year normal mild severe 1 5.8 98 0 0 2 15.0 51 2 1 3 21.5 34 6 3 4 27.5 35 5 8 5 33.5 32 10 9 6 39.5 23 7 8 7 46.0 12 6 10 8 51.5 4 2 5