In 2011, the first clinical trial assessing Sanaria® PfSPZ Vaccine (radiation attenuated, aseptic, purified, cryopreserved Plasmodium falciparum (Pf) sporozoites (SPZ))  demonstrated that when PfSPZ Vaccine was injected subcutaneously (SC) or intradermally (ID), it was safe and well tolerated, but induced minimal immune responses and minimal protection against controlled human malaria infection (CHMI). We had hoped to induce highly reactive PfSPZ-specific, interferon gamma (IFN-g)-producing CD8+ T cells, which would kill the parasites during their development in the liver. However, there was no evidence of these responses in the study subjects.
This same lack of response was seen in non-human primates given PfSPZ Vaccine SC, not only in the periphery, but also in the livers, which were examined after immunization. However, when the same dose of PfSPZ Vaccine was administered by direct venous inoculation (DVI), results improved dramatically: 3% of liver-resident CD8+ T cells in non-human primates were specific for PfSPZ and produced IFN-g. These data demonstrated that PfSPZ Vaccine was indeed immunogenic provided the PfSPZ could be administered directly into the bloodstream and reach the liver.
Based on these findings, PfSPZ Vaccine was then administered IV to humans, protecting 6/6 (100%) subjects against CHMI . This startling improvement confirmed the importance of the route of administration. This was the first time >90% protection had ever been demonstrated in a malaria vaccine trial.
Since then, Sanaria and partners from the International PfSPZ Consortium (I-PfSPZ-C) , have systematically assessed how to further optimize the vaccine dose and regimen in order to consistently confer high-level, durable protection against both homologous (same Pf strain as the vaccine) and heterologous (different Pf strain to vaccine) CHMI, as well as against naturally transmitted malaria in field studies, where the parasites are extremely heterogeneous. Long-term protection (at least 6 months) is critical, if the vaccine is to become a tool in the malaria elimination armory, so our trials have focused on this (or even longer) intervals. Protection for at least 14 months was shown in 2016 against homologous CHMI . Now, in January-February 2017, four more papers have expanded our knowledge of the protective efficacy and safety of PfSPZ Vaccine [5-7] and of the similar vaccine approach PfSPZ-CVac, where infectious PfSPZ are administered to volunteers taking an antimalarial drug) .
The first three publications focused on the question of heterologous protection and reduced numbers of doses. PfSPZ Vaccine is composed of PfSPZ from the NF54 strain of Pf, which is thought to have originated in West Africa. Epstein et al.  and Lyke et al.  showed that PfSPZ Vaccine can protect volunteers against CHMI by the heterologous Pf7G8 (originating from Brazil) and that this protection could extend for at least 33 weeks after vaccination (long-term). They also showed that 3 dose regimens could be effective. The third publication describes a trial conducted in a natural endemic setting in Mali, where the immunized study subjects were challenged in nature by mosquito bites transmitting genetically heterogeneous parasites for 24 weeks after the last dose of vaccine. The clinical trial led by Sissoko and Healy  demonstrated 52% protection by time to event analysis and 29% by proportional analysis despite using a vaccine dosage regimen now known to be sub-optimal. This was done in a high Pf transmission area, where 93% of normal saline control volunteers became infected during the 24 weeks of follow-up. Together these publications show that PfSPZ Vaccine has the capacity to provide long-term protection as well as protection against diverse parasite strains. More work is needed to achieve the consistent high-level, durable protection required for a traveler’s vaccine and for a vaccine for malaria elimination.
The fourth paper by Mordmüller et al.  tested the PfSPZ-CVac approach (PfSPZ Chemoprophylaxis Vaccine) , administering infectious PfSPZ (Sanaria® PfSPZ Challenge) [10-14] to volunteers taking the antimalarial drug chloroquine. At approximately 10% of the dose used for PfSPZ Vaccine, 9/9 volunteers (100%) were protected again homologous CHMI at 10 weeks (moderate term) after the last dose of vaccine, and a vaccine regimen administered in just 10 days total was shown to be 63% protective at 10 weeks. Protection against heterologous (CHMI) and heterogeneous Pf (field exposure), durability of protection and improved shortened dosage regimens are now being investigated.
Here, the lead authors of these papers explain their results:
From left to right: Judy Epstein; Kirsten Lyke; Mahamadou Sissoko; Sara Healy; Benjamin Mordmüller; Peter Billingsley
What are the major findings of your paper in JCI Insight? This was the first demonstration that PfSPZ Vaccine could protect against a strain of Pf that was different from the one used to make the vaccine. The results are really encouraging even though the CHMI was done on a limited group of malaria-naive adult subjects. We obtained similar protection against both short-term (3 week) and long-term (24 weeks) CHMI with both 3- and 5-dose regimens against homologous CHMI.
