How to determine protective immunity in the post-vaccine era

The ability to determine an individual's susceptibility to infection relies heavily on the assay used, and the ability to correlate results of the assay to a clinical interpretation. Current rubella immunity screening methods identify total rubella IgG antibodies circulating in the serum, however both humoral and cell mediated immune responses have been shown to contribute to protection from infection. Therefore, antibody screening assays may under-estimate immunity in some populations. In fact, waning antibody titers over time in a large prenatal population were recently documented in North America, and the trend has been echoed in other countries that have achieved elimination through universal rubella vaccination. Despite decreasing antibody titers, the number of acute rubella cases has not increased in these populations, suggesting that the lower antibody levels may still be protective. Based on the changing epidemiology in universally vaccinated populations, it may be time to reassess the level of antibody that indicates immunity to rubella infection.