Duration of residency in a non-endemic area and risk of severe malaria in African immigrants

In malaria-endemic areas, adults very rarely succumb to severe malaria, suggesting that immunity to severe disease is life-long under conditions of repeated exposure. To what extent this protection persists in the absence of exposure remains to be established. The aim of this study was to assess whether duration of residency in a malaria-free country affects the risk for severe malaria in immigrants originating from sub-Saharan Africa. We conducted a retrospective chart review of 948 cases of malaria diagnosed in Stockholm, Sweden in 1995-2013. Among 501 adult patients with Plasmodium falciparum (315 of endemic origin and 186 of non-endemic origin, mainly Sweden), 41 (8.2%) had severe malaria according to WHO criteria (including 5% with parasitaemia), 22 (4.4%) had factors prognostic of poor outcome, and 35 (7.0%) were admitted to intensive care. Overall, patient origin did not affect the odds of severe malaria, according to any of these definitions. However, when the immigrants were stratified with regard to their duration of residency in Sweden, the risk of factors prognostic for poor outcome was associated with duration of prior residency in a malaria-free country among patients of endemic origin (p 0.02), and immigrants who had lived for ≥15 years in Sweden had a similar risk as non-immune travellers. The results of this explorative study suggest that, although immunity to severe malaria is maintained for several years in African adults, this protection might be lost with time without repeated re-exposure. A larger study, preferably including multiple centres, will be needed to confirm our findings.