Our aim was to evaluate the safety, reactogenicity, and immunogenicity of an investigational malaria vaccine.
This was an age-stratified phase Ib, double-blind, randomized, controlled, dose-escalation trial. Children were recruited into one of three cohorts (dosage groups) and randomized in 2:1 fashion to receive either the test product or a comparator.
The study was conducted in a rural population in Kombewa Division, western Kenya.
Subjects were 135 children, aged 12–47 mo.
Subjects received 10, 25, or 50 μg of falciparum malaria protein 1 (FMP1) formulated in 100, 250, and 500 μL, respectively, of AS02A, or they received a comparator (Imovax® rabies vaccine).
We performed safety and reactogenicity parameters and assessment of adverse events during solicited (7 d) and unsolicited (30 d) periods after each vaccination. Serious adverse events were monitored for 6 mo after the last vaccination.
Both vaccines were safe and well tolerated. FMP1/AS02A recipients experienced significantly more pain and injection-site swelling with a dose-effect relationship. Systemic reactogenicity was low at all dose levels. Hemoglobin levels remained stable and similar across arms. Baseline geometric mean titers were comparable in all groups. Anti-FMP1 antibody titers increased in a dose-dependent manner in subjects receiving FMP1/AS02A; no increase in anti-FMP1 titers occurred in subjects who received the comparator. By study end, subjects who received either 25 or 50 μg of FMP1 had similar antibody levels, which remained significantly higher than that of those who received the comparator or 10 μg of FMP1. A longitudinal mixed effects model showed a statistically significant effect of dosage level on immune response (F3,1047 = 10.78, or F3, 995 = 11.22, p < 0.001); however, the comparison of 25 μg and 50 μg recipients indicated no significant difference (F1,1047 = 0.05; p = 0.82).
The FMP1/AS02A vaccine was safe and immunogenic in malaria-exposed 12- to 47-mo-old children and the magnitude of immune response of the 25 and 50 μg doses was superior to that of the 10 μg dose.
Background: Malaria is thought to kill between 1 and 2 million people each year in sub-Saharan Africa; most of these are young children under the age of five, who are particularly prone to developing clinical malaria because their immunity is not yet developed. Many groups of researchers around the world are developing candidate vaccines of different types that it is hoped would protect against malaria. One of these types is a “blood-stage” vaccine, which would prevent parasite multiplication in red blood cells. A candidate blood-stage vaccine is FMP1/AS02A, which is designed to raise an immune response against a particular protein (merozoite surface protein-1) on the surface of the blood-stage infectious form of the malaria parasite. In early-stage clinical trials performed in people not exposed to malaria (healthy volunteers in the United States) and in African adults who were exposed to malaria, this candidate vaccine has already been shown to be safe and to bring about an immune response. As part of the next stage in developing this vaccine, a group of researchers next wanted to see whether the vaccine was also safe and brought about an immune response in the population most in need of a vaccine: young children living in an African region with very intense malaria transmission. Therefore, as reported here, this group performed a small trial in western Kenya, recruiting 135 children under 5 y of age to receive either the FMP1/AS02A vaccine (at three different doses) or rabies vaccine for comparison (thus ensuring that children in the control arm got some benefit from being in the trial). The outcomes that the researchers were interested in were primarily adverse events, which they categorized using a standard questionnaire at up to 7 d after vaccination; unsolicited events reported up to 30 d after vaccination; and, finally, any serious events occurring up to 8 mo later. The researchers also examined antibody responses to the FMP1/AS02A vaccine.
What this trial shows: Participants who received the FMP1/AS02A vaccine (as compared to the rabies vaccine) experienced more immediate symptoms, such as pain and swelling at the injection site. Most participants reported unsolicited events during follow-up, but the proportion of participants with adverse events did not seem to be different between the FMP1/AS02A vaccine groups and the rabies vaccine group. Unsolicited outcomes that were reported included, for example, clinical malaria, upper respiratory tract infections, and a few events that were thought to be related to the vaccines, such as fever and eczema. A few serious adverse events occurred up to 8 mo after vaccination, but the numbers did not seem to be different between the FMP1/AS02A and rabies vaccine groups, and the events were not judged to be related to vaccination. Finally, participants who received the FMP1/AS02A vaccine raised an antibody response to the vaccine, which was highest in those who received the highest vaccine dose. The researchers concluded that this vaccine was safe and brought about an immune response in the group of malaria-exposed children studied.
Strengths and limitations: The trial was conducted in a population that is likely to benefit from the vaccine, if it is shown to be effective in further studies. Therefore, the data obtained from this study will be informative in helping to design future trials on FMP1/AS02A. The randomization procedures used in this study were appropriate, and in particular participants of different ages were equally distributed to the different intervention groups, helping to minimize bias. Procedures were also set up to prevent participants and staff giving the vaccines and collecting data from knowing which interventions participants had received. However, the number of participants recruited into the trial was small, and it therefore was not powered to detect anything other than large differences in rates of adverse events between the study groups.
Contribution to the evidence: This study extends evidence from prior trials on the safety and immunogenicity of the FMP1/AS02A vaccine to a population that is representative of those most in need of an effective vaccine—young African children. The results suggest that the vaccine candidate should undergo further evaluation in trials examining vaccine efficacy in a similar population.