Breakthrough infection
Infants have antibodies from their mother. This limits the efficacy of many vaccines. Maternal antibodies can bind to the proteins produced by the virus in the vaccine. Maternal antibodies can also neutralize the virus. The maternal antibodies help the immune system of the infant, which isn't very active yet: the infant produces fewer antibodies. This means that few memory B cells are produced. The level of memory B-cells is not adequate to ensure an infant's lifelong resistance to the pathogen.