
Appendix 3 for Busch et al. manuscript in March 2006 EID West Nile Virus Seasonal incidence The seasonal incidence or proportion of a population infected with West Nile virus during 2003 is a measure of the incident infections in the population at risk over a time period, t, and can be estimated using minipoolNAT data and an estimate of the average period of time during which RNA is detectable by minipoolNAT (T_{MPNAT}). For a month, i, the monthly proportion of donations with West Nile virus RNA (monthly minipoolNAT yield) can be derived by dividing the number of West Nile virus minipoolNAT positives in the month by the number of donations screened in the month:
The monthly average incidence rate can be estimated from the monthly West Nile virus minipoolNAT data and from a monthly persontime estimate (34, 35). On average, each donation can be considered at risk for being minipoolNAT positive during the average period of time in which RNA is detectable by minipoolNAT (T_{MPNAT}). Thus, monthly persontime can be estimated as the number of donations in the month multiplied by T_{MPNAT}. Hence, the monthly average incidence rate can be estimated as:
The estimated monthly average incidence rate, , is a rough approximation to the actual average monthly incidence rate but is considered adequate for this study (e.g., although some minipoolNAT positives may be incidents from the previous month, this is counterbalanced by incidents occurring late in the month that would not be identified by minipoolNAT). The monthly West Nile virus incidence (i.e., proportion of blood donors who were infected during the month) is estimated from the monthly average incidence rate. That is, the monthly West Nile virus incidence is equal to the monthly average incidence rate times the number of days in the month:
The seasonal West Nile virus incidence (i.e., proportion of blood donors who were infected during the 2003 season) can then be estimated by summing the estimated monthly West Nile virus incidences for these four months (36);

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