Malaria is a parasitic disease causing substantial morbidity and mortality globally. Malaria is a potentially severe and fatal disease in non-immune individuals. In areas of intense transmission infections individuals acquired immunity that protect against clinical disease. Nonetheless, immunity is not regarding sterilizing and repeated infections often result in an asymptomatic carriage of malaria parasites. These chronic apparently asymptomatic infections have been associated with anemia, cognitive dysfunction and adverse events during pregnancy.

Global migration has increased over the last decade and has resulted in an increasing number of migrants from malaria endemic regions arriving in non-endemic countries. Migrants from malaria endemic countries may carry asymptomatic infections with malaria parasites, as well as other parasitic infections such as strongyloides and schistosomiasis, with a possible negative impact on health in this group. The prevalence of asymptomatic malaria and other parasites is not fully elucidated in migrants from different regions. Moreover, the longevity of asymptomatic carriage of malaria parasites in absence of re-exposure is not known.

The aim of this study is to assess the prevalence of malaria parasites and other parasitic infections in migrants in Sweden, both newly arrived and migrants with longer residency, and intend to evaluate the need for screening for various parasitic infections in migrants arriving in Sweden. Moreover, this study will also assess antibody responses to malaria and other parasitic diseases.

Study participants are recruited in different ways, participation in the study is offered when attending an asylum health clinic at several sites in the Stockholm county. Secondly, patients attending the Infectious Disease outpatient clinic for non-febrile diseases (e.g. follow up of hepatitis, treatment for latent tuberculosis). Thirdly, quota refugees from Democratic Republic of the Congo or Uganda with arrival in Sweden in the years 2015-2019 with postal address in Stockholm, will be invited by letter to participate in the study .

A venous blood sample (EDTA tube) is collected from study participants at one occasion and a questionnaire including questions about patient origin and previous malaria are completed with the aid of a translator.

Blood samples are analysed in the research laboratory, where a malaria rapid diagnostic test (RDT) is performed and haemoglobin concentrations are measured (using Hemocue, point of care test) the same day as sample collection. The samples are then centrifuged, aliquoted, and stored frozen at the research laboratory. Presence of malaria parasites are analysed by realtime-PCR of the species specific 18SRNA gene in a multiplex assay. Serologic markers for other parasitic diseases (eg. schistosomiasis and strongyloides) are analysed in plasma. These analyses are performed at the reference laboratory at the Public Health Agency of Sweden.

Projected sample size is estimated using the formula n=(Z^2xP(1-P))/d^2 where P is the expected prevalence based on previous studies (5%) and d is the precision (1.5%) and Z is the confidence interval (95%). Based on these assumptions, the required sample size is n=715.

All RDT or PCR positive individuals are referred to the Department of Infectious Diseases or the Department of Pediatrics at the regional hospital for further investigations and treatment. Results from complete blood count are assessed and followed up according to clinical routine at each centre where samples were collected. Contact information to the study participant are included as part of the recruitment and consent procedures in order to be able to contact the study participants for referral.

Moreover, antibody responses to crude parasite extracts and specific antigens are assessed by ELISA and Luminex. In a subset of participants peripheral blood mononuclear cells, PBMC, are prepared in follow up samples to assess the cellular immune response. The magnitude and breadth of antibody and cellular responses are assessed in relation to duration of residency in a malaria free country. An estimation of the long- term decay kinetics using a sero-epidemiological mathematical model is performed.

All data will be handled according to the EU GDPR (General Data Protection Regulation) superseded all EU member states' data protection laws based on the 1995 Data Protection Directive (DPD), (GDPR, EU 2016/679 and SFS 2018:218). The patient register, containing all collected data and analysis results, is kept pseudonymizised and stored in a secure server at Karolinska University Hospital. The key to pseudonumization is kept in a secure locker at the institution.

Collected blood specimens are stored in Stockholm Medical Biobank located at Karolinska University Hospital in Solna. Handling of these stored samples is governed by the Act on Biobanks in Health Care (SFS 2002: 297).

Inclusion Criteria:

• Born in a malaria endemic country (a country with reported indigenous spread of malaria according to World Malaria Report 2019)

Exclusion Criteria:

• Inability to understand study information or sign informed consent, except for children where legal guardian is asked for consent.

1. Migrants born in countries where malaria is endemic are invited to participate in the study at migration health care units and in the antenatal care during pregnancy.
2. Immigrants from malaria endemic countries living in Sweden are invited to participate with voluntary blood sampling on one occasion. The study is advertised in posters in waiting areas at health clinics.
3. Family members to individuals that are diagnosed with malaria at Karolinska University Hospital or other hospitals in Stockholm and Västerås are invited to the study at in-patient clinics.
4. Persons with origin in Democratic Republic of the Congo and Uganda arrived in Sweden between 2015-2019 are invited by an addressed letter