H1N1 Monitoring Questions & Answers

Why has CDC stopped reporting confirmed and probable novel H1N1 flu cases?
Because only a small proportion of persons with respiratory illness are tested for novel H1N1, at this time, confirmed and probable case counts represent a significant underestimation of the true number of novel H1N1 flu cases in the U.S., so the true benefit of reporting these numbers to track the course of the epidemic is questionable. In addition, because of the extensive spread of novel H1N1 flu within the United States, it has become extremely resource-intensive for states to count individual cases.

What monitoring system will CDC use to replace counting confirmed and probable novel H1N1 flu cases?
Instead of reporting confirmed and probable novel H1N1 flu cases, CDC has transitioned to using its traditional flu surveillance systems to track the progress of both the novel H1N1 flu pandemic and seasonal influenza. These systems work to determine when and where flu activity is occurring, track flu-related illness, determine what flu viruses are circulating, detect changes in flu viruses and measure the impact of flu on hospitalizations and deaths in the U.S

What do CDC’s traditional flu surveillance systems entail?
Several systems are in place to address CDC’s flu surveillance needs. These include:

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.

Routine seasonal surveillance does not count individual flu cases, except in the case of pediatric influenza deaths, but instead, monitors activity levels and trends and virus characteristics through this nationwide surveillance system.

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 is continuing 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.

Will CDC expand its flu surveillance to track the progress of novel H1N1 flu for the upcoming flu season?
CDC continues to work with the Council of State and Territorial Epidemiologists (CSTE) and other partners to determine ways to enhance surveillance for novel H1N1 flu during the 2009-10 flu season. This may include establishing new systems, and will include more detailed information and easier to understand graphics. Also a subset of sites reporting ILI data, deaths, and laboratory data through CDC’s traditional influenza surveillance systems have moved from weekly to daily reporting.