Vaccination of koalas with a prototype chlamydial vaccine is safe, does not increase the incidence of lymphoma-related disease and maybe associated with increased lifespan in captive koalas

Abstract

ObjectivesTo assess the impact of Chlamydia vaccination on survival of captive koalas, and to compare the incidence of lymphomas and neoplasias between vaccinated and unvaccinated koalas. MethodsSurvival analysis using Cox and Weibull regressions on 54 vaccinated and 52 matched unvaccinated koalas, and chi-square contingency table for incidence of lymphomas/neoplasias. ResultsVaccination was found to have a significant positive effect on koala lifespan (P=0.03), with vaccinated koalas having a median lifespan of 12.25 years compared to 8.8 years for unvaccinated ones. The effect of sex on lifespan was not significant (P=0.31). The risk ratio of unvaccinated over vaccinated koalas was 2.2 with both Cox and Weibull regressions. There was no association between the incidence of lymphoma/neoplasias and vaccination status (P=0.33). ConclusionsKoalas vaccinated with a prototype Chlamydia vaccine may live longer than unvaccinated ones. There was no known Chlamydia infection among koalas, so our interpretation is that vaccination may have boosted the innate and adaptive immune systems to protect against a wide spectrum of bacteria, fungi and parasites. Vaccinated koalas did not show negative physiological effects of the vaccine, for example, the frequency of deaths due to lymphomas/neoplasias was the same in both vaccinated and unvaccinated animals.