Mathematical modeling of occupational needlestick injury reduction in a U.S. Army mass immunization program through universal serologic screening for pre-existing immunity

BACKGROUND/OBJECTIVES: Needlestick injuries are an occupational hazard in mass immunization programs where large numbers of immunizations are rapidly administered. New accessions to the U.S. Army receive universal immunization to a number of diseases to which pre-existing immunity may exist. We investigated the potential to reduce the administration of unnecessary immunizations and the risk of occupational needlestick injury through universal serologic screening for pre-existing immunity in a theoretical annual cohort of 125,000 new accessions. METHODS: A mathematical model was constructed using TreeAge™ software to predict the differences in mean numbers of vaccine doses administered per accession between current policy and proposed policies that would include universal serologic screening for pre-existing immunity to various combinations of hepatitis A and B, measles, rubella, and varicella. Parameters for the model included rates of pre-existing immunity to each disease, the sensitivity and specificity of each screening test, and rates of attrition between immunization doses. The model simulated the administration of a two-dose series of live virus vaccine where serologic tests demonstrated susceptibility, and the administration of a three-dose series of the bivalent hepatitis vaccine when indicated by serology. Due to the complexity of the decision tree, expected values for mean number of immunization doses were calculated via 10,000 runs of microsimulation repeated 1000 times to construct confidence intervals. RESULTS: All policy options employing universal serologic screening demonstrated significant reductions in mean numbers of administered vaccine doses relative to current policy. A policy option involving universal screening for hepatitis A and B, measles, rubella, and varicella reduced the mean number of administered vaccine doses by 0.9860 per accession relative to current policy (95% CI 0.9852–0.9868). CONCLUSIONS: Due to high rates of pre-existing immunity, accessions to the U.S. Army currently receive multiple unnecessary immunizations. In a typical annual cohort of 125,000 accessions, implementation of universal serologic screening would result in the administration of over 123,000 fewer vaccine doses, leading to a reduced risk of occupational needlestick injury. The collection of serum for serologic testing does not increase the risk of needlestick injury as phlebotomy for other purposes is already performed on all accessions.