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Monitoring Influenza Activity, Including 2009 H1N1

This website is archived for historical purposes and is no longer being maintained or updated. For updated information on the current flu season, see the CDC Seasonal Flu website.

May 28, 2010, 12:00 PM ET

Surveillance for Influenza and Pneumonia-Associated Hospitalizations and Deaths for the 2009-2010 Season

How are influenza hospitalizations and deaths going to be tracked this season?

CDC has asked states to report either laboratory confirmed hospitalizations and deaths or syndromic cases, i.e. cases of presumed influenza and/or pneumonia based on ICD-9 coded hospitalizations or death reports each week. These will be reports of all influenza and pneumonia-related hospitalizations and deaths, not just those due to 2009 H1N1.

Is reporting hospitalizations and deaths associated with flu new?

Updated Nov 06

Routine seasonal surveillance does not count individual flu cases, hospitalizations or deaths (except for pediatric influenza deaths) but instead monitors activity levels and trends and virus characteristics through a nationwide surveillance system. The reporting of hospitalizations and deaths by state health departments was a new surveillance system that was initiated at the beginning of the 2009 H1N1 outbreak.

Beginning August 30, 2009, CDC modified this surveillance and asked states to report either laboratory confirmed hospitalizations and deaths or syndromic cases, i.e. cases of presumed influenza and/or pneumonia based on ICD-9 coded hospitalizations or death reports each week. This is a new system will be used to monitor trends in hospitalizations and deaths. CDC believes this system will provide a fuller picture of the burden of serious flu illness and deaths during this pandemic. This number will be cross-checked periodically against modeling studies to assess its validity. CDC has provided guidance for states on how to count and report these cases.

Exactly how are states reporting influenza and pneumonia-associated hospitalizations and deaths to CDC?

CDC has developed a web-based data application with which states can submit their influenza and pneumonia-associated hospitalization and death reports. Using this online system, data from each state is due by midnight each Tuesday and compiled and analyzed by CDC for publication the following Friday of the same week. Data from each reporting week runs from Sunday to Saturday, which is consistent with the Morbidity and Mortality Weekly Report (MMWR).  So, data from Sunday through Saturday is reported to CDC on the Tuesday of the next week and reported in the FluView report three days later, on Friday.

Will tracking of 2009 H1N1 hospitalizations and deaths after August 30 be the same as it was in the spring and summer?

No, tracking of 2009 H1N1 hospitalizations and deaths will not be the same after August 30, 2009. In an effort to add additional structure to the national 2009 H1N1 reporting, new case definitions for influenza-associated hospitalizations and deaths were implemented on August 30, 2009. The new definitions allow states to report to CDC hospitalizations and deaths (either confirmed OR probable) resulting from all types of influenza, not just those from 2009 H1N1 flu. This is a broader set of data than states were previously reporting as it now includes 1) laboratory-confirmed influenza for all types of influenza, and 2) pneumonia and influenza cases identified from hospital records, most of which will not be laboratory confirmed.

Reporting of Influenza and Pneumonia-Associated Hospitalizations and Deaths for the 2009-2010 Season

Why was the definition of what states should report broadened?

Updated Nov 06

The definition of what states should report was broadened to include all laboratory-confirmed influenza and influenza and pneumonia syndrome for three reasons:

  1. CDC believes that regular seasonal influenza viruses will co-circulate with 2009 H1N1 influenza and capturing all laboratory-confirmed influenza will provide a fuller picture of the burden of all flu during the pandemic.
  2. There are too many cases of flu to test and confirm so laboratory-confirmed data is a vast underestimate of the true number of cases and this bias would be exacerbated over the course of the pandemic as more and more people become ill.
  3. Influenza and pneumonia syndrome are diagnostic code used by all hospitals. Capturing this number will reflect a fuller picture of influenza and influenza-related serious illness and deaths in the United States during the pandemic. However, influenza and pneumonia syndrome hospitalizations and deaths may be an overestimate of actual number of flu-related hospitalizations and deaths because they are less specific for influenza, but these data can be helpful in following disease trends.

Do the numbers reported now include 2009 H1N1 cases in the spring and summer?

No. The number of reported hospitalizations and deaths was “re-set” to zero on August 30. The report of the first week of data for the newly defined system will appear in the September 11, 2009 FluView.

Why is CDC re-setting reported hospitalizations and deaths numbers for the 2009-10 influenza season?

At the request of the states, and in preparation for what is expected to be an early 2009-2010 flu season, CDC began a new reporting season for flu-associated hospitalizations and deaths on August 30, 2009. The first counts of the new reporting season appeared on September 11, 2009.

When will the rest of CDC’s surveillance systems be “re-set”?

CDC’s regular flu surveillance systems (not including the hospitalizations and deaths reporting system) are long-standing systems that run year-round. These regular reporting systems “re-set” each October (during Morbidity and Mortality Weekly Report (MMWR) Week 40 – October 4-10, 2009) in anticipation of a new flu season. CDC’s standard influenza surveillance, which includes viral surveillance, sentinel physician surveillance for influenza-like illness (ILI), deaths from the 122 Cities Mortality Monitoring System, and the number of laboratory-confirmed deaths from influenza among children, will all re-set starting with MMWR Week 40 as usual.

If states are reporting aggregate influenza hospitalizations and deaths, how will you tell what percentage of cases is due to 2009 H1N1 and what percentage is due seasonal influenza viruses?

Updated Nov 6

Due to CDC’s new case definitions, there will be no definitive way to differentiate between hospitalizations and deaths due to seasonal influenza versus those due to 2009 H1N1 influenza from aggregate reporting. However, information on the proportion of influenza viruses that are 2009 H1N1 versus seasonal influenza will continue to be reported in FluView from the virologic surveillance system. (For example, as of October 31, 2009, mare than 99% of all subtyped influenza A viruses being reported to CDC were 2009 H1N1 viruses.)

