The primary objectives of this study are to assess the safety and feasibility of blood-stage controlled human P. vivax malaria infection (CHMI) in healthy adult Thai volunteers through experimental injection of cryopreserved P. vivax infected erythrocytes, and to choose the optimal inoculation dose for future P. vivax CHMI studies. In this study, blood-stage CHMI will be conducted in 8 volunteers per inoculum stock who will each be infected with P. vivax by experimental injection with cryopreserved P. vivax infected erythrocytes, which were collected from the controlled human Plasmodium vivax malaria infection model through experimental sporozoite infection in Thai adults (NCT04083508) . There are currently 4 stocks of inocula from 6 volunteers in the NCT04083508 study, which have differing quantities and stages of parasites.

The total number of volunteers of this study will be up to 48 (8 volunteers per inocula stock). The volunteers will be monitored closely as in-patients in the Hospital for Tropical Diseases, and will be treated according to the Research Proposal.

This study is funded by the UK Wellcome Trust. The grant reference number are Oxford/MORU: 212336/Z/18/Z and 212336/Z/18/A, and Mahidol University: 212336/A/18/Z and 212336/A/18/A.

This study is a blood-stage P. vivax human challenge study with the primary aim of assessing the safety and feasibility of a challenge model using banks of cryopreserved P. vivax infected erythrocytes produced from NCT04083508 study to identify the dose of the inocula to be used in the future CHMI studies.

Forty-eight healthy Thai adults, aged between 20 and 55 years will be recruited at the Clinical Therapeutics Unit (CTU) in the Hospital for Tropical Diseases, Faculty of Tropical Medicine, Mahidol University, Bangkok. The overall period of each volunteer's participation will be 13 months: 2-week screening process prior to the Day 0 challenge, about 2-week inoculum process until reaching malarial treatment criteria, and follow-up period for 1 year after malarial treatment. All inclusion and exclusion criteria will be checked to ensure eligibility criteria have been met prior to Day 0.

Volunteers will be admitted to the hospital one day prior to challenged day. All eligible volunteers will have physical examinations. Serum pregnancy test (women only), malaria diagnosis, complete blood count (CBC), and biochemistry will be tested. Glucose -6-phospate dehydrogenase (G6PD) test and malaria immunological profiles will be tested for baseline information.

On the challenged day (Day 0), four different doses of inoculum (one whole vial,1:5 dilution, 1:10 dilution, and 1:20 dilution) will be assessed. Each dose of inoculum will be tested in 2 volunteers to identify the lowest concentration producing a reliable infection within a practicable timeframe. Therefore, there will be 8 volunteers enrolled per inoculum bank.

The assessment will be repeated in each inoculum bank. There are currently 4 inoculum banks and 20 volunteers have been enrolled to date so 28 volunteers will be enrolled into this study.

From Day 1 after challenge, the volunteers will be assessed once daily until malaria qPCR becomes positive. The assessment includes a clinical well-being check, physical examination, vital signs, and blood drawn for parasitaemia (malaria blood film, qPCR, and gametocyte qPCR) and membrane feeding to assess the transmissibility of gametocyte. Malaria immunology and CBC will be performed on day 4 and the day that qPCR become positive.

After qPCR becomes positive the monitoring of clinical well-being will continue. Blood will be drawn twice daily to monitor blood parasitaemia and allow membrane feeding to assess the transmissibility of gametocyte. Malaria immunology, CBC, and blood biochemistry will be performed on day that volunteer reach malaria treatment criteria.

When the malaria slide positivity and/or symptoms thresholds have been reached study physician will immediately prescribe antimalarial treatment with chloroquine according to local standard guidelines. Blood will be collected to test for malaria (blood films and qPCR) once daily until clinically recovered and two consecutive malaria blood films are negative (completing of the chloroquine treatment course) and volunteers will be discharged from the hospital. Before discharge from the hospital, HIV-I and HIV-II antigen, HBV and HCV serology, COVID-19 testing (RT-PCR) and COVID-19 serology test will be carried out to confirm that volunteers did not acquire COVID-19 during their inpatient stay.

If any volunteer reaches day 21 post-challenge without a positive malaria blood film, they shall be started on 3-day course of antimalarial treatment (chloroquine).

If a volunteer withdraws/is withdrawn from the study after challenge but before reaching the criteria for malaria treatment, then a complete, appropriate, curative course of antimalarial therapy must be completed

After discharge from the hospital, there will be out-patient visits on day 7, 28, 60, 90, 180, and 1 year post antimalarial treatment initiation. Blood will be collected to detect malaria parasites by blood film and qPCR, and for malaria gametocyte qPCR, membrane feeding assays (MFA), HIV-I and HIV-II antigen, HBV, HCV, CMV, and EBV, malaria immune response, CBC, and biochemistry according to the study protocol.

Data analysis

The safety of the CHMI will be assessed by descriptive analysis of the frequency, incidence and nature of adverse events and serious adverse events arising during the study. Since this is a feasibility study conducted in 2 volunteers per dosing group, formal statistical hypothesis testing will not be used for most analyses due to the limited sample size, and only a brief Statistical Analysis Plan (SAP) will be developed and finalized prior to database lock.

