Computation and Measurement of Risk - Mathematics and Statistics for the Environment - page 206

TABLE 8.10
Incidence of Varicella Among Schoolchildren
in Nine Affected Classrooms—Oregon, 2002

Varicella Non-case Total

Vaccinated a = 18 b = 134 152

Unvaccinated c = 3 d = 4 7

Total 21 138 159

Source: Data from CDC (2012) and Tugwell et al. (2004).

The risk ratio is less than 1.0, indicating a decreased risk or protective effect for the exposed (vac-
cinated) children. The risk ratio of 0.28 indicates that vaccinated children were only approximately
one-fourth as likely (28%, actually) to develop Varicella as were unvaccinated children.

8.6.2 r ate r atio
A rate ratio compares the incidence rates, person-time rates, or mortality rates of two groups. As
with the risk ratio, the two groups are typically differentiated by demographic factors or by expo-
sure to a suspected causative agent. The rate for the group of primary interest is divided by the rate
for the comparison group:

Rate forgroup of primaryinterest
Rate for comparison group

The interpretation of the value of a rate is similar to that of the risk ratio. That is, a rate ratio of 1.0
indicates equal rates in the two groups, a rate ratio greater than 1.0 indicates an increased risk for
the group in the numerator, and rate ratio less than 1.0 indicates a decreased risk for the group in
the numerator.

■ EXAMPLE 8.22
Problem: Public health officials were called to investigate a perceived increase in visits to ships'
infirmaries for acute respiratory illness (ARI) by passengers of cruise ships in Alaska in 1998
(Uyeki et al., 2003). The officials compared passenger visits to ship infirmaries for ARI during
May-August 1998 with the same period in 1997. They recorded 11.6 visits for ARI per 1000 tour-
ists per week in 1998, compared with 5.3 visits per 1000 tourists per week in 1997. Calculate the
rate ratio.

Solution:

Rate ratio = 11.6/5.3 = 2.2

Passengers on cruise ships in Alaska during May-August 1998 were more than twice as likely to
visit ships' infirmaries for ARI as were passengers in 1997. ( Note: Of 58 viral isolates identified
from nasal cultures from passengers, most were influenza A, making this the largest summertime
influenza outbreak in North America.)