Briefly describe the experimental design. The trial assessed tolerability, safety, immunogenicity, and protective efficacy of PfSPZ Vaccine administered by direct venous inoculation (DVI). Volunteers were given a total dosage of 13.5 x 105 PfSPZ in either 5 (2.7 × 105 PfSPZ/dose) or 3 (4.5 × 105 PfSPZ/dose) doses. Protection was assessed by the bite of five mosquitoes infected with sporozoites of homologous (PfNF54) or heterologous (Pf7G8) parasites at either 3 or 24 weeks after the last vaccination, then examining blood for the presence of parasites.
What are the implications of the results in terms of malaria vaccine development? These results provide a foundation for developing an optimized immunization regimen for preventing malaria. They take us an important step closer to having a PfSPZ Vaccine regimen that will suitable for travelers, including military, diplomats, aid workers, tourists and people posted to malaria endemic areas for work purposes. Theoretically, the same regimen could be used in vulnerable populations in endemic areas.
What are your next steps? We are currently assessing higher doses of the vaccine against long-term heterologous CHMI. The long-term goal here is to have a vaccine that provides durable protection against any Pf strain.
What are the major findings of your paper in PNAS? After CHMI with homologous Pf parasites 19 weeks after final immunization, nine of 14 vaccinated volunteers (64%) were non parasitemic compared with none of the six non-vaccinated controls. Of the nine protected subjects, six were subjected to a second CHMI by mosquito bite with Pf7G8 at 33 weeks after the final immunization. Five (83%) of six remained protected while all six non-vaccinated controls were infected after CHMI. PfSPZ-specific T-cell and antibody responses were detected in all vaccine recipients. PfSPZ- specific T cell responses were highest in blood after the 1st immunization and not boosted in blood thereafter.
Briefly describe the experimental design. After establishing dose-dependent, durable protective efficacy against homologous CHMI in a prior study, we continued PfSPZ Vaccine dose escalation to a three-dose regimen of 9.0 × 105 PfSPZ per dose at 8 week intervals. Protection was assessed by CHMI with five mosquitoes infected with PfSPZ of PfNF54 or Pf7G8 strains at 19 or 33 weeks after the last vaccination, then examining blood for the presence of parasites.
What are the implications of the results in terms of malaria vaccine development? A three-dose regimen of a live attenuated whole-parasite PfSPZ Vaccine conferred durable sterile protection through 33 weeks in ∼50% of subjects against a CHMI with a heterologous strain to that in the vaccine. The immune response generated to the vaccine recognized both the homologous and the heterologous Pf strain.
What are your next steps? On the basis of the favorable safety profile of the doses tested to date and the immune data reported, additional changes to the vaccine regimen using different doses, intervals and number of immunizations will be tested to determine whether even higher protective efficacy can be achieved, which will be preferable for travelers and for mass vaccination strategies aimed at interrupting transmission in endemic regions.
Mahamadou Sissoko and Sara Healy
What are the major findings of your paper in Lancet Infectious Diseases? PfSPZ Vaccine administered by direct venous inoculation (DVI) was safe and well tolerated. This was the first trial which evaluated the efficacy of a whole malaria sporozoite vaccine in a malaria endemic area. PfSPZ Vaccine provided protective efficacy for at least 6 months against natural infections of malaria in an endemic area where over 90% of the controls became infected.
Briefly describe the experimental design. After an open-label dose-escalation study in a pilot safety cohort, we did a double-blind, randomized, placebo controlled clinical trial. Participants were randomly assigned (1:1) in a double-blind manner, with stratification by village and block randomization, to receive either five doses of 2·7 × 10⁵ PfSPZ or of normal saline at days 0, 28, 56, 84, and 140 during the dry season. Participants received artemether and lumefantrine (Coartem) to eliminate malaria parasites before the first and last vaccinations. We collected blood smears every 2 weeks and during any illness for 24 weeks after the fifth vaccination.
What are the implications of the results in terms of malaria vaccine development? This trial justifies our continued development of PfSPZ Vaccine to validate a 3 doses regimen and to immunize individuals who are highly exposed to malaria. The work furthers our efforts to understand the mechanism of protection by PfSPZ Vaccine, especially considering the difference between antibody responses to PfCSP and efficacy in the USA and in Mali.
What are your next steps? We plan to conduct further studies to refine the protective dose regimen and immunize children and pregnant women who are at the highest risk of malaria in endemic areas.
