CDC: Flu Season Ongoing with Tens of Thousands Hospitalized So Far

CDC continues to recommend flu vaccination and prompt antiviral treatment of severely ill and high-risk persons

January 11, 2019 – With the 2018-2019 flu season well underway, CDC today estimated that so far this season, between about 6 million and 7 million people have been sick with flu, up to half of those people have sought medical care for their illness, and between 69,000 and 84,000 people have been hospitalized from flu. CDC expects flu activity to continue for weeks and continues to recommend flu vaccination and appropriate use of antiviral medications.

Flu vaccination is the first line of defense to prevent flu and its potentially serious complications, including death in children. Flu vaccines have been shown to be life-saving in children, in addition to having other benefits. Flu vaccination has been shown in several studies to reduce severity of illness in people who get vaccinated but still get sick. Antiviral drugs are a second line of defense that can be used to treat flu illness. CDC recommends that people who are very sick or people who are at high risk of serious flu complications who develop flu symptoms should see a health care provider early in their illness for possible treatment with a flu antiviral drug.

CDC’s weekly FluView reports when and where influenza activity is occurring, what influenza viruses are circulating and their properties, and reports the impact influenza is having on hospitalization and deaths in the United States based on data collected from eight different surveillance systems.

So far this season, H1N1 viruses have predominated nationally, however in the southeast, H3N2 viruses have been most commonly reported. The number of states reporting widespread activity increased this week to 30 from 24 states last week. While levels of influenza-like-illness (ILI) declined slightly over the previous week in this week’s report, ILI remains elevated and 15 states and New York City continue to experience high flu activity. There also was a decline in the percent of respiratory specimens testing positive for flu at clinical laboratories however this number remains elevated also. During some previous seasons, drops in ILI and the percent of specimens testing positive for flu have been observed following the holidays.

Current Severity Lower Than Last Season

Flu severity is assessed by looking at levels of influenza-like-illness (ILI), flu hospitalization rates and the percentage of deaths resulting from pneumonia or influenza that occurred during each season. It’s not possible to say definitively at this time how severe the 2018-2019 season will be since there are still weeks of flu activity to come, but at this time, severity indicators are lower than they were during a similar time-frame last season.

ILI this week is 3.5%. Last season ILI peaked at 7.5%. [Over the past 5 seasons, peak of ILI has ranged from 3.6% (2015-2016) to 7.5% (2017-2018).]
The highest percentage of respiratory specimens testing positive for flu at clinical laboratories this season was almost 17% during the week ending December 29, 2018. 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%.
This week, the overall hospitalization rate is 9.1 per 100,000. For the same week last season, the overall hospitalization rate was 30.5 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 102.8 per 100,000 (2017-2018).]
So far this season, pneumonia and influenza (P & I) deaths have not yet exceeded the epidemic threshold. Last season, P&I was at or above the epidemic threshold for 16 consecutive weeks. [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).]

Despite these lower severity indicators, flu is still taking a serious toll this season. Today CDC reported an additional 3 flu pediatric deaths. To date there have been 16 flu pediatric deaths. Given that significant influenza activity is likely to continue to occur, severity indicators are expected to rise.

Modeling Provides National Case Estimates

Data presented in FluView allows CDC to track flu activity, but does not provide exact case counts, with the exception of flu-associated pediatric deaths and human infections with novel influenza A viruses, which are nationally notifiable. To round out the picture of the full burden of flu in the United States, CDC has estimated the total number of illnesses, medical visits and hospitalizations annually using a mathematical model. These estimates are being provided in-season for the first time.

To date, CDC estimates that this season in the United States, flu has caused between:

6.2 million to 7.3 million flu illnesses,
2.9 to 3.5 million medical visits, and
69,300 to 83,500 hospitalizations.

To put these numbers into context, total, end-of-season estimates for flu illnesses, medical visits and hospitalizations going back to 2010 have ranged from:

9.3 million to 49 million illnesses annually
4.3 million to 23 million medical visits annually
140,000 to 960,000 hospitalizations annually.

For all of these estimates, the low end of the range occurred during the 2011-2012 flu season, while the high end of the range occurred last season (2017-2018). CDC has classified 2011-2012 as a “low” severity season, while 2017-2018 was a “high” severity season overall and across all age groups.

The preliminary flu burden estimates are based on adjusted rates of laboratory-confirmed influenza-associated hospitalizations collected through FluSurv-Net, a hospitalization surveillance network that covers approximately 8.5% of the U.S. population, or about 27 million people. Hospitalization rates were then used to estimate a range of the number of illnesses, medical visits and hospitalizations, using mathematical multipliers.

All preliminary flu burden estimates are cumulative and will be updated weekly on the CDC website. A preliminary in-season estimate of flu deaths in the United States will be provided, pending availability of data.

More information is available on the CDC Seasonal Influenza (Flu) website.

Note: Delays in reporting may mean that data changes over time. The most up to date data for all weeks during the 2018-2019 season can be found on the current FluView and FluView Interactive.