CDC Flu Update: Overall Activity Decreasing

April 1, 2019

According to this week’s FluView report, overall influenza activity decreased since last week, but remains relatively high for this time of year. Thirty-four states and Puerto Rico continue to report widespread flu activity and 20 states are still experiencing high ILI activity. Influenza A(H1N1)pdm09 viruses remain predominant for the flu season overall nationally; however, influenza A(H3) viruses have been reported more frequently than A(H1N1)pdm09 viruses in all regions for the last three weeks. Also, one flu-related pediatric death occurring during the 2018-2019 season was reported by CDC, bringing the total to 77 flu-related pediatric deaths this season.

Influenza-like-illness levels have been at or above baseline for 18 weeks this season. By this measure, the last five seasons have averaged 16 weeks, with a range of 11 to 20 weeks. CDC expects flu activity to remain elevated for a number of weeks. While CDC continues to recommend influenza vaccination as long as influenza viruses are circulating, influenza antiviral drugs are an important second line of defense that can be used to treat flu illness. A(H3) viruses are typically associated with more severe illness in older adults, and flu vaccine may protect less well against A(H3) illness in older adults than other virus components, making prompt treatment with flu antivirals in this age group especially important during the current period of A(H3) predominance. A clinician alert on this was issued on March 28 and is available at: https://emergency.cdc.gov/han/han00419.asp

Below is a summary of the key flu indicators for the week ending March 23, 2019:

Influenza-like Illness Surveillance: For the week ending March 23 (week 12), the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased from 4.3% to 3.8% and is above the national baseline of 2.2%. The most recent data indicates that ILI activity for the current season peaked the week ending February 16 (week 7) at 5.1%.
All regions show decreasing ILI, with the exception of regions 9 and 10, which are remaining level.
All 10 regions continue to experience a proportion of outpatient visits for ILI at or above their region-specific baseline levels.
For the past five flu seasons, the peak percent of visits due to ILI has ranged between 3.6% (2015-2016) and 7.5% (2017-2018).
Additional ILINet data, including national, regional, and select state-level data for the current and previous seasons, can be found at http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html.
Influenza-like Illness (ILI) State Activity Indicator Map: The number of states experiencing high ILI activity decreased from 26 states last week to 20 states this week. Additional data, including data for previous seasons, can be found at https://gis.cdc.gov/grasp/fluview/main.html. https://gis.cdc.gov/grasp/fluview/main.html.
Geographic Spread of Influenza Viruses: The number of jurisdictions reporting widespread influenza activity decreased from 44 states last week to 34 states and Puerto Rico this week. Geographic spread data reflect how many areas within a state or territory are seeing flu activity. Additional data are available at: https://gis.cdc.gov/grasp/fluview/FluView8.html.

Flu-Associated Hospitalizations: Since October 1, 2018, 15,165 laboratory-confirmed influenza-associated hospitalizations have been reported through the Influenza Hospitalization Network (FluSurv-NET), a population-based surveillance network for laboratory-confirmed influenza-associated hospitalizations covering approximately 9% of the U.S. This translates to a cumulative overall rate of 52.5 hospitalizations per 100,000 people in the United States.
The highest hospitalization rate is among adults aged 65 years and older (167.0 per 100,000) followed by adults aged 50-64 years (67.4 per 100,000), and children younger than 5 years (63.6 per 100,000). During most seasons, adults 65 years and older have the highest hospitalization rates.
For comparison purposes:
The final, cumulative overall hospitalization rate for week 12 last season was 95.4 per 100,000.
Over the past five seasons, cumulative end-of-season hospitalization rates have ranged from 31.4 per 100,000 (2015-2016) to 102.9 per 100,000 (2017-2018).
Additional data, including hospitalization rates during previous influenza seasons, can be found at http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html.
FluSurv-Net data is used to generate national estimates of the total numbers of flu cases, medical visits, and hospitalizations. This season, CDC is reporting preliminary cumulative in-season estimates, which are available at https://cdc.gov/flu/about/burden/preliminary-in-season-estimates.htm.

Mortality Surveillance: The proportion of deaths attributed to pneumonia and influenza (P&I) was 7.4% during the week ending March 16, 2019 (week 11). This percentage is above the epidemic threshold of 7.2% for week 11 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. P&I has been at or above threshold for a total of 6 weeks this season (weeks 1-3, weeks 7-8, and week 11).
For comparison purposes, over the last five seasons, P&I has been at or above epidemic threshold for a range of four weeks (2015-2016) to 16 weeks (2017-2018).
Additional P&I mortality data for current and past seasons and by geography (national, HHS region, or state) are available at https://gis.cdc.gov/grasp/fluview/mortality.html

Pediatric Deaths: One influenza-associated pediatric death was reported to CDC during week 12 (the week ending March 23, 2019).
This death was associated with an influenza A(H1N1)pdm09 virus and occurred during week 12 (the week ending March 23, 2019).
A total of 77 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: https://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html. 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 March 23 was 22.1%, a decrease from 26.3% the prior week.
For comparison purposes, since laboratory data from clinical and public health laboratories was disaggregated three seasons ago, the peak percent of respiratory specimens testing positive for flu at clinical laboratories has ranged from 23.6% to 27.4%.
Nationally, during the week ending March 23, influenza A(H3) viruses were reported more frequently than influenza A(H1N1)pdm09 viruses and were predominant in all 10 regions
However, overall for the 2018-2019 flu season, influenza A(H1N1)pdm09 viruses remain predominant nationally and in all regions of the U.S., except for Region 4 (Southeast) where A(H3) has predominated and Regions 6 and 7 (South Central and Midwest) where A(H1N1)pdm09 and A(H3) viruses have circulated in roughly equal proportions.
The A(H1N1)pdm09, B/Victoria and B/Yamagata influenza viruses collected in the United States during September 30, 2018 through March 23, 2019, are still characterized antigenically and genetically as being similar to their cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses; However, an increasing proportion of influenza A(H3) viruses are antigenically distinguishable from A/Singapore/INFIMH-16-0019/2016 (3C.2a1), a cell-propagated reference virus representing the A(H3N2) component of 2018-19 Northern Hemisphere influenza vaccines.
The vast majority (>99%) of influenza viruses tested showed susceptibility to oseltamivir, zanamivir and peramivir. This week, one additional influenza A(H1N1)pmd09 virus with highly reduced susceptibility to the antiviral drugs oseltamivir and peramivir were reported. So far this season, three (0.2%) influenza A(H1N1)pdm09 viruses displayed highly reduced inhibition by oseltamivir and peramivir. An additional two (0.1%) influenza A(H1N1)pdm09 viruses showed reduced inhibition by oseltamivir. All influenza viruses tested showed susceptibility to zanamivir.

Source: CDC