The study will be conducted in accordance with the current approved protocol, the International Conference on Harmonisation-Good Clinical Practice (ICH GCP), relevant regulations, and standard operating procedures. Data will be evaluated for compliance with the protocol and accuracy in relation to source documents. Following written standard operating procedures, the monitors will verify that the clinical study is conducted and data are generated, documented and reported in compliance with the protocol.

Inclusion Criteria:

The volunteer must meet all of the following criteria to be eligible for the study:

1. Healthy Thai adult aged 20 to 55 years with weight at least 50 kg.
2. Red blood cells positive for the Duffy antigen/chemokine receptor (DARC)
3. Women only: Must practice continuous effective contraception for the duration of study period until 3 months post-challenge.
4. COVID-19 vaccination at least two doses of COVID-19 vaccines approved by WHO.
5. Agreement to refrain from blood donation during the course of the study and for 1 year after the initiation of antimalarial treatment.
6. Willing to be admitted in the Hospital for Tropical Diseases for clinical monitoring, until antimalarial treatment is completed and their symptoms are settling.
7. Willing to take a curative antimalarial treatment following CHMI.
8. Willing to reside in Bangkok and its vicinity for 2 months after malarial treatment initiation.
9. Able to read and write in Thai.
10. Provide written informed consent to participate in the trial
11. Answer all questions on the informed consent quiz correctly
12. Educational level: has at least an undergraduate degree

Exclusion Criteria:

The volunteer must NOT enter the study if any of the following apply:

1. Positive malaria qPCR OR malaria film

2. Presence of any medical condition (either physical or psychological) which in the judgment of the investigator would place the participant at undue risk or interfere with the results of the study (e.g. serious underlying cardiac, renal, hepatic or neurological disease; severe malnutrition; congenital defects or febrile condition)

3. Presence of chronic disease or chronically use of medication

4. Use of systemic antibiotics with known antimalarial activity in the 30 days before challenge (e.g. trimethoprim-sulfamethoxazole, doxycycline, tetracycline, clindamycin, erythromycin, fluoroquinolones and azithromycin)

5. Use of immunoglobulins or blood products (e.g. blood transfusion) at any time in the 1 year preceding enrolment

6. Receipt of an investigational product, any vaccine in the 30 days preceding enrolment (D0), or planned receipt during the study period

7. Prior receipt of an investigational vaccine likely to impact on interpretation of the trial data or the P. vivax parasite as assessed by the Investigator.

8. Any confirmed, or suspected immunosuppressive, or immunodeficient state, including HIV infection, asplenia, history of splenectomy, recurrent, severe infections, and chronic infection

9. Immunosuppressant medication within the past 6 months preceding enrolment (D0) (inhaled and topical steroids are allowed)

10. History of allergic disease or reactions likely to be exacerbated by malaria infection

11. Female participant who is pregnant as evidenced by positive beta-human chorionic gonadotropin (β-HCG) test, lactating, or planning pregnancy during the course of the study

12. Contraindications to the use of antimalarial treatment (e.g. chloroquine, atovaquone / proguanil or dihydroartemisinin/piperaquine)

13. Use of medications known to have a potentially clinically significant interaction with the antimalarial drug that will be used in this study (chloroquine, atovaquone / proguanil or dihydroartemisinin/piperaquine)

14. Known existing positive family history in both 1st AND 2nd degree relatives < 50 years old for cardiac disease

15. History of cardiac arrhythmia, including clinically relevant bradycardia

16. Family history of congenital QT prolongation or sudden death

17. Any clinical condition, including using medications, known to prolong the QT interval.

18. Screening electrocardiogram (ECG) demonstrates a QTc interval ≥ 450 ms.

19. Suspected or known or history of alcohol abuse

20. Suspected or known or history of drug abuse.

21. Concurrently participating in another clinical study, at any time during the study period

22. Haemoglobin < 11 g/dL

23. Positive hepatitis B surface antigen or seropositive for hepatitis C virus

24. Finding on safety laboratory values as defined below:

    • Abnormal AST (AST > 40 U/L for male, and > 32 U/L for female [upper normal range])
    • Abnormal ALT (ALT > 41 U/L for male, and > 33 U/L for female [upper normal range])
    • Abnormal serum creatinine (Scr) (Creatinine [Cr] > 1.17 mg/dL for male, and > 0.95 mg/dL for female [upper normal range])
    • Abnormalities corrected calcium and magnesium blood levels

25. Blood group Rhesus negative

26. Blood incompatibility to the inoculum

27. Positive for COVID-19 diagnosed by RT-PCR

Volunteer's data and results from blood analyses stored in our database may be shared with other researchers to use in the future. However, the other researchers will not be given any information that could identify the subject.

This is a double-blinded study; volunteers, clinical investigators, and study staff who perform laboratory study endpoint will be blinded to inoculation arm allocation.