A preliminary evaluation of a recombinant circumsporozoite protein vaccine against plasmodium falciparum malaria 

Authors Stoute J, Slaoui M, Heppner G, Momin P, Kester K, Desmons P, et al (RTS,S malaria vaccine evaluation group). 
Source New England Journal of Medicine. 336:86-91. January 9, 1997. 
Institutions Walter Reed Army Institute of Research; SmithKline Beecham, Belgium. 
Support US Army Medical Research and Development Command; Belgian Walloon Region. 



There is currently no effective, practical vaccine against malaria. Protective immunity has been achieved by irradiating infected Anopheles mosquitos and then allowing them to bite subjects repeatedly. Although this is not a feasible approach, it demonstrates the feasibility of producing protective antibodies in the pre-erythrocytic stage of the disease, when sporozoites are introduced into the body and migrate to the liver.

The circumsporozoite protein on the sporozoite is considered to be the principal antigen responsible for these antibodies. In previous trials, however, circumsporozoite vaccines have not been found to be sufficiently immunogenic. In this trial, the authors report on a recombinant circumsporozoite vaccine (consisting of the circumsporozoite protein fused to hepatitis B surface antigen), in three different formulations with three different adjuvants. 



Healthy volunteers, without prior exposure to malaria and without significant medical problems. 


The RTS,S vaccine, a recombinant polypeptide produced by yeast cells and consisting of two components:

  • RTS is a polypeptide chain consisting of part of the circumsporozoite protein fused to HBsAg.
  • S is the HBsAg polypeptide alone.
This vaccine was prepared in three different formulations, with different adjuvants. Vaccine 1 consisted of RTS,S with alum and monophosphoryl lipid A. Vaccine 2 was RTS,S in an oil-water emulsion. Vaccine 3 was RTS,S in an oil-water emulsion plus the immune stimulants QS21 and monophosphoryl lipid A.

The vaccines were administered i.m. at 0, 4 and 28 weeks. Thus, there were three vaccines, each vaccine to be given in three doses. 

Follow-up and measurements

Patients were monitored clinically at days 1, 2, 7 and 14 after each vaccination.

Blood samples were obtained for the evaluation of humoral immunity on the day of each vaccination, and at days 1 and 14 thereafter. Humoral immunity was evaluted by determining antibodies to circumsporozoite protein (total IgG levels and IgG subclass levels) and by an indirect fluorescence antibody assay to air dried sporozoite. Antibodies to HBsAg were also determined.

Blood was obtained for the evaluation of cellular immunity (proliferative and cytolytic assays) before the first vaccination and after the third one.

Challenge with malaria

22 subjects who had received all three vaccine doses and 6 unimmunized controls were challenged with falciparum malaria. This involved five bites from laboratory-bred Anopheles mosquitos that had been infected with sporozoites. Follow-up included frequent blood-smear examinations and treatment as needed if infection developed. 



46 subjects, men and women (mean age 31, range 18-45), were randomized to one of the three vaccines in roughly equal numbers. The first dose was well tolerated (mild local discomfort).

41 subjects went on to receive the second dose. Reactions to vaccines 2 and 3 were more severe after the second dose (mainly constitutional symptoms).

27 subjects received the third vaccine dose. The reasons for not completing all three doses were mainly scheduling problems or noncompliance. Because of the severity of the reaction after the second dose, two subjects (one for vaccine 2 and one for vaccine 3) were not given the third dose and the remainder received a reduced dose.

Of the 27 subjects who received all three doses, 22 agreed to undergo sporozoite challenge (8 given vaccine 1 and 7 given vaccines 2 and 3). 


Antibodies to circumsporozoite developed in all three vaccine groups, but were higher in subjects who received vaccines 2 and 3. Similarly, subjects who had been HBsAb negative before vaccination seroconverted, but antibody response was higher in groups 2 and 3. 

Vaccine efficacy

Sporozoite challenge in 6 unimmunized controls produced parasitemia in all 6 after 11-13 days. 7 out of the 8 subjects given vaccine 1 became parasitemic (12.6 days after exposure). 5 of the 7 subjects given vaccine 2 became parasitemic (after 15.2 days). Only 1 out of the 7 subjects given vaccine 3 developed malaria, for a relative risk of infection of 0.14 (95% CI 0.02 to 0.88).

Antibody titers were higher in subjects who were protected. Measurements of cellular immunity did not clearly predict protection, however. 

Authors' discussion

In the past, the only successful immunization against malaria had been achieved using irradiated Anopheles mosquitos. This approach, although impractical, showed that protective immunity to the pre-erythrocytic phase of the disease (sporozoites) could be achieved and was the basis for the development of the recombinant vaccine described here. The authors note that although strong antibody response seems to be important, it is not the only factor, since both vaccines 2 and 3 produced such responses but only vaccine 3 was effective. They speculate that some effect of the adjuvant in vaccine 3 on cellular immunity, perhaps on CD4+ T cells, may be responsible.

Although these results are very encouraging, they are preliminary, and many more studies of safety and efficacy will be needed, particularly in the field, in the setting of repeated exposure to multiple strains of plasmodium. 


In an editorial, Nussenzweig and Zavala give an excellent summary of the work that has been done so far in developing a malaria vaccine, of the problems that have been overcome and the problems that remain to be dealt with. 


Although the results of this study are very preliminary and may not be borne out in practice, they are encouraging and fascinating. Malaria takes a tremendous toll in lives and morbidity worldwide, and an effective vaccine would be a major advance.

From a purely intellectual point of view, the scientific process leading up to the vaccine described here and detailed in the editorial makes for very interesting reading. For anyone without expertise in molecular biology or immunology, reading the "methods" section of this article is a sobering experience. 

January 28, 1997


References related to this article from the NLM's PubMed database. 

Reader Comments

June 12, 1997

Letters to the Editor about this article, from the NEJM website. 

Topics include the ethics of administering malaria to human subjects and the exact nature of the adjuvant used.

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