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At the beginning of 2011/12 season (week 36 of 2011), an outbreak of gastroenteritis occurred in a kindergarten in Osaka Prefecture and norovirus genogroup (G) I/4 was detected. After week 39, detection of norovirus GII from sporadic cases and outbreak cases increased and peaked in week 50. From week 2 to week 3 of 2012, detection of norovirus GII increased again (Fig. 1).
From week 36 of 2011 to week 13 of 2012, detection of norovirus GI from 148 cases in 15 of 47 prefectures (GI/4 from 46 cases, GI/14 from 11 cases, GI/8 from 10 cases, and GI/1, GI/2 and GI/3 from 2 cases each, and GI/NT from 75 cases), norovirus GII from 1,400 cases in 44 prefectures (GII/4 from 295 cases, GII/2 from 47 cases, GII/12 from 24 cases, GII/13 from 22 cases, GII/6 from 20 cases, GII/3 from 19 cases, GII/7 from one case, and GII/NT from 1,012 cases), sapovirus from 107 cases in 24 prefectures (including GI from 46 cases and GII from 4 cases), group A rotavirus from 99 cases in 21 prefectures (including G1 from 17 cases, G2 from 5 cases, G3 from 3 cases, and G9 from one case), group C rotavirus from 5 cases in Niigata Prefecture, and astrovirus from 8 case in 4 prefectures were reported (Fig. 2).
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