Safety, tolerability and immunogenicity of a condensed, multi-dose prime regimen of PfSPZ Vaccine for the prevention of Plasmodium falciparum malaria infection.

backgroundThe World Health Organization (WHO) has called for new malaria vaccines with > 90% efficacy against Plasmodium falciparum infection to expand the anti-disease benefit provided by the RTS,S/AS01 and R21/Matrix M subunit vaccines currently administered to infants and young children in sub-Saharan Africa. Attenuated P. falciparum sporozoites (PfSPZ) are being developed as a traveller's vaccine and to fulfill WHO's call for high-level efficacy in endemic countries to support malaria elimination.

methodsPfSPZ Vaccine, comprised of radiation-attenuated PfSPZ, was compared with normal saline placebo in a randomized, double-blind trial targeting 60 malaria-naive US adults to assess safety, tolerability, immunogenicity, and efficacy against heterologous controlled human malaria infection three and twelve weeks after immunization. Pharmacists provided syringes to blinded clinicians using 3:1 (vaccine:placebo) blocked randomization, for administration by direct venous inoculation on days 1 and 8 (multidose prime) and day 29 (boost), a condensed regimen with superior efficacy. Primary outcomes included adverse events and antibody responses to the P. falciparum circumsporozoite protein (PfCSP).

results31 participants were screened, randomized and immunized twice (V1, V2) 5-7 days apart, with one withdrawal after an intercurrent adverse event. A vial issue, later traced to the vial manufacturer, halted further immunizations. Solicited local and systemic adverse events recorded for 2 and 7 days after immunizations, respectively, occurred with equal frequency and severity in the 23 vaccinees and 7 controls receiving two immunizations, as did unsolicited adverse events recorded for 28 days and laboratory abnormalities 1 and 5 weeks after V2. Four of 23 vaccinees and one of 7 controls (p = 1.00) developed grade 2 adverse events including subjective fever, headache, malaise, fatigue, rigors, arthralgia and myalgia after V2 but not V1, these symptoms generally resolving within 24 h. Twenty-two of 23 (96%) vaccinees developed IgG (median 99-fold increase over baseline) and IgM (median 1,110-fold increase) antibodies to PfCSP one week after V2. Antibody responses were not associated with reactogenicity.

conclusionsThe two-dose priming immunization regimen was safe, well tolerated and highly immunogenic. Larger studies may better define the adverse event profile of condensed regimens of PfSPZ Vaccine in malaria-naive adults. TRIAL REGISTRATION NUMBER: clinicaltrial.gov NCT05604521.