Vitamin D levels
Two-thirds (73%) of migrant children are deficient in vitamin D at arrival in Switzerland, one-third (28%) of them severely. This is higher than the rate in Swiss children (40–50%) [12]. It is consistent with other studies, although reference values for deficit can vary between authors. For example, 72.3% of refugees in Canada were deficient (< 50 nmol/L) [29] and 77.4% in Italy (< 75 nmol/L) [30]. Supplementation is needed to prevent impact on bone health [1–4].
Besides the overall hypovitaminosis D, we found that the presence and severity of deficit varies according to origin, age and gender.
Even though hypovitaminosis D is classically associated with dark skin, a great majority of children with fairer complexion also presented with mild and severe deficit. For example, Eastern Mediterranean children were predominantly deficient (80%), a third of them (33%) severely. In particular, Palestinian, Iraqi and Afghan children showed a large proportion of deficit. This might be linked to cultural/religious practices associated with a more covering dress code. Another hypothesis may be related to the cause of migration (i.e armed conflict might be associated with more time spent indoors).
As expected, African children were largely deficient (75%), although slightly less severely (21%). Similarly to Switzerland, 50% of European children had vitamin D deficit [12]. We found 75% of sufficient values in children from the region of the Americas but had a limited number of patients (n = 8). Similar results for geographic distribution of hypovitaminosis D were found in a Canadian study [29]. Another Norwegian study [31] had comparable results, except for a lower prevalence of deficit in the East Asian region (approximately our Western Pacific region), but the analysed countries were different and they included adults. Close results were also found for African and Asian children in an Italian study, but they found a higher deficit in America and Europe [30]. These results are however difficult to compare as vitamin D ranges were different and geographic zones not clearly defined.
Hypovitaminosis D significantly increased with age. Highest prevalence of deficit (86%) and severe deficit (41%) were found in children older than 10 years old. Other studies found concordant results in Italy [30] and Canada [29], with better vitamin D status in younger patients.
These results highlight the need for vitamin D prophylaxis for all migrants children and not only the young (< 3 years old) and dark-skinned children, as suggested in the current Swiss guidelines [11, 17].
Additionally, we found a significant difference in severe deficit linked to gender, which was more frequent in females (34%) than males (23%), especially if older than 10 years old. This result might be linked to cultural/religious dressing practices appearing with the first menstruations, limiting sun exposure. Gender differences in activities are also possible, with girls spending more time indoors than boys. Another Canadian study also found that Middle East, Asian and African female were particularly at risk of hypovitaminosis D [29]. Similar results were found in a Norwegian study, with a greater risk of hypovitaminosis D in females, especially if adolescent [31].
As expected due to the difference in sun exposure, vitamin D status largely varied between seasons, with a great majority of hypovitaminosis D in winter (96%) and a small majority of normal values in summer (56%). Half of the children (53%) were severely deficient in winter. These results, although consistent with current knowledge [3, 10] and several studies [30, 32, 33], emphasize the importance of wintertime supplementation, in particular in the migrant population. Although the specific impact of winter deficit is not yet clearly defined, it seems to negatively affect bone health [34].
Two children (0.4%) presented with biological rickets. Both had concomitant severe vitamin D deficit. Although rare in Europe, rickets is still present, especially in refugees and dark-skinned children and is mainly linked to vitamin D deficiency [35, 9].
Other biological manifestations related to vitamin D deficit were found (hyperparathyroidism 17%, hypocalcaemia 5% and hypophosphatemia 8%), all strongly correlated to hypovitaminosis D. These results were expected and emphasize the significant impact of decreased vitamin D levels and need for supplementation.
Moreover, it underlines the importance of vitamin D dosage and supplementation in case of musculoskeletal symptoms or more subtle symptoms (irritability, tiredness, weakness), in particular in teenagers (Table 1) [11, 9].
Cost analysis
Our data suggest that systematic vitamin D supplementation (three months at arrival followed by winter substitution) for all migrant children older than 3 years old without level check (strategy 2) is more cost-efficient than level-based supplementation, reducing costs by 25% over 10 years. As nurse care, number of appointments and blood draw costs associated to strategy 1 were not considered, the gap is presumably wider.
Being less discriminative, this strategy is nonetheless associated with a substantial number (27%) of unnecessary substitution. The supplementation with 600 IU per day of cholecalciferol should however not induce toxicity in children with normal vitamin D status [12, 36, 37]. Winter substitution with two doses of 100’000 IU (corresponding approximately to 1100 IU/day) also stays below the safe upper intake concentrations [22, 11, 37, 38]. Moreover, it limits the need for an “unnecessary” blood drawn (vitamin D measure in deficient children, who will need supplementation anyway), which was a predominant drawback in the vitamin D level-based supplementation strategy (strategy 1).
However, unlike strategy 1, strategy 2 (systematic substitution) does not allow a control of the vitamin D correction in severely deficient children, as there is no base value. This might result in partial correction, especially if compliance is not adequate. It underlines the importance of symptom-based vitamin D level check, especially in high-risk groups, to avoid missing and under-treating cases of rickets. Indeed, severe or symptomatic deficit might needs higher doses for correction [9, 11].
Finally, there is a higher risk of loss of follow-up cases and thus lack of supplementation in strategy 1 due to the need for a second appointment to give the patient the vitamin D results before starting replacement therapy.
Limitations
This study is subject to various limitation. Although the number of patients in this study were relatively high, our population was heterogeneous and not all results were statistically significant. Some countries of origin and geographic zones were poorly represented. Furthermore, migrant population being subject to current refugee patterns, it might differ in other centres and change over time. Vitamin D levels might also vary according to type and duration of the refugee route taken or prior vitamin D substitution.
As our study was retrospective, we also lacked data about cultural/religious habits and outdoor time.
The nurse encounters in our consultation being subject to hourly rates regardless of exams performed, the costs of nurse care, number of appointments, blood draw and other collaterals were not considered, as not comparable. Newly arrived migrant children also often benefit from blood draws and follow-up appointments in other contexts, making the individual costs linked to vitamin D difficult to isolate. The price of vitamin D measuring and supplementation may also vary between centres and must be taken into account individually.
For these reasons, this study’s results and the supplementation strategy proposed should be confirmed in a larger and if possible multicentred trial and adapted locally.