CDC Flu Update: Season Has Peaked and is Winding Down

According to the CDC's FluView report for the week ending April 29, 2017 (week 17), flu activity continues to decrease in the United States. Levels of flu-like illness have been below the national baseline for three weeks now.  While the 2016-2017 flu season has peaked and is winding down, 3 states continue to report widespread flu activity and another seven flu-related pediatric deaths were reported. This brings the total number of flu deaths in children reported to CDC this season to 89. Sporadic flu activity may continue for a number of weeks. While influenza A (H3N2) viruses have been most common overall this season, influenza B viruses accounted for 71% of the viruses reported by public health laboratories during week 17. 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 29, the proportion of people seeing their health care provider for influenza-like illness (ILI) was 1.4% and has now been below the national baseline of 2.2% for the third consecutive week since early December. All 10 regions reported a proportion of outpatient visits for ILI below their region-specific baseline levels. For the 2016-2017 season, ILI was 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: No states experienced high or moderate flu activity. Four states (Arizona, Colorado, Louisiana, and South Carolina) experienced low ILI activity. New York City, Puerto Rico and 46 states (Alabama, Alaska, Arkansas, California, Connecticut, Delaware, Florida, Georgia, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, 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, Rhode Island, South Dakota, Tennessee, 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 3 states (Connecticut, New Hampshire, and New York).  Regional influenza activity was reported by Guam and 8 states (Alaska, Arizona, Maine, Massachusetts, New Jersey, Ohio, Rhode Island, and South Carolina). Local influenza activity was reported by the District of Columbia, Puerto Rico and 20 states (California, Delaware, Florida, Iowa, Kansas, Louisiana, Maryland, Michigan, Minnesota, Missouri, Montana, New Mexico, North Dakota, Oklahoma, Pennsylvania, Tennessee, Texas, Washington, West Virginia, and Wisconsin). Sporadic activity was reported by 19 states (Alabama, Arkansas, Colorado, Georgia, Hawaii, Idaho, Illinois, Indiana, Kentucky, Mississippi, Nebraska, Nevada, North Carolina, Oregon, South Dakota, Utah, Vermont, Virginia and Wyoming). 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 17,871 laboratory-confirmed influenza-associated hospitalizations have been reported. This translates to a cumulative overall rate of 63.8 hospitalizations per 100,000 people in the United States. This is higher than the cumulative hospitalization rate for the 2012-2013 flu season (44.0 per 100,000), when influenza A (H3N2) viruses also predominated, and is similar to the cumulative hospitalization rate during 2014-2015 (64.1 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 285.3 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 183.9 per 100,000. For week 17 during 2014-2015, it was 308.8 per 100,000.
The hospitalization rate among adults 50-64 years is 63.3 per 100,000. This is the highest hospitalization rate ever observed since this type of surveillance began. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 40.9 per 100,000 and 53.4 per 100,000 respectively.
The hospitalization rate among children younger than 5 years is 44.7 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 67 per 100,000 and 57.2 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 6.6% for the week ending April 15, 2017 (week 15). This percentage is below the epidemic threshold of 7.2% for week 15 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. The weekly percentage of deaths attributed to P&I was at or exceeded the epidemic threshold for 12 consecutive weeks this season.

Pediatric Deaths: Ten influenza-associated pediatric deaths are being reported by CDC for the week ending April 29, 2017, including three that occurred during the 2015-2016 season.
One death was associated with an influenza A (H3) virus and occurred during week 17 (the week ending April 29, 2017).
One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 10 (the week ending March 11, 2017).
Five deaths were associated with an influenza B virus and occurred during weeks 7, 15, and 16 (the weeks ending February 18, April 15, and April 22, 2017, respectively).
One death that was reported earlier this season was reclassified by the reporting jurisdiction.
A total of 89 influenza-associated pediatric deaths have been reported for the 2016-2017 season.
Three influenza-associated pediatric deaths that occurred during the 2015-2016 season were reported to CDC during the week ending April 29, 2017. One death was associated with an influenza A (H1N1)pdm09 virus.
One death was associated with an influenza A virus for which no subtyping was performed.
One death was associated with an influenza B virus
This brings the total number of reported influenza-associated pediatric deaths occurring during the 2015-2016 flu season to 92

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 29 was 6.9%.
Regionally, the three week average percent of specimens testing positive for influenza in clinical laboratories ranged from 5.6% to 17.7%.
During the week ending April 29, of the 938 (6.9%) influenza-positive tests reported to CDC by clinical laboratories, 257 (27.4%) were influenza A viruses and 681 (72.6%) 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 29, 52 (29.4%) of the 177 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 125 (70.6%) were influenza B viruses. Of the 50 influenza A viruses that were subtyped, 48 (96.0%) were H3N2 viruses and 2 (4.0%) 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 3,123  specimens (296 influenza A (H1N1)pdm09, 2,099 influenza A (H3N2), and 728 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.

One human infection with a novel influenza A virus was reported by the state of Texas. The person was infected with an influenza A (H3N2) variant (H3N2v) virus. The patient became ill with respiratory symptoms in February 2017, was not hospitalized, and has fully recovered from their illness. Swine contact at an agricultural event was reported in the week preceding illness onset. This is the first H3N2v virus infection detected in the United States in 2017.

Source: CDC