CDC Flu Update: Levels of Flu-Like Illness Below the National Baseline for the First Time This Season

According to the CDC's FluView report for the week ending April 15, 2017 (week 15), flu activity decreased in the United States. Levels of flu-like illness are now below the national baseline for the first time this season since early December.  While the 2016-2017 flu season has peaked, 10 states continue to report widespread flu activity and additional five flu-related pediatric deaths were reported. This brings the total number of flu deaths in children reported to CDC this season to 77. Sporadic flu activity is expected to continue for a number of weeks. While influenza A (H3N2) viruses have been most common overall this season, influenza B viruses accounted for 69% of the viruses reported by public health laboratories during week 15. 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 15, the proportion of people seeing their health care provider for influenza-like illness (ILI) was 2.0%.and is now below the national baseline of 2.2% for the first time this season since early December. Three regions (Regions 1, 2, and 4) reported ILI at or above their region-specific baseline level. For the 2016-2017 season, ILI has been at or above baseline for 17 consecutive weeks. 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: Two states (Rhode Island and South Carolina) experienced moderate ILI activity. New York City and 3 states (Alabama, Georgia and Tennessee) experienced low ILI activity. Puerto Rico and 45 states (Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, South Dakota, Texas, Utah, Vermont, Virginia, 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 10 states (Alaska, Connecticut, Delaware, Maine, Massachusetts, New Hampshire, New York, North Carolina, Ohio, and Rhode Island).  Regional influenza activity was reported by Guam, Puerto Rico and 15 states (Arizona, Florida, Iowa, Kentucky, Michigan, Minnesota, Missouri, Nevada, New Jersey, North Dakota, Oklahoma, Pennsylvania, South Carolina, Texas, and Vermont). Local influenza activity was reported by the District of Columbia and 19 states (Arkansas, California, Georgia, Idaho, Kansas, Louisiana, Maryland, Mississippi, Montana, Nebraska, New Mexico, Oregon, South Dakota, Tennessee, Virginia, Washington, West Virginia, Wisconsin, and Wyoming). Sporadic activity was reported by six states (Alabama, Colorado, Hawaii, Illinois, Indiana, 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 17,055 laboratory-confirmed influenza-associated hospitalizations have been reported. This translates to a cumulative overall rate of 60.9 hospitalizations per 100,000 people in the United States. This is higher than the hospitalization rate for week 15 (43.6 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 (63.4 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 266.6 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 181.7 per 100,000. For week 14 during 2014-2015, it was 302.8 per 100,000. The hospitalization rate among children younger than 5 years is 40.7 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 65.4 per 100,000 and 55.7 per 100,000 respectively.
The hospitalization rate among adults 50-64 years is 60.2 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 40.4 per 100,000 and 52.8 respectively.
The hospitalization rate among children younger than 5 years is 42.1 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 65.9 per 100,000 and 56.3 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 7.2% for the week ending April 1, 2017 (week 13). This percentage is below the epidemic threshold of 7.3% for week 13 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. The weekly percentage of deaths attributed to P&I has at or exceeded the epidemic threshold for 12 consecutive weeks this season.

Pediatric Deaths: Five influenza-associated pediatric deaths were reported to CDC for the week ending April 15, 2017.
Four deaths were associated with an influenza A (H3) virus and occurred during weeks 10, 14 and 15 (the weeks ending March 11, April 8, and April 15, 2017, respectively).
One death was associated with an influenza B virus and occurred during week 48 (the week ending December 3, 2016, respectively).
A total of 77 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 15 was 12.6%.
Regionally, the three week average percent of specimens testing positive for influenza in clinical laboratories ranged from 6.6% to 23.3%.
During the week ending April 15, of the 2,249 (12.6%) influenza-positive tests reported to CDC by clinical laboratories, 648 (28.8%) were influenza A viruses and 1,601 (71.2%) were influenza B viruses.
While influenza A (H3N2) viruses have predominated this season, the most frequently identified influenza virus type reported by public health laboratories since mid-March  was influenza B viruses.
During the week ending April 15, 105 (31.2%) of the 337 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 232 (68.8%) were influenza B viruses. Of the 100 influenza A viruses that were subtyped, 99 (99.0%) were H3N2 viruses and 1 (1.0%) was (H1N1)pdm09 virus.
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 2,998  specimens (289 influenza A (H1N1)pdm09, 2,043 influenza A (H3N2), and 666 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