"The delivery of measles vaccine in endemic areas must contend with a biological catch-22. From birth to about nine months of age, most infants have maternal antibodies that protect them from measles infection. But these antibodies also prevent the measles vaccine from conferring lifelong immunity. Those children whose maternal immunity wears off early are at risk of infection at an age when measles infection can be most severe. Thus, health systems in endemic regions, like the DRC, employ a first dose at a relatively early age (nine months) to immunise these vulnerable children. Later they provide a second dose to catch those for whom the first dose didn’t provide protection."