CDC cuts US SARS case count in half

Jul 17, 2003 (CIDRAP News) – Because of a change in the case definition for SARS (severe acute respiratory syndrome), the United States has had only half as many suspected and probable cases of the illness as previously reported, federal health officials said today.

The Centers for Disease Control and Prevention (CDC) said the total case count is now 211 instead of 418, a 49.5% reduction. The official tally now is 175 suspected and 36 probable cases, down from 344 suspected and 74 probable cases.

The change is a result of excluding all cases in which convalescent blood samples—those collected more than 21 days after illness onset—tested negative for the SARS coronavirus, the CDC said in a news release. "Exclusion of these cases with negative convalescent serum provides a more accurate accounting of the epidemic in the U.S.," the agency said.

The Council of State and Territorial Epidemiologists recommended changing the SARS case definition to exclude cases with negative convalescent serum tests. The recommendation is based on evidence that 95% of SARS patients mount a detectable antibody response in the convalescent phase, the CDC said.

The revised case definition and case count are detailed in the Jul 18 issue of Morbidity and Mortality Weekly Report, published online today.

"Serologic testing results suggest that a small proportion of persons who had illness consistent with the clinical and epidemiologic criteria for a U.S. case of suspect or probable SARS actually had SARS," the article states. "The case definition captures an array of respiratory illnesses that cannot be easily distinguished from SARS until laboratory testing results for SARS and other agents are performed." The sensitive case definition allowed for rapid investigation of possible cases and public health steps to prevent spread of the disease, the report adds.

The MMWR article also recommends changing the time for collecting convalescent-phase serum specimens to test for the SARS virus from more than 21 days to more than 28 days after illness onset. The CDC said recent data indicate that some people who have SARS do not have a detectable antibody response until 28 days after the onset of symptoms. However, test results from serum previously collected between 22 and 28 days are acceptable, the agency said.

The CDC lifted the SARS-related travel alert for Taiwan—the last remaining alert—Jul 15. "With removal of all SARS travel alerts and completion of an incubation period (10 days), U.S. travelers with respiratory illness will no longer meet the current case definition for SARS," the agency said in today's statement. "Reports of suspect or probable cases, therefore, are expected to end by July 31, 2003."

See also:

CDC news release

MMWR article

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