![]() This certainly provides strong evidence that a Ron's Grill was the source of the outbreak, and further investigation confirmed that one of the food handlers at Ron's had recently had a subclinical case of hepatitis A. The other three restaurants that had been suspects had odds ratios that were close to 1.0. This suggests that patrons of Ron's Grill had 75 times the risk of being a case compared to those who did not eat at Ron's. In contrast, consider the findings for Ron's Grill:įor Ron's Grill the odds ratio would be computed as follows: In this case the 95% confidence interval is 0.37 to 3.35, and p= 0.85. This certainly provides no compelling evidence to suggest an association with Papa Gino's, but, as we did with the risk ratio, we could compute a 95% confidence interval for the odds ratio, and we could also compute a p value. The odds ratio can be interpreted the same way as a risk ratio. These odds are quite similar, and the odds ratio is close to 1.0. Given these hypothetical results, the odds that someone who ate a Papa Gino's was a case were 10/19, while the odds that someone not exposed to Papa Gino's became a case were 9/19. ![]() An odds ratio can be computed for each of the possible sources. However, one can calculate the odds of disease in exposed and non-exposed subjects, and these can be expressed as an odds ratio, which is a good approximation of a risk ratio in a situation like this, i.e., when the outcome is rare. ![]() When using a case-control strategy for sampling, it is not possible to calculate the incidence ( attack rate ) in exposed and non-exposed subjects, because the denominators of the exposure groups are unknown. Investigators then ascertained the prior exposures of subjects in each group, focusing on food establishments and other possibly relevant exposures they had had during the past two months. In this case, the "controls" were non-diseased people who were matched to the cases with respect to age, gender, and neighborhood of residence. The investigators identified as many cases as possible (19 agreed to answer the questionnaire), and they selected a sample of 38 non-diseased people as a comparison group (the controls). In a situation like this a case-control design is a much more efficient option. Moreover, the disease was rare, so that even if they interviewed a sample of patrons at each of the restaurants, it is most likely that few, if any would have had recent hepatitis, even from the responsible restaurant. Hypothesis-generating interviews resulted in five food establishments that were candidate sources. The epidemic curve suggested a point source epidemic, and the spot map showed the cases to be spread across the entire South Shore of Massachusetts, although the pattern suggested a focus near Marshfield. Within a short period of time 20 cases of hepatitis A were identified in the Marshfield area. "The increase in the number of reported cases … during February in a confined geographic area was an indication of a possible outbreak of hepatitis A infection."Marshfield had 1 case in 2002 and 0 cases in 2003.".In addition, a case of hepatitis A in a Plymouth resident, employed in Marshfield, was reported." "Between February 25 six cases of HAV infection in Marshfield residents were reported to …MDPH. ![]() ![]() A good example of this is an actual outbreak of hepatitis A that occurred in Marshfield, MA in 2004.Įxcerpts from introduction of the report by the Massachusetts Department of Health However, in most outbreaks the population is not well defined, and cohort studies are not feasible. A cohort study design works well in these circumstances. The Salmonella outbreak above occurred in a small, well-defined cohort, and the overall attack rate was 58%. ![]()
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