BACKGROUND: The presence of imported Zika virus (ZIKV) cases and a potentially competent Aedes albopictus vector in New York City (NYC) suggests that local transmission of ZIKV in NYC is a theoretical possibility. Urosurveys can be used to detect ZIKV infections in the event of suspected local transmission. In many parts of NYC, the population is too large to feasibly include all local residents in a urosurvey. Therefore, the NYC Department of Health and Mental Hygiene developed a sampling plan.
METHODS: If two cases of ZIKV infection with no history of travel to a ZIKV-affected area or sexual contact with an infected person were to occur in the same geographic area, then a 150-meter radius circle around each residence or suspected exposure location will be delineated as a suspected risk site. To estimate the underlying population size within a suspected risk site, an in-house GIS application will be used to anonymously extract the number of residential units (from United States Postal service data) and the mean household size (from US Census data). The urosurvey sample size will be calculated using freedom from disease methods (R package FFD, function computeOptimalSampleSize) adapted from veterinary epidemiology. This approach requires inputs of the underlying population size, the maximum number of allowable missed cases within the risk site (10), the alpha level (0.05), and the test screening sensitivity (93%). The sample size will be inflated to account for predicted non-response (50%). Five NYC communities varying in population density and housing types were selected as illustrative examples based on local numbers of imported ZIKV cases and trap counts of Aedes mosquitoes.
RESULTS: In a 150-meter radius in each of the five communities, the number of urine samples needed ranged from 133 (estimated population of 479, predominantly in low-rise buildings) to 1,244 (estimated population of 4,453, predominantly in high-rise buildings). In the former risk site, 266 people would need to be approached to achieve the required sample size, corresponding to 83 households. In the latter risk site, 2,488 people would need to be approached, corresponding to 968 households.
CONCLUSIONS: Sampling is necessary to determine the extent of locally acquired ZIKV virus transmission in a dense urban environment like NYC. Preparing for such a urosurvey requires detailed residential population geodata and specialized methods for rare events. Finally, there is a necessary tradeoff of resource allocation and the degree of confidence that cases will not be missed.