International Journal of Epidemiology, Vol 28, 787-792, Copyright © 1999 by International Epidemiological Association
M Reilly and E Lawlor
BACKGROUND: In 1994 a small cluster of hepatitis-C cases in Rhesus-
negative women in Ireland prompted a nationwide screening programme for
hepatitis-C antibodies in all anti-D recipients. A total of 55 386 women
presented for screening and a history of exposure to anti-D was sought from
all those testing positive and a sample of those testing negative. The
resulting data comprised 620 antibody-positive and 1708 antibody-negative
women with known exposure history, and interest was focused on using these
data to estimate the infectivity of anti-D in the period 1970-1993.
METHODS: Any exposure to anti-D provides an opportunity for infection, but
the infection status at each exposure time is not observed. Instead, the
available data from antibody testing only indicate whether at least one of
the exposures resulted in infection. Using a simple Bernoulli model to
describe the risk of infection in each year, the absence of information
regarding which exposure(s) led to infection fits neatly into the framework
of 'incomplete data'. Hence the expectation-maximization (EM) algorithm
provides estimates of the infectiousness of anti-D in each of the 24 years
studied. RESULTS: The analysis highlighted the 1977 anti-D as a source of
infection, a fact which was confirmed by laboratory investigation. Other
suspect batches were also identified, helping to direct the efforts of
laboratory investigators. CONCLUSIONS: We have presented a method to
estimate the risk of infection at each exposure time from multiple exposure
data. The method can also be used to estimate transmission rates and the
risk associated with different sources of infection in a range of
infectious disease applications.
ARTICLES
A likelihood-based method of identifying contaminated lots of blood product
Department of Statistics, University College Dublin, Ireland.
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