What are the major findings of your paper in Nature? Protection against CHMI following PfSPZ-CVac is strictly dose-dependent between the infective doses of 3,200 PfSPZ and 51,200 PfSPZ. The highest tested dose was well tolerated and fully protected against CHMI. Shortening time for administration of the full dosage regimen down to 5 days led to high-grade protection, although full protection was not achieved.
Briefly describe the experimental design. Two-stage, placebo-controlled, double-blind trials of PfSPZ-CVac in healthy, malaria-naïve volunteers with CHMI by DVI of homologous PfSPZ as measure of efficacy. Dose escalation (1st Stage) of three dose regimens (three injections of 3.2 x103, 1.28 x 104, 5.12 x 104, respectively) at 4-week intervals, followed by assessment of accelerated schedules of 14-day and 5-day intervals (2nd Stage).
What are the implications of the results in terms of malaria vaccine development? The study shows that PfSPZ-CVac is a feasible approach to immunization, and much more potent on a PfSPZ basis than PfSPZ Vaccine. It is currently being further developed for use in endemic areas and travelers.
What are your next steps? We are currently assessing alternative antimalarial agents for chemoprophylaxis which would allow simplified regimens, and testing increased doses of PfSPZ and studying protection against heterologous CHMI. Later this year we will start a randomized controlled trial in African children with PfSPZ-CVac and PfSPZ Vaccine under natural exposure.
- Epstein JE et al. Live attenuated malaria vaccine designed to protect through hepatic CD8+ T cell immunity. Science 2011 334:475-80
- Seder RA, et al. Protection against malaria by intravenous immunization with a nonreplicating sporozoite vaccine. Science 2013 341:1359-65
- Richie et al. Progress with Plasmodium falciparum sporozoite (PfSPZ)-based malaria vaccines. Vaccine 2015 33:7452-61
- Ishizuka A et al. Protection against malaria at 1 year and immune correlates following PfSPZ vaccination. Nature Medicine 2016 22:614-23
- Epstein JE et al. Protection against Plasmodium falciparum malaria by PfSPZ Vaccine. JCI Insight 2017 2:e89154. doi:10.1172/jci.insight.89154
- Lyke K et al. PfSPZ vaccine induces strain-transcending T cells and durable protection against heterologous controlled human malaria infection. PNAS 2017 doi: 10.1073/pnas.1615324114
- Sissoko MS, Healy SA, et al. Safety and efficacy of PfSPZ Vaccine against Plasmodium falciparum via direct venous inoculation in healthy malaria-exposed Malian adults: a randomised, double-blind trial. Lancet Infectious Diseases 2017 doi:10.1016/S1473-3099(17)30104-4
- Mordmüller B et al. Sterile protection against human malaria by chemoattenuated PfSPZ vaccine. Nature 2017 542:445–449
- Bastiaens GJ et al. Safety, immunogenicity, and protective efficacy of intradermal immunization with aseptic, purified, cryopreserved Plasmodium falciparum sporozoites in volunteers under chloroquine prophylaxis: a randomized controlled trial. Am J Trop Med Hyg 2016 94:663-73
- Roestenberg M et al. Controlled Human Malaria Infections by Intradermal Injection of Cryopreserved Plasmodium falciparum Sporozoites. Am J Trop Med Hyg 2013 88:5-13
- Gómez-Pérez GP et al. Controlled human malaria infection by intramuscular and direct venous inoculation of cryopreserved Plasmodium falciparum sporozoites in malaria-naïve volunteers: effect of injection volume and dose on infectivity rates. Malaria Journal 2015 14:306
- Mordmüller B et al. Direct venous inoculation of Plasmodium falciparum sporozoites for controlled human malaria infection: a dose-finding trial in two centres. Malaria Journal 2015 14:117
- Shekalaghe S et al. Controlled human malaria infection of Tanzanians by intradermal injection of aseptic, purified, cryopreserved Plasmodium falciparum sporozoites. Am J Trop Med Hyg 2014 91:471–480
- Hodgson SH et al. Evaluating controlled human malaria infection in Kenyan adults with varying degrees of prior exposure to Plasmodium falciparum using sporozoites administered by intramuscular injection. Frontiers in Microbiology 2014 5:686
By Dr. Peter F. Billingsley, Vice President International Projects and Strategy, Sanaria Inc., 9800 Medical Center Drive, Rockville, MD 20850, USA. firstname.lastname@example.org
To find out more about these results you can contact Sanaria at www.sanaria.com