Efficacy and Safety of the RTS,S/AS01 Malaria Vaccine during 18 Months after Vaccination: A Phase 3 Randomized, Controlled Trial in Children and Young Infants at 11 African Sites

Abstract

Mary Hamel and colleagues in the RTS,S Clinical Trials Partnership report updated safety and efficacy results from an ongoing Phase 3 trial, including calculations of vaccine impact (malaria cases prevented).

Please see later in the article for the Editors' Summary

Abstract

Background

A malaria vaccine could be an important addition to current control strategies. We report the safety and vaccine efficacy (VE) of the RTS,S/AS01 vaccine during 18 mo following vaccination at 11 African sites with varying malaria transmission.

Methods and Findings

6,537 infants aged 6–12 wk and 8,923 children aged 5–17 mo were randomized to receive three doses of RTS,S/AS01 or comparator vaccine.

VE against clinical malaria in children during the 18 mo after vaccine dose 3 (per protocol) was 46% (95% CI 42% to 50%) (range 40% to 77%; VE, p<0.01 across all sites). VE during the 20 mo after vaccine dose 1 (intention to treat [ITT]) was 45% (95% CI 41% to 49%). VE against severe malaria, malaria hospitalization, and all-cause hospitalization was 34% (95% CI 15% to 48%), 41% (95% CI 30% to 50%), and 19% (95% CI 11% to 27%), respectively (ITT).

VE against clinical malaria in infants was 27% (95% CI 20% to 32%, per protocol; 27% [95% CI 21% to 33%], ITT), with no significant protection against severe malaria, malaria hospitalization, or all-cause hospitalization.

Post-vaccination anti-circumsporozoite antibody geometric mean titer varied from 348 to 787 EU/ml across sites in children and from 117 to 335 EU/ml in infants (per protocol).

VE waned over time in both age categories (Schoenfeld residuals p<0.001). The number of clinical and severe malaria cases averted per 1,000 children vaccinated ranged across sites from 37 to 2,365 and from −1 to 49, respectively; corresponding ranges among infants were −10 to 1,402 and −13 to 37, respectively (ITT). Meningitis was reported as a serious adverse event in 16/5,949 and 1/2,974 children and in 9/4,358 and 3/2,179 infants in the RTS,S/AS01 and control groups, respectively.

Conclusions

RTS,S/AS01 prevented many cases of clinical and severe malaria over the 18 mo after vaccine dose 3, with the highest impact in areas with the greatest malaria incidence. VE was higher in children than in infants, but even at modest levels of VE, the number of malaria cases averted was substantial. RTS,S/AS01 could be an important addition to current malaria control in Africa.

Trial registration

www.ClinicalTrials.gov NCT00866619

Please see later in the article for the Editors' Summary

Editors' Summary

Background

Every year, more than 200 million cases of malaria occur worldwide, and more than 600,000 people, mainly children living in sub-Saharan Africa, die from this parasitic disease. Malaria parasites are transmitted to people through the bites of infected night-flying mosquitoes and cause fever that needs to be treated promptly with anti-malarial drugs to prevent anemia (a reduction in red blood cell numbers) and life-threatening organ damage. Malaria transmission can be prevented by using long-lasting insecticides sprayed on the indoor walls of homes to kill the mosquitoes that spread the malaria parasite or by sleeping under insecticide-treated nets to avoid mosquito bites and further reduce mosquito numbers. Widespread use of these preventative measures, together with the introduction of artemisinin combination therapy (an effective anti-malarial treatment), has reduced the global burden of malaria by 45% in all age groups, and by 51% among young children, since 2000.

Why Was This Study Done?

Unfortunately, the emergence of insecticide and drug resistance is threatening this advance in malaria control. Moreover, additional interventions—specifically, effective malaria vaccines—will be needed to eliminate malaria in the large areas of Africa where malaria transmission remains high. Currently, there is no licensed malaria vaccine, but RTS,S/AS01, the most advanced malaria vaccine candidate, is undergoing phase 3 clinical trials (the last stage of testing before licensing) in infants and children in seven African countries. The RTS,S Clinical Trials Partnership reported encouraging results on the efficacy and safety of RTS,S/AS01 during 12 months of follow-up in 2011 and 2012. Here, researchers report on the 18-month efficacy and safety of RTS,S/AS01. Vaccine efficacy (VE) is the reduction in the incidence of a disease (the number of new cases that occur in a population in a given period) among trial participants who receive the vaccine compared to the incidence among participants who do not receive the vaccine.

What Did the Researchers Do and Find?

The researchers randomly assigned 6,537 infants aged 6–12 weeks and 8,923 children aged 5–17 months to receive three doses of RTS,S/AS01 or a control vaccine. During 18 months of follow-up, there were 0.69 episodes of clinical malaria (a high temperature and parasites in the blood) per person-year among the children who received all the planned doses of RTS,S/AS01 (the “per protocol” population) and 1.17 episodes per person-year among the control children—a VE against clinical malaria in the per-protocol population of 46%. A similar VE was seen in an intention-to-treat analysis that included all the enrolled children, regardless of whether they received all of the planned vaccine doses; intention-to-treat analyses reflect the real-life situation—in which children sometimes miss vaccine doses—better than per-protocol analyses. In intention-to-treat analyses, the VE among children against severe malaria (fever, parasites in the blood, and symptoms such as anemia) and hospitalization for malaria was 34% and 41%, respectively. Among infants, the VE against clinical malaria was 27% in both per-protocol and intention-to-treat analyses; the vaccine showed no protection against severe malaria or hospitalization. In both infants and children, VE waned with time since vaccination. Across all the study sites, RTS,S/AS01 averted an average of 829 and 449 cases of clinical malaria per 1,000 children and infants vaccinated, respectively. Finally, the serious adverse event meningitis (inflammation of the tissues lining the brain and spinal cord) occurred more frequently in trial participants given RTS,S/AS01 than in those given the control vaccine, but the incidence of other serious adverse events was similar in both groups of participants.

What Do These Findings Mean?

These and other findings show that, during 18 months of follow-up, vaccination of children and young infants with RTS,S/AS01 prevented many cases of clinical and severe malaria and that the impact of vaccination was highest in regions with the highest incidence of malaria. They indicate, as in the earlier analysis, that the VE against clinical and severe malaria is higher in children than in young infants and suggest that protection wanes over time. Whether or not the vaccine played a causal role in the observed cases of meningitis cannot be determined from these results, and the occurrence of meningitis will be followed closely during the remainder of the trial. Other study limitations (for example, variations in the clinical characteristics of participants from one center to another) may also affect the accuracy of these findings and their interpretation. However, by showing that even a modest VE can avert a substantial number of malaria cases, these findings suggest that vaccination with RTS,S/AS01 could have a major public health impact in sub-Saharan Africa.

Additional Information

Please access these websites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1001685.

• Information is available from the World Health Organization on all aspects of malaria (in several languages), including malaria immunization; the World Malaria Report 2013 provides details of the current global malaria situation; the World Health Organization also provides information on its Global Immunization Vision and Strategy (in English and French)

• The US Centers for Disease Control and Prevention provides information on malaria, including a selection of personal stories about malaria

• Information is available from the Roll Back Malaria Partnership on the global control of malaria and on the Global Malaria Action Plan (in English and French); its website includes a fact sheet about malaria in Africa

• The latest results from the phase 3 trial of RTS,S are available on the website of the PATH Malaria Vaccine Initiative, a global program of the international nonprofit organization PATH that aims to accelerate the development of malaria vaccines and ensure their availability and accessibility in the developing world

• MedlinePlus provides links to additional information on malaria and on immunization (in English and Spanish)