The direct benefit of a second modified live core vaccine for adult shelter dogs and cats: a decision-analytic model

Abstract Background. Guidelines suggest revaccinating adult shelter dogs and cats 2 to 3 weeks after a modified live core vaccine at intake, especially when disease risk is high. A second dose on day 14 can prevent disease only in adults that are susceptible, fail their first dose, respond to the second dose and are still present and uninfected when it becomes effective. How many adults qualify, and so the benefit, has not been quantified. We built a decision-analytic model of direct protection to estimate the number of second doses needed to avert one additional case (number needed to vaccinate, NNV) of feline panleukopenia, canine distemper or canine parvovirus. Cases were infections acquired during shelter care, including those diagnosed after departure. Assuming constant daily infection risk, a low-risk approximation estimated the NNV from shelter records. Infection pressure came from the window case rate, adult cases diagnosed 8–21 days after intake per 1,000 adult-days in care on those days. This window falls after most pre-intake infections appear and before a day-14 second dose takes effect. Lengths of stay came from records at six shelters and from illustrative profiles. Inputs were varied alone and together. Results. At a base of 0.10 window cases per 1,000 adult-days and 25% of first-dose failures converted, about 6,000–25,000 second doses were needed per case averted at the six shelters ($35,000–$138,000 at $5.50 a dose). The second dose averted only 1%–5% of shelter-acquired adult cases, and 74%–91% arose from infection before the intake dose could protect. A larger or longer slow track lowered the NNV, to 2,500–3,350 when half of adults averaged 90 days. First-dose failure and the share converted, which shelter records cannot measure, each moved the NNV about as much as the window case rate, and further when varied together. Conclusions. Under our base assumptions, a routine second dose prevents very few shelter-acquired cases of panleukopenia, distemper or parvovirus. The benefit is larger where adults stay longer. Shelters can estimate their own NNV from intake and outcome dates and adult case counts with the accompanying browser tools. These estimates depend on first-dose failure and second-dose conversion, which remain uncertain.