Malaria Vaccine RTS, S/AS01: Results of a Seven Year Study

• Several vaccines are being evaluated to prevent falciparum malaria
• A forerunner among these is the RTS, S/AS01 malaria vaccine, which is in clinical trials
• The vaccine’s effectiveness appears to reduce in subsequent years

A study published in the New England Journal of Medicine studied the long-term effects of the candidate malaria vaccine RTS, S/AS01. Falciparum malaria is a potentially fatal infection that is particularly prevalent in Africa. The parasitic disease caused by Plasmodium falciparum can have particularly serious consequences especially in small children. Several vaccines are being developed to prevent infection from this mosquito-borne disease. The forerunner among these is the RTS,S/AS01, developed by GlaxoSmithKline Biologicals (GSK) and the PATH Malaria Vaccine Initiative (MVI).

A large study that was previously published showed that the vaccine was effective in reducing malaria cases in the first year after it was administered. But the effectiveness of the vaccination reduced considerably in the subsequent years till the fourth year. The babies who received three doses of the vaccine between 5 and 17 months of age showed an overall efficacy of 28.3% over a follow-up period of 4 years. Administration of a booster dose at 18 months improved the protection to 36.3%. The efficacy was less when vaccination was started at a younger age at 6 to 12 weeks of age.

Nearly half of the world’s population is at risk of malaria. Children under the age of 5 are particularly susceptible to malaria infection, illness and deaths. Early diagnosis and prompt treatment could prevent malaria deaths.

There was a concern that the vaccine could have a negative effect as the time passes. Therefore researchers studied the effect of the vaccine over seven years and published their results in The New England Journal of Medicine. Though the duration of the study was longer, the number of children studied was comparatively much less than the above-mentioned study.

The study included data from 447 African children from Kenya who were vaccinated when they were between 5 to 17 months of age. Some children received the malaria vaccine, while others received the rabies vaccine and were used for comparison. Blood tests were carried out regularly to detect the presence of the malarial parasite.

The present study was conducted to check if there is a possibility of a rebound effect on long-term follow-up after administration of the vaccine. The researchers found the following:

• The efficacy of the vaccine against the first episode of clinical malaria was 27%
• The efficacy of the vaccine against all episodes of clinical malaria was estimated at 4.4%
• The efficacy of the vaccine reduced from 35.9% in the first year to 3.6% in the seventh year
• A negative efficacy was observed in the high-exposure group in the year 5 of the study
• The estimated number of cases of malaria prevented due to the vaccine over 7 years were 317 cases per 1000 children

The efficacy of the vaccine was estimated to be lower in those who were highly exposed to the parasite as compared to those in lower exposure areas. No significant complications were noted as due to the vaccine.

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Though the vaccine does not appear to provide long-term protection, it may benefit the children at an age where they may be most susceptible to the complications of the malarial infection. There were several factors that could have affected the results of the study. Larger studies will probably give better insights into the usefulness of the vaccine, which is the need of the hour. In the meantime, other methods to prevent mosquito bites and reduce the mosquito population should be continued on a firm footing.

Reference:

1. Olotu A et al. Seven-Year Efficacy of RTS,S/AS01 Malaria Vaccine among Young African Children. N Engl J Med 2016; 374:2519-2529

Source-Medindia