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Monitoring Over-the-Counter Pharmacy Sales
for Early Outbreak Detection --- New York City,
August 2001--September 2003

Debjani Das, F. Mostashari, D. Weiss, S. Balter, R. Heffernan
New York City Department of Health and Mental Hygiene, New York, New York

Corresponding author: Debjani Das, New York City Department of Health and Mental Hygiene, 125 Worth Street, Room 318, CN#22A, New York, NY 10013. E-mail:


Introduction: Because over-the-counter medications (OTCs) are commonly taken before patients seek medical care, OTC sales data might serve as an early indicator of communitywide illness. Since August 2002, the New York City Department of Health and Mental Hygiene (DOHMH) has tracked OTC sales from New York City pharmacies to enhance detection of natural and intentional infectious disease outbreaks.

Objectives: First-year surveillance results on OTC sales were summarized and compared with results from an emergency department (ED) syndromic surveillance system.

Methods: A file containing the number of OTC units sold the previous day, by drug name and retail store, is transmitted to DOHMH daily from a central pharmacy database. The influenza-like illness (ILI) drug category includes cough and influenza medications whose sales correlate strongly with annual influenza epidemics. The antidiarrheal drug category includes generic and brand-name loperamide. Citywide trends are evaluated by using a linear regression model, controlling for seasonality, day of week, promotional sales, and temperature and are compared with ED data. Spatial clustering by store is evaluated by using the spatial scan statistic (SaTScan software, available at

Results: Citywide ILI drug sales were highest during annual influenza epidemics and elevated during the spring and fall allergy seasons, similar to trends in the ED system (Figure). Loperamide sales peaked during influenza season but did not increase substantially during the November 2002 viral gastroenteritis season. A spike in loperamide sales occurred after the August 2003 New York City blackout. Spatial signals for ILI sales occurred on 277 of 365 days.

Conclusions: The effect of allergies and asthma on respiratory illness should be considered when interpreting trends in OTC sales for ILI. Loperamide sales were not a useful indicator of a large 2002 norovirus outbreak detected by ED surveillance. Spatial cluster analysis was sensitive to variability in sales data by store and has not proven useful. OTC sales indicate correlation with other syndromic surveillance systems, but methods require refinement.


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