CDC Flu Update: Season Has Peaked but 24 States Continue to Report Widespread Activity

According to the CDC's FluView report for the week ending April 1, 2017 (week 13), flu activity decreased but remains elevated in the United States. While the 2016-2017 flu season has peaked, 24 states continue to report widespread flu activity and significant flu activity is likely to continue for a number of weeks. While influenza A (H3N2) viruses have been most common overall this season, influenza B viruses accounted for 56% of the viruses reported by public health laboratories during week 13. Interim vaccine effectiveness (VE) estimates indicate flu vaccines this season reduced a vaccinated person’s risk of getting sick and having to go to the doctor because of flu by about half (48%). Estimated VE against H3N2 viruses was 43% while VE against B viruses was 73%. CDC recommends annual flu vaccination for everyone 6 months of age and older. Vaccination efforts should continue for as long as influenza viruses are circulating.

Influenza-like Illness Surveillance: For the week ending April 1, the proportion of people seeing their health care provider for influenza-like illness (ILI) was2.9%. This remains above the national baseline of 2.2%. Six regions (Regions 1, 2, 3, 4, 5, and 7) reported ILI at or above their region-specific baseline level. This is the 16th week during the 2016-2107 flu season that influenza-like-illness has been at or above baseline. For the last 15 seasons, the average duration of a flu season by this measure has been 13 weeks, with a range from one week to 20 weeks.

Influenza-like Illness State Activity Indicator Map: 19 states (Alabama, Georgia, Kentucky, New York, North Carolina, Rhode Island, South Carolina, Tennessee, and Virginia) experienced high ILI activity. Five states (Maryland, Minnesota, New Jersey, Oklahoma, and Pennsylvania) experienced moderate ILI activity. New York City and nine states (Alaska, Arizona, Arkansas, Illinois, Louisiana, Massachusetts, Mississippi, Missouri, and North Dakota) experienced low ILI activity. Puerto Rico and 27 states (California, Colorado, Connecticut, Delaware, Florida, Hawaii, Idaho, Indiana, Iowa, Kansas, Maine, Michigan, Montana, Nebraska, Nevada, New Hampshire, New Mexico, Ohio, Oregon, South Dakota, Texas, Utah, Vermont, Washington, West Virginia, Wisconsin, and Wyoming) 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.

Geographic Spread of Influenza Viruses: Widespread influenza activity was reported by 24 states (Alaska, Connecticut, Delaware, Iowa, Kentucky, Maine, Maryland, Massachusetts, Michigan, Mississippi, Missouri, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Vermont, Virginia, and Wisconsin).  Regional influenza activity was reported by Guam, Puerto Rico and 19 states (Alabama, Arizona, Arkansas, California, Florida, Georgia, Idaho, Illinois, Indiana, Kansas, Louisiana, Minnesota, Nebraska, Nevada, New Mexico, Tennessee, Texas, Washington, and West Virginia). Local influenza activity was reported by the District of Columbia and five states (Colorado, Montana, Oregon, South Dakota, and Wyoming). Sporadic activity was reported by two states (Hawaii and Utah). 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.

Flu-Associated Hospitalizations: Since October 1, 2016, a total of 16,021 laboratory-confirmed influenza-associated hospitalizations have been reported. This translates to a cumulative overall rate of 57.2 hospitalizations per 100,000 people in the United States. This is higher than the hospitalization rate for week 13 (42.6 per 100,000) during the 2012-2013 flu season, when influenza A H3N2 viruses also predominated, but lower than the cumulative hospitalization rate during 2014-2015 (61.6 per 100,000) which also was an H3N2 predominant season. Vaccine effectiveness during 2012-13 was 49%, similar to interim estimates for the current season, but was 19% during 2014-2015 as a result of a high proportion of drifted influenza viruses during that season. The hospitalization rate among people 65 years and older is 256.0 per 100,000. This is the highest rate of any age group. The hospitalization rate for people 65 and older for the same week during the 2012-2013 flu season was 180.1 per 100,000. For week 13 during 2014-2015, it was 298.6 per 100,000.
The hospitalization rate among adults 50-64 years is 56.5 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 39.4 per 100,000 and 50.8 respectively.
The hospitalization rate among children younger than 5 years is 39.5 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 63.7 per 100,000 and 54.7 per 100,000 respectively.
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. 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.

Mortality Surveillance:  The proportion of deaths attributed to pneumonia and influenza (P&I) was 7.3% for the week ending March 18, 2017 (week 11). This percentage is below the epidemic threshold of 7.4% for week 11 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. The weekly percentage of deaths attributed to P&I has exceeded the epidemic threshold for eight consecutive weeks this season.

Pediatric Deaths: Seven influenza-associated pediatric deaths are being reported by CDC for the week ending April 1, 2017.
Two deaths were associated with an influenza A (H3) virus and occurred during weeks 50 and 11 (the weeks ending December 17, 2016, and March 18, 2017, respectively).
One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 13 (the week ending April 1, 2017).
Four deaths were associated with an influenza B virus and occurred during weeks 1, 7, 10, and 12 (the weeks ending January 7, February 18, March 11, and March 25, 2017, respectively).
A total of 68 influenza-associated pediatric deaths have been reported for the 2016-2017 season.
Additional information on pediatric deaths for the 2016-2017 season is available on FluView Interactive at: https://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html.

Laboratory Data: Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending April 1 was 18.4%.
Regionally, the three week average percent of specimens testing positive for influenza in clinical laboratories ranged from 7.5% to 24.9%.
During the week ending April 1, of the 4,086 (18.4%) influenza-positive tests reported to CDC by clinical laboratories, 1,596 (39.1%) were influenza A viruses and 2,490 (60.9%) were influenza B viruses.
While influenza A (H3N2) viruses have predominated circulation this season, the most frequently identified influenza virus type reported by public health laboratories during the week ending April 1  was influenza B viruses.
During the week ending April 1, 322 (44.1%) of the 730 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 408 (55.9%) were influenza B viruses. Of the 316 influenza A viruses that were subtyped, 307 (97.2%) were H3N2 viruses and 9 (2.8%) 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 2,331 specimens (240 influenza A (H1N1)pdm09, 1,525 influenza A (H3N2), and 566 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.

Source: CDC