CDC Flu Update: Influenza-Like Illness Above National Baseline

According to this week’s FluView report, seasonal influenza activity remains elevated overall and increased again over last week’s report. Influenza-like-illness (ILI) is 4.3%, which is above the national baseline of 2.2% and the highest it has been so far this flu season, but is still substantially lower than the 7.5% peak recorded last season. Severity indicators are increasing as expected, but remain well below what indicators observed last season. However, this week another four flu-related pediatric deaths were reported to CDC, bringing the total to 28 flu-related deaths in children for the 2018-2019 flu season. H1N1 viruses have been the most commonly identified flu viruses nationally. However, H3N2 viruses have predominated in the southeastern region of the United States.

CDC expects flu activity to remain elevated for a number of weeks. An annual flu vaccine is the best way to protect against influenza and its potentially serious complications. There are many benefits to vaccination, including reducing the risk of flu illness, doctor’s visits, hospitalization, and even death in children. Flu vaccination also has been shown to reduce severity of illness among people who get vaccinated but still get sick. For anyone 6 months or older who has not yet been vaccinated this season, CDC recommends that they get vaccinated now. There also are flu antiviral drugs that can be used to treat flu illness. Below is a summary of the key flu indicators for the week ending February 2, 2019:

Influenza-like Illness Surveillance: For the week ending February 2 (week 5), the proportion of people seeing their health care provider for influenza-like illness (ILI) increased from 3.8% to 4.3%, which is above the national baseline of 2.2%. This is the highest ILI activity has been this flu season.
All 10 regions reported a proportion of outpatient visits for ILI at or above their region-specific baseline level.

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 state experiencing high ILI activity increased from 23 states plus NYC last week to 24 states plus New York City. The 24 states are Alabama, Alaska, Arkansas, Colorado, Connecticut, Georgia, Indiana, Kansas, Kentucky, Massachusetts, Mississippi, Nebraska, New Jersey, New Mexico, North Carolina, Oklahoma, Rhode Island, South Carolina, South Dakota, Texas, Utah, Vermont, Virginia and West Virginia. Puerto Rico and 10 states (Arizona, California, Hawaii, Illinois, Louisiana, Missouri, New York, North Dakota, Pennsylvania, and Tennessee) experienced moderate ILI activity. The District of Columbia and 13 states (Florida, Idaho, Iowa, Maine, Maryland, Michigan, Minnesota, Montana, Nevada, New Hampshire, Oregon, Washington, and Wisconsin) experienced low ILI activity. Three states (Delaware, Ohio, and Wyoming) experienced minimal ILI activity. Additional data, including data for previous seasons, can be found at https://gis.cdc.gov/grasp/fluview/main.html.

Geographic Spread of Influenza Viruses: The number of states reporting widespread influenza activity rose from 45 states to Puerto Rico and 47 states (Alabama, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, 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, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, Wisconsin and Wyoming). Regional influenza activity was reported by two states (Alaska and West Virginia). Local influenza activity was reported by the District of Columbia and one state (Hawaii). Sporadic influenza activity was reported by the U.S. Virgin Islands. Guam did not report. 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, 5,791 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 20.1 hospitalizations per 100,000 people in the United States.

The highest hospitalization rate is among adults aged 65 years and older (53.0 per 100,000) followed by children younger than 5 years (33.5 per 100,000), and adults aged 50-64 years (27.2 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 5 last season was 65.5 per 100,000.
Over the past 5 seasons, cumulative end-of-season hospitalization rates have ranged from 35.1 per 100,000 (2013-2014) 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 6.9% during the week ending January 26, 2019 (week 4). This percentage is below the epidemic threshold of 7.2% for week 4 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. P&I has been at or above threshold for three weeks this season.

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 5 (the week ending February 2, 2019).

Two deaths were associated with influenza A(H1N1)pdm09 viruses and occurred during weeks 4 and 5 (the weeks ending January 26, 2019 and February 2, 2019). Two deaths were associated with influenza A viruses for which no subtyping was performed and also occurred during weeks 4 and 5 (the weeks ending January 26, 2019 and February 2, 2019).

A total of 28 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 February 2 was 21.6%, an increase from 19.0% the prior week. This is the highest proportion of specimens testing positive for flu viruses in clinical laboratories this season.

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%.

Regionally, the three-week average percent of specimens testing positive for influenza in clinical laboratories ranged from 13.7% to 25.5%.

During the week ending February 2, of the 7,205 (21.6%) influenza-positive tests reported to CDC by clinical laboratories, 7,080 (98.3%) were influenza A viruses and 125 (1.7%) were influenza B viruses.

The most frequently identified influenza virus type reported by public health laboratories was influenza A(H1N1)pdm09 virus.

During the week ending February 2, 829 (98.3%) of the 843 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 14 (1.7%) were influenza B viruses. Of the 806 influenza A viruses that were subtyped, 282 (35.0%) were H3N2 viruses and 524 (65.0%) were (H1N1)pdm09 viruses.

The majority of the influenza viruses collected from the United States during September 30, 2018 through February 2, 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 (0.4%) influenza A(H1N1)pdm09 viruses displayed highly reduced inhibition by oseltamivir and peramivir. An additional two (0.4%) influenza A(H1N1)pdm09 viruses showed reduced inhibition by oseltamivir. All influenza viruses tested showed susceptibility to zanamivir.

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 (0.5%) influenza A(H1N1)pdm09 viruses have demonstrated highly reduced antiviral susceptibility to oseltamivir and peramivir.  An additional two (0.5%) influenza A(H1N1)pdm09 viruses demonstrated reduced antiviral susceptibility to just oseltamivir. All influenza viruses tested showed susceptibility to zanamivir.

Source: CDC