Monovalent rotavirus vaccine provides protection against an emerging fully heterotypic G9P[4] rotavirus strain in Mexico.

After the introduction of monovalent rotavirus vaccine (RV1) in Mexico in 2006-2007, diarrhea mortality and morbidity declined substantially among Mexican children under 5 years of age. In January 2010, surveillance identified the emergence of a novel G9P[4] rotavirus strain nationwide. We conducted a case-control study to assess the field effectiveness of RV1 against severe rotavirus gastroenteritis caused by this unusual strain and to determine whether the G9P[4] emergence was related to vaccine failure or failure to vaccinate. RV1 was 94% effective (95% confidence interval, 16%-100%) against G9P[4] rotavirus-related hospitalization, indicating that its emergence was likely unrelated to vaccine pressure.