Effect of different definitions of numerators and denominators to estimate the incidence rates of diarrhea and upper respiratory tract infections (URTI) in day-care centers (DCC)

Introduction / Objective: Public health issues have been raised about the increased incidence rates (IR) of diarrheal and upper respiratory tract (URTI) infections in day-care (DCC) settings. However, because of the variety of measures used, it is difficult to make comparisons and uptake of research results into public health practice may suffer. We assessed the impact of different definitions for episodes of illness and person-time on IR estimation.

Methods: Data from a 13-month-long cohort study of 2408 toddlers were used. Daily occurrence of illness, expected days of attendance, and daily absences, were recorded by DCC educators. Four definitions of IR using different lag times to define independent episodes (numerator) and at-risk time units to define child-at-risk (denominator) were compared.

Results: For diarrhea, the four different IR estimates were 1.41 episodes / 365 child-days at risk, 0.79 episodes / 52 child-weeks at risk, 0.81 episodes / 26 child-fortnights at risk, and 0.88 episodes / 12 child-months. For URTI, the four IRs were 7.64 episodes / 365 child-days at risk, 4.72 episodes / 52 child-weeks at risk, 5.05 episodes / 26 child-fortnight at risk, and 4.22 episodes / 12 child-months.

Conclusion: Different episodes and population-time definitions affect the estimation of the IR of diarrhea and URTI. Future research will investigate how they may also affect the estimation of the strength of risk factors for infection. This research will contribute to the development of a more standard approach to the measurement and reporting of incidence rates in epidemiological, occupational and environmental health studies.