CDC Flu Update: Influenza-Like Illness Climbs Above Baseline

According to the CDC's FluView report for the week ending Dec. 17, 2016 (week 50), flu activity increased in the United States. The proportion of people seeing their healthcare provider for influenza-like-illness (ILI) is above the national baseline for the first time this season. Influenza A (H3) viruses were most commonly reported during week 50 and have been the predominant virus so far this season. While the timing of influenza activity varies and is unpredictable, flu activity is expected to increase further in the coming weeks. CDC recommends annual flu vaccination for everyone 6 months of age and older. Anyone who has not gotten vaccinated yet this season should get vaccinated now.

For the week ending Dec. 17, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) increased to 2.3% (ILI was 1.9% during the week ending December 10, 2016). This is above the national baseline of 2.2%. Five regions (regions 2, 4, 8, 9 and 10) reported ILI at or above their region-specific baseline level.

One state (Oklahoma) and Puerto Rico experienced high ILI activity. Two states (Arizona and Georgia) and New York City experienced moderate ILI activity. Ten states (Alabama, Colorado, Hawaii, Louisiana, Mississippi, Nevada, New Jersey, North Carolina, South Carolina and Virginia) experienced low ILI activity. 37 states experienced minimal ILI activity. The District of Columbia did not have sufficient data to calculate an activity level. ILI activity data indicate the amount of flu-like illness that is occurring in each state.

Widespread influenza activity was reported by Puerto Rico. Regional influenza activity was reported by Guam, the U.S. Virgin Islands and 13 states (Alabama, Alaska, Connecticut, Massachusetts, New Hampshire, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, South Carolina, Virginia, and Washington). Local flu activity was reported by the District of Columbia and 26 states (Arizona, Arkansas, Colorado, Delaware, Florida, Georgia, Hawaii, Idaho, Indiana, Kentucky, Louisiana, Maine, Maryland, Michigan, Minnesota, Mississippi, Missouri, Nebraska, Nevada, New Jersey, North Dakota, Ohio, Tennessee, Texas, Utah, and Wyoming). Sporadic flu activity was reported by 11 states (California, Illinois, Iowa, Kansas, Montana, New Mexico, Rhode Island, South Dakota, Vermont, West Virginia, and Wisconsin). Geographic spread data show how many areas within a state or territory are seeing flu activity.

Since Oct. 1, 2016, a total of 676 laboratory-confirmed influenza-associated hospitalizations have been reported. This translates to a cumulative overall rate of 2.4 hospitalizations per 100,000 people 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 highest hospitalization rates are among people 65 years and older (9.8 per 100,000), followed by adults 50-64 years (2.8 per 100,000) and children younger than 5 years (1.7 per 100,000). During most seasons, children younger than 5 years and adults 65 years and older have the highest hospitalization rates. 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.

The proportion of deaths attributed to pneumonia and influenza (P&I) was 5.9% for the week ending December 3, 2016 (week 48). This percentages is below the epidemic threshold of 6.9% for week 47 in the National Center for Health Statistics (NCHS) Mortality Surveillance System.

No influenza-associated pediatric deaths for the 2016-2017 season have been reported to CDC.

Two influenza-associated pediatric deaths that occurred during the 2015-2016 season were reported to CDC during week 50. One death was associated with an influenza A (H3) virus and one was associated with an influenza B virus. These deaths bring the total number of reported influenza-associated pediatric deaths occurring during that season to 89.

Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending December 17 was 7.1%.

Regionally, the three week average percent of specimens testing positive for influenza in clinical laboratories ranged from 2.0% to 11.6%.

During the week ending December 17, of the 1,130 influenza-positive tests reported to CDC by clinical laboratories, 986 (87.3%) were influenza A viruses and 144 (12.7%) were influenza B viruses.

The most frequently identified influenza virus type reported by public health laboratories during the week ending December 17 was influenza A viruses, with influenza A (H3) viruses predominating. During the week ending December 17, 319 (94.7%) of the 337 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 18 (5.3%) were influenza B viruses. Of the 296 influenza A viruses that were subtyped, 291 (98.3%) were H3 viruses and 5 (1.7%) were (H1N1)pdm09 viruses.

Since October 1, 2016, antigenic and/or genetic characterization shows that the majority of the tested viruses remain similar to the recommended components of the 2016-2017 Northern Hemisphere vaccines.

Since October 1, 2016, CDC tested 236 specimens (35 influenza A (H1N1)pdm09, 154 influenza A (H3N2), and 47 influenza B viruses) for resistance to the neuraminidase inhibitors antiviral drugs. None of the tested viruses were found to be resistant to oseltamivir, zanamivir, or peramivir.