CDC Flu Update: Illness Driven by a Wave of H3N2 Virus Activity

According to this week’s FluView report, overall influenza activity decreased slightly but remains high, driven by a wave of H3N2 virus activity. While this week saw small declines in levels of influenza-like-illness (ILI) and the proportion of clinical laboratory specimens testing positive for flu, 46 states and Puerto Rico continue to report widespread flu activity and 30 states are still experiencing high ILI activity. Influenza A(H1N1)pdm09 viruses remain predominant for the flu season nationally, however, during the week ending March 9, influenza A(H3) viruses were reported more frequently that H1N1pdm09 viruses for the second week in a row. Also, another four flu-related pediatric deaths occurring during the 2018-2019 season were reported by CDC, bringing the total to 68 flu-related pediatric deaths this season.

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. H3N2 viruses are typically associated with more severe illness in older adults, and flu vaccine may protect less well against H3N2 illness in older adults, making prompt treatment with flu antivirals in this age group especially important during the current period of H3N2 predominance.

Below is a summary of the key flu indicators for the week ending March 9, 2019:

Influenza-like Illness Surveillance: For the week ending March 9 (week 10), the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased slightly to 4.5% from 4.6%, but remains 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 10 regions reported a proportion of outpatient visits for ILI at or above their region-specific baseline levels.
For comparison purposes, over 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 32 states last week to 30 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 Puerto Rico and 48 states last week to Puerto Rico and 46 states 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, 11,922 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 41.3 hospitalizations per 100,000 people in the United States.
The highest hospitalization rate is among adults aged 65 years and older (123.9 per 100,000) followed by children younger than 5 years (54.8 per 100,000), and adults aged 50-64 years (54.0 per 100,000). During most seasons, adults 65 years and older have the highest hospitalization rates, followed by young children.
For comparison purposes:
The final, cumulative overall hospitalization rate for week 10 last season was 90.3 per 100,000.
Over the past 5 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 7.2% during the week ending March 2, 2019 (week 9). This percentage is below the epidemic threshold of 7.3% for week 9 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. P&I has been at or above threshold for a total of 5 weeks this season (weeks 1-3, and weeks 7 and 8).
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: Four influenza-associated pediatric deaths were reported to CDC during week 10 (the week ending March 9, 2019).
One death was associated with an influenza A(H1N1)pdm09 virus and occurred during week 6 (the week ending February 9, 2019). Two deaths were associated with an influenza A(H3) virus and occurred during weeks 9 and 10 (the weeks ending March 2 and March 9, 2019, respectively). One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 9 (the week ending March 2, 2019).
A total of 68 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 9 was 25.8%, a slight decline from 26.7% 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 9, influenza A(H3) viruses were reported more frequently than influenza A(H1N1)pdm09 viruses and have been predominant for the most recent three weeks in Regions 2, 4, 5, 6, 7, 8 and 10. However, overall for the 2018-2019 flu season, influenza A(H1N1)pdm09 viruses are still predominant nationally and in all regions of the U.S., except for the Southeast where H3N2 has predominated.
The majority of the influenza viruses collected from the United States during September 30, 2018, through March 9, 2019, were characterized antigenically and genetically as being similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses.
The vast majority (>99%) of influenza viruses tested showed susceptibility to oseltamivir, zanamivir and peramivir. This week, no new viruses with reduced susceptibility to antiviral drugs were reported. So far this season, two (0.1%) 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