Risk ratios can be used to compare the risk of acquiring the
disease in individuals who were exposed to a particular factor (e.g. consumed
a particular food at an event, or were in a particular age group), compared to
those who were not exposed to the factor (e.g. did not eat the food at the event,
or were in a different age group). This is useful to compare risks (attack rates)
in the two groups and, therefore, whether the factor may
be a risk factor for transmission of the agent and development of the disease. This
is illustrated in the following exercise, concerning a COVID 19 outbreak.
Risk (Number ill in group/total in group)
Risk ratio (Risk in exposed group/Risk in unexposed group)
Exercise
In the Ebola outbreak in 2000-2001 in Uganda, investigators conducted an epidemiological study to identify the risk factors associated with transmission of the virus (Francesconi et al, 2003). 83 contacts of index cases were interviewed about their actions and contact with sick individuals during the time of the outbreak, and whether they experienced symptoms themselves. Contact with bodily fluids of the ill person was identified as the strongest risk factor (the risk ratio was 5, which means that individuals who had contact with bodily fluids of the patient were around 5 times as likely to become infected and develop Ebola virus disease, compared to individuals who had not had any contact with bodily fluids). Below are a couple of adapted examples from their data, including the example for bodily fluids:
Table illustrating number of people who developed symptoms of Ebola relative to two potential risk
factors in an epidemiological investigation in Uganda (adapted from Francesconi et al, 2003)
Question: Referring
to the completed example for bodily fluids, calculate the risks
(attack rates) for touching the body and not touching the body of the
patient, and use these to calculate the risk ratio for contracting
Ebola for touching the body of the corpse (compared to not touching
the corpse).
