Summary: This is a phase 1/2, single-centre, double-blind, double-dummy, randomized, active-controlled, age de-escalation trial. Age de-escalation will be based on a review of the safety data from the preceding cohort (adults for toddlers and toddlers for infants) up to day 14 post study product administration by a data monitoring committee (DMC). All participants will receive either the MRV-MNP and a plac...