K - College of Natural Resources - University of California, Berkeley

tracing, with the stated goal of determining the original “primary” cases of the outbreak
(i.e. those who had acquired infection directly from the zoonotic reservoir). Cases (or
their next of kin) were asked to identify persons from whom they had probably acquired
the disease, who were in turn asked to identify who had infected them. Primary cases
were defined as those whose sources of infection could not be identified. Prospective
contact tracing was conducted to the extent that lists of contacts of identified cases
(information that was “routinely collected”) were matched with a list of reported cases.
This data collection technique may bias the dataset toward surviving chains of
transmission, since these are the ones that lead to the later-generation cases from which
contact tracing began. The effort at prospective contact tracing would have mitigated
this to some extent, but the level of tracing effort was certainly lower than for the
retrospective work. The resulting dataset is conspicuously low in Z=0 entries, just as
we would expect for a methodology that is biased against detecting chains that have
died out.
Measles, US 1997-1999 (Gay et al. 2004)
In this summary of measles elimination efforts in the United States, 165 separate
chains of measles transmission were identified (of which 107 were classified as
importations). 122 outbreaks consisted of a single case with no secondary transmission
(yielding an estimate of p0=122/165). Insufficient data were reported to estimate R
directly, but estimation of R was a major goal of the source paper so we used their
estimate and 95% confidence interval. These estimates of R were derived from three
approaches, all based on the assumption that Z~Poisson(R). Our analysis shows that the
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