J Neurol (2011) 258:1140–1149 of immigration were much earlier in the Group I with a mean of 1928 versus 1962 for Group II. The years of birth (1918 early, 1939 late) and ages in 1981 (63 early, 42 late) also reﬂected expectations. What was unanticipated were the major differences found in the other features: age at immigration (9.5 years early, 23.4 late); age at onset (AAO) (39 early, 33 late); and the number of years between immigration and onset (30 years early, 10 years late). In the next paper these differences will be considered in fur- ther detail. Age at immigration and interval to MS onset in Group II UKI immigrants The characteristics of the MS cases in the Group II migrants for the four states combined are noted by age- groups at immigration (Table 4). About half of them resi- ded in NSW, and the proportions by age at immigration did not differ notably among the four states. Of the 258 cases, 56 or over one-ﬁfth had entered Australia under age 15, while nearly three-quarters were then age 15–39. The ratio for these two age groups was 0.30 (56/189); an apparent difference with a higher ratio in QLD was not signiﬁcant with the small numbers available. This ratio of 0.30 is artiﬁcially high because of the absence of cases at older ages with onset before immigration. The year of onset (YOO) did not differ notably by age at immigration (AAI), but younger immigrants entered ear- lier, with a mean of 1954 versus 1963 for the older ones. With increasing AAI, mean AAO increased steadily as the mean interval between immigration and onset decreased, Table 4 Overview of MS Group II UKI immigrants by age at immigration, Four States combined AAO AAO–AAI AAI age at immigration, N number of cases, F number of females, YOI Year of Immigration, YOO Year of onset, AAO Age at onset, AAO–AAI interval between age at immigration and age of onset AAI 0–4 5–9 10–14 0–14 15–19 20–24 25–29 15–29 30–34 15–34 35–39 15–39 40? N (F) 6 (5) 26 (21) 24 (21) 56 (48) 30 (19) 43 (33) 56 (42) 129 (94) 39 (19) 168 (113) 21(15) 189 (128) 13 (7) YOI 1953.2 1953.8 1954.9 1954.2 1960.8 1964.7 1964.5 1963.7 1961.6 1963.2 1964.9 1963.4 1962.8 YOO 1973.2 1974.2 1973.0 1973.6 1971.4 1973.4 1972.1 1972.4 1968.2 1971.5 1969.5 1971.3 1967.5 22.3 23.9 27.5 25.3 27.9 30.7 35.1 31.9 38.9 33.6 40.7 34.3 47.8 20.0 17.0 15.8 16.8 14.9 8.6 7.8 9.7 7.0 9.1 4.6 8.6 4.6 1145 both a consequence of the requirement of symptom onset only after immigration (data not presented). Figure 3 shows the intervals between immigration and clinical onset of MS for each AAI in the Group II cases. There is a relative paucity of cases with entry under age 15, and especially under age 8, while there seems to be notable clustering for those age 20–35 or so. There is a trend to shorter intervals with increasing AAI, compatible with the relatively ﬁxed mean AAO common to this disease seen world wide. Risk of MS by age at immigration The population at risk for the denominator and the number of MS cases for the numerator used to calculate the risk ratios are given in 5-year age groups age 0–79 for each state in Table 5. New South Wales contained 47% of the cases (122/258) and 39% of the population at risk (272,813/696,735). Similar percentages for QLD were 16% of cases and 17% of population; for SA 15 and 20%; and for WA 22 and 23%. The absolute risk ratios for MS cases per 100,000 pop- ulation for each state among the immigrants under age 15 at entry vs those age 15–39 were 31.0 versus 61.4 for NSW; 29.0 versus 44.3 for QLD; 10.7 versus 50.1 for SA; and 15.0 versus 50.7 for WA. Table 6 summarizes the risk ratios with their 95% conﬁdence intervals for the combined four states. For the 56 immigrants who entered Australia under age 15, the risk of MS was 22.3/100,000, with a 95% conﬁdence interval of 16.9–29.0. Even for the next age group, the 30 entrants age 15–19, the risk was signiﬁcantly higher at 57.3 (95% CI 38.7–81.8), and consecutive older 5-year age groups retained both the differential and the signiﬁcance. For example the risk ratio for those age 15–39 was 53.9 (CI 46.5–62.1) and for all age 15? it was 45.3 (CI 39.2–52.0). The preplanned comparisons for this reassessment had been for MS in immigrants under age 15 at entry versus those then older, but the more appropriate comparison is between those age 0–14 versus—at most—age 15–39, since the exclusion of those already affected before immigration, an increasing number of which are seen with increasing age, artiﬁcially minimizes the contrast. But even with the oldest immigrants included, the results still indi- cate clearly a highly signiﬁcant decrease in the risk of MS for migrants under age 15 at entry into Australia. Discussion The calculated risk of MS in migrant populations is dependent not only on a sufﬁciency of people who change their residence from one risk area to another but also on 123