 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. ILI has been at or above the national baseline for six weeks so far this season. During recent seasons, ILI has remained at or above baseline for 13 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 26 states experienced high ILI activity (Alabama, Arizona, Arkansas, California, Georgia, Illinois, Indiana, Kansas, Kentucky, Louisiana, Maryland, Michigan, Mississippi, Missouri, Nebraska, Nevada, New Jersey, New Mexico, Ohio, Oklahoma, Oregon, South Carolina, Texas, Virginia, Washington, and West Virginia). Puerto Rico and nine states (Alaska, Colorado, Hawaii, Iowa, Massachusetts, North Carolina, Pennsylvania, Tennessee, and Wyoming) experienced moderate ILI activity. The District of Columbia and six states (Florida, Minnesota, New York, South Dakota, Utah, and Wisconsin) experienced low ILI activity. Nine states experienced minimal ILI activity (Connecticut, Delaware, Idaho, Maine, Montana, New Hampshire, North Dakota, Rhode Island, and Vermont). Additional data, including data for previous seasons, can be found at

Geographic Spread of Influenza Viruses: Widespread influenza activity was reported by 46 states (Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin, and Wyoming). Regional influenza activity was reported by 4 states (Hawaii, Maine, New Hampshire, and New Jersey). Local influenza activity was reported by the District of Columbia. Guam, Puerto Rico and the U.S. Virgin Islands did not report. 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, 3,927 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 13.7 hospitalizations per 100,000 people in the United States.
The highest hospitalization rates are among people 65 years and older (56.6 per 100,000), followed by adults aged 50-64 years (15.4 per 100,000), and children younger than 5 years (9.9 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 and

Mortality Surveillance:
The proportion of deaths attributed to pneumonia and influenza (P&I) was 6.7% for the week ending December 16, 2017 (week 50). This percentage is below the epidemic threshold of 6.9% for week 50 in the National Center for Health Statistics (NCHS) Mortality Surveillance System.
Region and state-specific data are available at

Pediatric Deaths:
One influenza-associated pediatric deaths were reported to CDC during week 52.
This death was associated with an influenza A virus for which no subtyping was performed and occurred during week 52 (the week ending December 30, 2017).
A total of 13 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:

Laboratory Data:
Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending December 30 was 25.5%.

Regionally, the three week average percent of specimens testing positive for influenza in clinical laboratories ranged from 10.2% to 30.7%.
During the week ending December 30, of the 9,228 (25.5%) influenza-positive tests reported to CDC by clinical laboratories, 7,818 (84.7%) were influenza A viruses and 1,410 (15.3%) 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 30, 784 (84.9%) of the 923 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 139 (15.1%) were influenza B viruses. Of the 742 influenza A viruses that were subtyped, 682 (91.9%) were H3N2 viruses and 60 (8.1%) were (H1N1)pdm09 viruses.

The majority of the influenza viruses collected from the United States during October 1 through December 30, 2017 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 111 influenza A(H1N1)pdm09, 462 influenza A(H3N2), and 127 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, one H1N1pdm09 virus was resistant to both oseltamivir and peramivir, but was sensitive to zanamivir.

Source: CDC