Complex sentence: Serologic studies have distinguished 66 human enterovirus serotypes on the basis of an antibody neutralization test, and additional antigenic variants have been defined within several of the serotypes on the basis of reduced or nonreciprocal cross-neutralization between prototype and variant strains.
Split into simpler sentences: Serologic studies have distinguished 66 human enterovirus serotypes on the basis of antibody neutralization tests. Additional antigenic variants have been defined within several of the serotypes on the basis of reduced or nonreciprocal cross-neutralization between variant strains.