Influenza Outbreak Detection Is Becoming Faster, More Detailed

Key Point

Influenza-related morbidity may be easier to track by patient age and viral strain using electronic influenza-related information from hospital emergency departments.

Influenza outbreaks and pandemics may appear to strike more quickly than they actually do because current surveillance systems are incapable of tracking them fast enough to detect them before they are in full swing. In the United States, these systems—which in part rely on clinicians to voluntarily report influenza-like illness—are fraught with reporting delays and low participation rates.

However, some regions of the country are bringing influenza surveillance into the 21st century by employing a so-called syndromic surveillance system. In this system, hospitals provide public health agencies with electronic data from the emergency department (ED). Indicators such as increased emergency department visits for fever or respiratory problems can provide an early warning of an influenza outbreak, Donald R. Olson, MPH, and colleagues said in the August PLoS Medicine. Mr. Olson is a research scientist at the New York City Department of Health and Mental Hygiene.

The investigators recently used the syndromic surveillance system in New York City to evaluate whether ED chief complaint data can help measure influenza morbidity by age and predominant circulating influenza strain. “The importance of understanding age when estimating the impact of influenza on hospitalizations and deaths has been well described, yet existing surveillance systems have not made adequate use of age-specific data,” the investigators pointed out.

SYNDROMIC SURVEILLANCE

During the study, the investigators analyzed ED fever and respiratory chief complaint and viral surveillance data from November 2001 through June 2006. Information on the predominant circulating influenza strains during that period were obtained from national surveillance reports. Hospitals submitted ED reports daily, and by study’s end, coverage had increased to include approximately 90% of all ED visits.

The researchers found that influenza-related morbidity in New York City was highly age specific, with increased ED visits for fever or respiratory symptoms during the influenza season occurring disproportionately and earliest among children. The rise in ED visits generally coincided with pneumonia and influenza hospitalization data and preceded pneumonia and influenza death by roughly one to two weeks, said the investigators. Influenza-related morbidity also appeared to be highly strain-specific, they added.

A total of 13.3 million ED visits were reported by participating New York City facilities during the study period. Of these, “2.3 million were categorized into a broad ‘fever and respiratory syndrome’ composed of the hierarchical and mutually exclusive syndromes ‘respiratory,’ ‘fever/flu,’ ‘common cold,’ and ‘sepsis,’” the investigators related. Specifically, the chief reporting complaints for these syndromes were described as follows:

Sepsis syndrome (included to capture visits describing potential severe influenza outcomes that would otherwise have been missed): sepsis bacteremia, cardiac arrest, unresponsive, unconscious, dead on arrival.

Common cold syndrome (not including visits captured within sepsis syndrome): stuffy nose or nasal or cold symptoms.

Respiratory syndrome (not including visits captured within the sepsis or common cold syndromes): pneumonia, shortness of breath, bronchitis, upper respiratory tract infection, difficulty breathing, pleurisy, croup, cough, dyspnea, chest cold.

Fever/flu syndrome (not including visits captured within the sepsis, common cold, or respiratory categories and not including key words representing acute gastroenteritis, enteritis, or diarrhea): fever, chills, malaise, body aches, viral syndrome, influenza.

The fever and respiratory syndrome category was thought to provide the most sensitive measure of ED visits potentially attributable to influenza. However, the researchers also created a narrower category—influenza-like-illness (ILI) syndrome—which was defined as an influenza keyword or a fever-related keyword with a mention of cough and/or sore throat. Of the 2.3 million ED visits that fit into the fever and respiratory syndrome category, 260,000 were categorized as ILI.

Incorporating both fever and respiratory syndrome and ILI syndrome into the model, the investigators found that ED visits for influenza appeared to spike during peak influenza periods, although this effect varied considerably by age-group and the predominant circulating strain. For example, an excess of 8.5 ED visits per 1,000 was observed for children ages 5 to 17 years during the influenza B/Victoria epidemic of early 2002. There was no increase in ED visits from adults during that epidemic, however.

By contrast, there were excess ED visits in all age-groups during the two influenza A/Fujian epidemics that occurred between 2003 and 2005 (excesses of 9.2 and 5.2 per 1,000 visits, respectively). However, “the relative increase was greatest and earliest among school-aged children,” reported the investigators. In fact, for every influenza epidemic that occurred during the study period, ED visits increased and the epidemic peaked sooner among school-age children.

Notably, there were increased ED visits for fever and respiratory symptoms among children younger than 5 during periods of respiratory syncytial virus circulation, regardless of influenza circulation. Furthermore, although the narrower illness category of ILI syndrome accounted for only 11% of the broader category of fever and respiratory syndrome, prevalence trends shown in the two categories were highly correlated (coefficient of determination, 0.96).

Based on their findings, the investigators suggested the incorporation of greater age detail into influenza surveillance and the use of electronic data to provide timely and representative information about the age-specific epidemiology of circulating influenza viruses. “For severe seasonal or even pandemic influenza, knowing the age pattern early on may be critical,” Mr. Olson told Pulmonary Reviews.           

—Timothy Begany

Reference
Olson DR, Heffernan RT, Paladini M, et al. Monitoring the impact of influenza by age: emergency department fever and respiratory complaint surveillance in New York City. PLoS Med. 2007;4(8):e247.

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