CDC Flu Update: Activity Slowly Increasing

According to this week’s FluView report, seasonal influenza activity continues to increase slowly in the United States. The proportion of people seeing their healthcare provider for influenza-like-illness (ILI) has been at the national baseline for two consecutive weeks. (This means that there were excess visits to health care providers most likely caused by influenza.) One state (Massachusetts) reported widespread flu activity and twenty-seven states are now reporting regional or local flu activity. That means those states are seeing outbreaks of flu and laboratory-confirmed flu in at least half of the regions of the state (widespread activity), in at least two regions but less than half of the regions of the state (regional activity), and in a single region (local activity). However 22 states, the District of Columbia, Puerto Rico, and the U.S. Virgin Islands continue to report only sporadic flu activity, which means those states are seeing small numbers of flu or one laboratory-confirmed flu outbreak. Influenza A(H1N1)pdm09 viruses have been the most commonly identified flu viruses since September 30, 2018. CDC analyzes flu data every week and declares the start of each “flu season” after sustained elevated activity is observed across key flu indicators for a number of weeks. While flu activity is elevated in parts of the country, it’s too early to say the 2018-2019 flu season has started nationally.

An annual flu vaccine is the best way to protect against influenza and its potentially serious complications. There are many benefits to vaccination, including reducing the risk of flu illness, doctor’s visits, hospitalization, and even death in children. For anyone 6 months or older who has not yet been vaccinated this season, CDC recommends that they get vaccinated now. Below is a summary of the key flu indicators for the week ending December 1, 2018:

Influenza-like Illness Surveillance: For the week ending December 1 (week 48), the proportion of people seeing their health care provider for influenza-like illness (ILI) was 2.2%, which is at the national baseline. Four of 10 regions (Regions 4, 7, 8 and 9) reported a proportion of outpatient visits for ILI at or above their region-specific baseline level. 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: Two states (Georgia and Louisiana) experienced high ILI activity. Two states (Colorado and South Carolina) experienced moderate ILI activity. New York City and eight states (Alabama, Arizona, Kentucky, Mississippi, New Jersey, North Carolina, Utah and Virginia) experienced low ILI activity. The District of Columbia, Puerto Rico and 38 states experienced minimal ILI activity. Additional data, including data for previous seasons, can be found at

Geographic Spread of Influenza Viruses: Widespread influenza activity was reported by one state (Massachusetts). Regional influenza activity was reported by nine states (California, Connecticut, Georgia, Kentucky, Louisiana, Nevada, New York, Oregon, and Vermont). Local influenza activity was reported by 18 states (Arizona, Colorado, Delaware, Florida, Idaho, Illinois, Michigan, Montana, New Hampshire, New Jersey, North Carolina, Ohio, Oklahoma,  Pennsylvania, South Carolina, Texas, Utah, and West Virginia). Sporadic influenza activity was reported by the District of Columbia, Puerto Rico, the U.S. Virgin Islands and 22 states (Alabama, Alaska, Arkansas, Hawaii, Indiana, Iowa, Kansas, Maine, Maryland, Minnesota, Mississippi, Missouri, Nebraska, New Mexico, North Dakota, Rhode Island, South Dakota, Tennessee, Virginia, Washington, Wisconsin, and Wyoming). 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:

Flu-Associated Hospitalizations: Data regarding influenza-associated hospitalizations for the 2018-2019 influenza season is now available. Since October 1, 2018, 383 laboratory-confirmed influenza-associated hospitalizations have now 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 1.3 hospitalizations per 100,000 people in the United States.
The highest hospitalization rates are among people 65 years and older (3.3 per 100,000) and children younger than 5 years (3.3 per 100,000), followed by adults aged 50-64 years (1.4 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 previous influenza seasons, can be found at and

Mortality Surveillance: The proportion of deaths attributed to pneumonia and influenza (P&I) was 5.7% during the week ending November 24, 2018 (week 47). This percentage is below the epidemic threshold of 6.5% for week 47 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. Additional P&I mortality data for current and past seasons and by geography (national, HHS region, or state) are available at

Pediatric Deaths: No influenza-associated pediatric deaths were reported to CDC during week 48 (the week ending December 1, 2018).
A total of five influenza-associated pediatric deaths have been reported for the 2018-2019 season.
Additional information on influenza-associated pediatric deaths reported during past seasons, including basic demographics, underlying conditions, bacterial co-infections, and place of death is available on FluView Interactive at: More detailed information about pediatric deaths reported during the current season will be available later in the season.

Laboratory Data:
Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending December 1 was 4.2%.
Regionally, the three-week average percent of specimens testing positive for influenza in clinical laboratories ranged from 1.0% to 9.6%.
During the week ending December 1, of the 925 (4.2%) influenza-positive tests reported to CDC by clinical laboratories, 846 (91.5%) were influenza A viruses and 79 (8.5%) were influenza B viruses.
The most frequently identified influenza virus type reported by public health laboratories was influenza A(H1N1)pdm09 virus.
During the week ending December 1, 170 (94.4%) of the 180 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 10 (5.6%) were influenza B viruses. Of the 152 influenza A viruses that were subtyped, 27 (17.8%) were H3N2 viruses and 125 (82.2%) were (H1N1)pdm09 viruses.
The majority of the influenza viruses collected from the United States during September 30, 2018 through December 1, 2018 were characterized antigenically and genetically as being similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses.

Source: CDC