IMRAS-A clinical trial of mosquito-bite immunization with live, radiation-attenuated P. falciparum sporozoites: Impact of immunization parameters on protective efficacy and generation of a repository of immunologic reagents

PLoS One. 2020 Jun 17;15(6):e0233840. doi: 10.1371/journal.pone.0233840. eCollection 2020.


Background: Immunization with radiation-attenuated sporozoites (RAS) by mosquito bite provides >90% sterile protection against Plasmodium falciparum (Pf) malaria in humans. RAS invade hepatocytes but do not replicate. CD8+ T cells recognizing parasite-derived peptides on the surface of infected hepatocytes are likely the primary protective mechanism. We conducted a randomized clinical trial of RAS immunization to assess safety, to achieve 50% vaccine efficacy (VE) against controlled human malaria infection (CHMI), and to generate reagents from protected and non-protected subjects for future identification of protective immune mechanisms and antigens.

Methods: Two cohorts (Cohort 1 and Cohort 2) of healthy, malaria-naïve, non-pregnant adults age 18-50 received five monthly immunizations with infected (true-immunized, n = 21) or non-infected (mock-immunized, n = 5) mosquito bites and underwent homologous CHMI at 3 weeks. Immunization parameters were selected for 50% protection based on prior clinical data. Leukapheresis was done to collect plasma and peripheral blood mononuclear cells.

Results: Adverse event rates were similar in true- and mock-immunized subjects. Two true- and two mock-immunized subjects developed large local reactions likely caused by mosquito salivary gland antigens. In Cohort 1, 11 subjects received 810-1235 infected bites; 6/11 (55%) were protected against CHMI vs. 0/3 mock-immunized and 0/6 infectivity controls (VE 55%). In Cohort 2, 10 subjects received 839-1131 infected bites with a higher first dose and a reduced fifth dose; 9/10 (90%) were protected vs. 0/2 mock-immunized and 0/6 controls (VE 90%). Three/3 (100%) protected subjects administered three booster immunizations were protected against repeat CHMI vs. 0/6 controls (VE 100%). Cohort 2 uniquely showed a significant rise in IFN-γ responses after the third and fifth immunizations and higher antibody responses to CSP.

Conclusions: PfRAS were generally safe and well tolerated. Cohort 2 had a higher first dose, reduced final dose, higher antibody responses to CSP and significant rise of IFN-γ responses after the third and fifth immunizations. Whether any of these factors contributed to increased protection in Cohort 2 requires further investigation. A cryobank of sera and cells from protected and non-protected individuals was generated for future immunological studies and antigen discovery.

Trial registration: NCT01994525.

Publication types

  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Animals
  • Anopheles / parasitology
  • Anopheles / physiology
  • Female
  • Gamma Rays
  • Humans
  • Insect Bites and Stings / immunology*
  • Malaria / immunology
  • Malaria / prevention & control*
  • Male
  • Middle Aged
  • Mosquito Vectors / parasitology
  • Mosquito Vectors / physiology
  • Plasmodium falciparum / growth & development
  • Plasmodium falciparum / immunology
  • Plasmodium falciparum / pathogenicity
  • Protozoan Proteins / immunology
  • Sporozoites / immunology*
  • Sporozoites / pathogenicity
  • Sporozoites / radiation effects
  • Vaccination / adverse effects
  • Vaccination / methods*
  • Vaccines, Attenuated / adverse effects*


  • Protozoan Proteins
  • Vaccines, Attenuated
  • circumsporozoite protein, Protozoan

Associated data


Grant support

The authors want to amend their FD to the following: The clinical trial was funded by the Bill and Melinda Gates Foundation, through Global Health Grant Number OPP1034596. < Caution- >. Awarded to the Naval Medical Research Center through a Cooperative Research & Development Agreement (NMRC-12-3941, TLR Principal Investigator and NCRADA-NMRC-15-0579, JEE Principal investigator). The Bill & Melinda Gates Foundation provided support in the form of salaries for authors JL, SR, MS, JGB, AR, AS & CV, reviewed and approved the study design but played no role in data collection and analysis, the decision to publish, or the preparation of the manuscript. The specific roles of these authors are articulated in the author contributions section. Sanaria Inc. also played no role in the study design, data collection and analysis, or the decision to publish. TLR transitioned from the Navy into the private sector from January 2014 prior to the initiation of the clinical trial, and thereafter received salary support from Sanaria Inc.