Previous studies have suggested that waning immunity to measles virus among vaccinated people may lead to attenuated illnesses, and is considered less infectious. The authors were interested in the epidemiological, clinical and laboratory profile of measles cases with waning immunity compared with other measles cases. They examined PCR-positive measles cases notified to health authorities in Victoria, Australia between 2008-2017 and classified them by their immunoglobulin (Ig)M and IgG levels as IgG negative [non-immune], IgM+/IgG+ [indeterminate], or IgM-/IgG+ [waning immunity]. 297 measles cases were notified, of which 64% were included in this analysis. 80% of these cases were non-immune at diagnosis, 4% had indeterminate immunity and 7% had waning immunity. Between 2008-2013 and 2014-2017, the proportion of cases with waning immunity increased significantly from 0% to 13%, and the diagnostic sensitivity of initial IgM fell significantly, from 93% to 81%. 54% of the waning immunity cases reported receiving measles-containing vaccines, one case had two documented doses, and three cases had one documented dose. Compared with non-immune and indeterminate cases, waning immunity cases were more likely to be male; less likely to report fever, coryza and cough; and had a lower burden of virus (higher Ct values on respiratory specimens). Among IgG+ cases, waning immunity cases had significantly higher IgG titres than indeterminate cases (mean optical density [OD] values 1.96 vs. 0.71). Onward transmission from one waning immunity case to two infant household contacts that were too young for vaccination was seen. Therefore waning immunity among measles cases (IgM-/IgG+ at diagnosis), associated with secondary vaccine failure and modified clinical illness, has been seen in Victoria, Australia.
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