RESUMO
We studied 96 mass outbreaks of infectious gastroenteritis due to Norovirus in winter 2006-2007. Of these, 56 occurred in welfare institutions for aging adults 31 in hospitals, and 9 in other facilities such as kinder gardens. Affected staff accounted for 25.9% and users (inpatients, etc.) for 74.1%. The shortest outbreak lasted 10 days and the longest 67 days. We found a positive corelation between periods from the beginning of an outbreak to warnings by public health centers and periods from the beginning to the end of outbreaks. The sooner advice was aired by public health centers, the sooner outbreaks ended. Dementia among users and insufficient knowledge and skills of staff were high risk factors in outbreaks. All 74 of specimens which we examined showed the GII4 genotype. We observed reoutbreaks at three institutions. We compared first and second specimens from the same institution. Two specimens from the second outbreak belonged to the same cluster as the first outbreak. We analyzed 310 bases of RT-PCR products in Capsid regions in both specimens, finding three point mutations accompanied by amino acids changes. This may change the antigenicity of Capsid protein, and may be why reoutbreaks occurred so quickly.