Trial Design

We conducted this trial as part of a double-blind, randomized, controlled, phase 2 trial of the RTS,S/AS01 vaccine in children who were 5 to 17 months of age at the time of the first vaccination and who lived in Kilifi, Kenya, or in Korogwe, Tanzania.3 The original two-site trial was initiated in March 2007 and was completed in August 2008 in Korogwe and, after a site-specific extension, in November 2008 in Kilifi. Additional follow-up was then conducted in Kilifi until April 2011 (4-year efficacy results were published in 2013).2 A further extension trial was conducted until November 2014, at which time follow-up was discontinued. The data reported here are from the Kilifi site only. The full details of the conduct of the trial are provided in the protocol (including the statistical analysis plan), available with the full text of this article at NEJM.org.

The trial extension was designed by the academic authors, and employees of GlaxoSmithKline Biologicals provided review of the protocol. Until November 1, 2008, the sponsorship, monitoring, and data management were the responsibility of GlaxoSmithKline Biologicals; after that date, these aspects of the trial were the responsibility of the Kenya Medical Research Institute–Wellcome Trust Research Programme. Data were gathered by the academic team, and the analysis was conducted by the first, second, and last authors. The first draft of the manuscript was written by the first author and revised by the last author, with comments from all the authors. Safety reporting to the regulatory authorities was undertaken by GlaxoSmithKline Biologicals.

Participants

In early 2007, we recruited 447 healthy children who were 5 to 17 months of age. We conducted three extensions: from the end of 12 months of follow-up until November 2008, then until April 2011, and then finally for an additional 3 years. Written informed consent for the extension was obtained from the parents or guardians of all the children with the use of approved consent forms provided in Swahili or Giriama. Nonliterate parents indicated consent by using a thumbprint, and a signature was obtained from an independent literate witness. The original trial and its extensions were approved by the Kenya Medical Research Institute National Ethics Committee, the Western Institutional Review Board, and the Oxford Tropical Research Ethics Committee. This article is published with the permission of the Director of Kenya Medical Research Institute.

Procedures

In the initial trial, three doses of the RTS,S/AS01 vaccine or rabies (control) vaccine were administered, at baseline and at 1 and 2 months. No vaccines were administered during the extended follow-up phase. Participants were followed up by means of both weekly active surveillance and passive surveillance to identify clinical malaria cases. We obtained blood samples for the assessment of asymptomatic parasitemia at 8, 12, 15, 25, 38, and 49 months (as reported previously2) and at 65, 78, and 91 months after vaccination. The participants and clinicians who were involved in the follow-up were unaware of the trial-group assignments. The principal investigators became aware of the trial-group assignments after the end of the initial phase of the trial but did not take part in the clinical evaluation of participants.

Malaria-Parasite Exposure

Malaria transmission shows fine-scale geographic heterogeneity.4 We previously found that fine-scale variations in exposure could be predicted by estimation of the prevalence of malaria infection among children who reside within a 1-km radius of each participant as an exposure index. Furthermore, the accuracy of this exposure index is refined by weighting according to the inverse of the distance between homes within the 1-km radius and the index child. Hence, homes that are near to the index child contribute more important information than do those on the edge of the 1-km radius.5 In an analysis that was not prespecified, we used data from 870 children who were under active surveillance in the same trial area to determine exposure indexes and categorized the participants into low-exposure and high-exposure groups according to whether they were at or below the cohort mean or above the cohort mean, respectively.

Statistical Analysis

The primary end point was clinical malaria caused by Plasmodium falciparum (temperature of ≥37.5°C and P. falciparum parasitemia density of >2500 parasites per cubic millimeter). The intention-to-treat cohort included all the children who had undergone randomization. The per-protocol cohort included children who received three doses of vaccine according to the trial protocol and for whom surveillance data were available at any time from 2 weeks after receipt of the third dose. We censored data from children at 7 years of follow-up.

We used Cox proportional-hazard regression for analysis of the first malarial episode. Multiple episodes were analyzed by means of negative binomial regression and the Andersen–Gill extension of Cox regression, with clustering by participant. The first-episode analysis was used as the primary analysis after 8 months of follow-up,3 but longer-term follow-up showed lower efficacy when all episodes were considered.2 Vaccine efficacy was defined as 1 minus the hazard ratio or the incidence-rate ratio multiplied by 100 in the RTS,S/AS01 group versus the control group.

Adjustments were made for age, bed-net use, and malaria exposure. We plotted the incidence-rate ratios over time by first aggregating the data into 4-month groups and then calculating the incidence rates in each trial group; we then divided the incidence in the RTS,S/AS01 group by the incidence in the control group. A quadratic equation was then fitted to these values.

As previously reported,1 we calculated the cases averted in each year of follow-up by subtracting the measured incidence per person-year among participants in the RTS,S/AS01 group from the incidence per person-year among participants in the control group and then multiplying by 1000 to express the result as the number of cases averted per 1000 children vaccinated with RTS,S/AS01. We calculated cumulative cases by summing the cases averted up to and including each given year. We used bootstrapping methods to obtain 95% confidence intervals by taking the 2.5th and 97.5th percentiles of 1000 iterations. All the analyses were performed with the use of Stata software, version 13 (StataCorp).