CDC hails Colorado's counting of flu-linked hospital cases

Jun 2, 2005 (CIDRAP News) – Colorado's new system for tracking hospital admissions related to influenza is a potential model for measuring the burden of serious flu complications and the severity of flu seasons, the Centers for Disease Control and Prevention (CDC) said today.

Last September Colorado became the first state to require the reporting of hospital admissions for laboratory-confirmed flu in all age-groups, the CDC says in the Jun 3 issue of Morbidity and Mortality Weekly Report. Though some existing surveillance systems yield estimates of flu-related hospitalizations, no other state or national system is designed to pick up all such cases.

Colorado's first season of experience with the system indicates that "implementation of statewide, population-based surveillance for influenza-associated hospitalizations is feasible and useful for assessing the age-specific burden of serious influenza-associated morbidity and the relative severity of influenza seasons," the CDC says.

For purposes of reporting, flu-associated hospitalization was defined as a hospital admission accompanied by laboratory confirmation of flu, including confirmation by a rapid diagnostic test, according to the article. Reporting is done mainly by hospital infection-control practitioners (ICPs), who either use the state's Web-based disease reporting system or fax reports to the state health department.

By Apr 16, 2005, 50 Colorado hospitals had reported a total of 964 flu-related hospitalizations, yielding a rate of 21.0 per 100,000 people, the CDC reports. Cases peaked in the week that ended Feb 19; the same week marked the peak for the percentage of patient visits attributed to flu-like illness, as reported by sentinel healthcare providers in Colorado.

By age-group, the highest rate of flu-related hospitalization was in people aged 80 and older, with 207.3 cases per 100,000. The next highest rates per 100,000 were in babies younger than 6 months, 183.0; 70- to 79-year-olds, 78.0; and children aged 6 to 23 months, 66.3. People aged 18 to 39 years had the lowest rate at 5.8 cases per 100,000.

The CDC says the Colorado figures are similar to estimates based on national hospital discharge data. Those estimates include an average of 36.8 cases per 100,000 population for pneumonia and flu hospitalizations for flu seasons from 1979-80 through 2000-01.

While national estimates based on hospital discharge data take at least a year to compile, the Colorado system yields "real-time, population-based" numbers. "The system provides improved ability to assess the severity of influenza seasons, track the time course of the season, determine which populations are most affected by severe influenza-related illness, and focus prevention and control efforts on those populations," the CDC states.

A national system like Colorado's could help guide flu immunization policy and help health agencies quickly determine if a flu season is causing high rates of hospitalizations, the article says. In addition, such a system could help identify a flu pandemic and guide the public health response.

Ken Gershman, MD, MPH, a Colorado epidemiologist who contributed much of the MMWR article, said Colorado had previously required healthcare providers to report all positive influenza tests. The switch to requiring only the reporting of cases involving hospitalization has lightened the workload for providers, he told CIDRAP News.

"Over the past 5 or 6 years we've seen the number of positive tests go up quite high, because the availability of tests has become widespread," said Gershman, who is chief of the communicable disease program in the Colorado Department of Public Health and Environment. "Two years ago, we had about 13,000 reports in our system, and that overwhelmed hospitals and public health." As a result, hospitals supported changing the requirement to cover only hospital admissions, because they could see it would be less burdensome, he added.

Gershman said that while Colorado is the only state that requires reporting of flu-related hospital admissions in all age-groups, a CDC colleague told him that New Jersey requires reporting of hospital cases among children.

The MMWR article says the surveillance program has some limitations that are likely to result in underestimates of cases. One is that not everyone hospitalized with a respiratory illness or a flu-related exacerbation of a chronic disease is likely to be tested for flu. A second is that rapid flu tests are less sensitive than viral culture tests.

The possibility of making flu-related hospitalizations reportable in more states will be discussed at next week's annual meeting of the Council of State and Territorial Epidemiologists, CDC spokeswoman Jennifer Morcone told CIDRAP News today.

CDC. Surveillance for laboratory-confirmed, influenza-associated hospitalizations—Colorado, 2004-05 influenza season. MMWR 2005 Jun 3;54(21):535-7

See also:

Sep 22, 2004, CIDRAP News story "Flu-related hospitalizations have risen among elderly"

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