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Will states be reporting confirmed cases?

In the surveillance guidance provided to states, CDC has asked states to report either laboratory confirmed hospitalizations and deaths or syndromic cases, i.e. cases of presumed influenza and/or pneumonia based on ICD-9 coded hospitalizations or death reports.

What does this mean?

Laboratory confirmed influenza hospitalizations and deaths are those that were confirmed as attributed to influenza infection by a laboratory test. Syndromic influenza-associated hospitalizations and deaths are those caused by suspected or probable influenza or pneumonia. 

So different states will be reporting according to different criteria?

Yes, the new definition of influenza-related hospitalizations and deaths will allow some states to report confirmed cases, and others to report suspected/probable cases based on surveillance systems available in individual states. 

How accurate a representation are these numbers?

Updated Nov 06

Laboratory-confirmed data is thought to be an underestimation of the true number of cases because most people will not be tested for influenza. However, influenza and pneumonia syndrome hospitalizations and deaths may be an overestimate of actual number of flu-related hospitalizations and deaths because that diagnostic category includes other illnesses. Influenza and pneumonia syndromic reports are less specific to influenza, but are helpful in following disease trends.

Will the old 2009 H1N1 counts prior to August 30, 2009 remain available?

Yes, the cumulative number of 2009 H1N1-related hospitalizations and deaths reported to CDC from April through August 2009 will be archived and available for future reference.

What hospitalizations and death information will CDC report publicly?

Added Nov 06

From August 30-October 24, 2009 CDC publicly posted reports of both laboratory-confirmed influenza for all types of influenza AND syndromic cases, i.e., cases of presumed influenza and/or pneumonia based on ICD-9 coded hospitalizations or death reports each week. From the week beginning October 25-31, 2009; however, CDC will only publicly report laboratory-confirmed influenza-associated hospitalizations and deaths. The reason for this change in public reporting is that the interpretation of syndromic data (most of which is not laboratory-confirmed) relative to laboratory-confirmed data has been challenging. CDC will continue to collect and analyze both laboratory-confirmed and syndromic data reported to CDC by states.

How many states are reporting laboratory-confirmed hospitalizations and deaths?

Added Nov 06

The number of states reporting laboratory-confirmed hospitalizations and deaths varies each week, however, for the week of October 25-31, 2009, more than 30 U.S. states were continuing to report laboratory-confirmed data.

Regular Surveillance Systems


When did CDC stop reporting flu activity for the 2009-10 flu season?

The final CDC influenza activity report for the 2009-10 season appeared on May 28, 2010. While influenza surveillance is carried out year-round, official reporting (FluView publication) routinely happens between October and May to coincide with normally occurring influenza activity in the United States. FluView production will resume during MMWR week 40 (October 3-October 9, 2010). This will be the first report of the 2010-2011 flu season, and will be distributed on October 15. Surveillance reports for all influenza viruses for the 2010-2011 season – including the 2009 H1N1 virus -- will appear on the seasonal influenza website.

What does regular influenza surveillance consist of?

Regular surveillance includes:

  1. Viral surveillance, which monitors
    • The percentage of specimens tested for influenza that are positive for influenza;
    • The types and subtypes of influenza viruses circulating;
    • Resistance to influenza antiviral medications, and
    • The emergence of new strains
  2. Sentinel physician surveillance for influenza-like illness (ILI), which monitors the percentage of doctor visits for symptoms that could be the flu.
  3. Hospitalization surveillance, which tracks numbers of hospitalizations with laboratory-confirmed flu infections among adults and children.
  4. Summary of the geographic spread of flu, which tracks the number of states affected by flu and the degree to which they are affected.
  5. Deaths from 122 Cities that report the total number of deaths and the percentage of those that are coded as influenza or pneumonia.
  6. The number of laboratory-confirmed deaths from influenza among children.

How is CDC’s traditional flu surveillance system reported?

CDC’s flu surveillance is reported in a weekly publication called FluView. The Epidemiology and Prevention Branch in the Influenza Division at CDC collects, compiles and analyzes information on flu activity in the U.S. year-round to produce and publish FluView every Friday. Usually FluView is published from October through mid-May, but in response to the ongoing novel H1N1 flu spread, weekly publication of FluView continued over the summer months.

Why is FluView dated a week earlier than the date it is posted?

Flu surveillance data collection is based on a reporting week that starts on Sunday and ends on Saturday of each week. Each surveillance participant is requested to summarize weekly data and submit it to CDC by Tuesday afternoon of the following week. Those data are then downloaded, compiled, and analyzed at CDC and posted on the web on Friday, 3 days later.

For more information about CDC’s surveillance systems, see Overview of Influenza Surveillance in the United States.

2009 H1N1 Individual Case Counts

Why did CDC stop reporting confirmed and probable 2009 H1N1 flu cases?

Individual case counts were kept early during the 2009 H1N1 outbreak when the 2009 H1N1 virus first emerged. As the outbreak expanded and became more widespread, individual case counts become increasingly impractical and not representative of the true extent of the outbreak. This isbecause only a small proportion of persons with respiratory illness are actually tested and confirmed for influenza (including 2009 H1N1) so the true benefit of keeping track of these numbers is questionable. In addition, the extensive spread of 2009 H1N1 flu within the United States made it extremely resource-intensive for states to count individual cases. On July 24, 2009, CDC discontinued reporting of individual cases of 2009 H1N1, but continued to track hospitalizations and deaths.

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