In four doses: On malaria vaccines

Beginning next year, the World Health Organisation will begin pilot tests of the injectable malaria vaccine RTS,S (or Mosquirix) on 750,000 children aged 5-17 months in Ghana, Kenya and Malawi. The vaccine has been successfully put through a Phase III trial, in which the drug is tested for safety and efficacy. Any decision on wider use will be taken based on the results of the pilot tests in the three countries. If the vaccine does indeed prove to be ready for large-scale use, it will be a milestone in the fight against malaria. Although the number of cases globally and in the African region came down by 21% between 2010 and 2015, in 2015 itself the number of deaths worldwide on account of the disease was as high as 429,000. According to WHO estimates, Africa accounted for 92% of these deaths, and 90% of the 212 million new cases that year. In such a scenario, even a vaccine with limited benefits could yield a substantial improvement. The vaccine, given in four doses, protects against Plasmodium falciparum, which is the most prevalent malaria parasite in Africa. The three countries have been chosen as they have settings with moderate-to-high transmission of malaria and already have in place malaria control programmes such as the use of bed-nets, rapid diagnostic tests and combination therapy. Each country is to decide where precisely to run the pilots. The first three doses of the vaccine will be administered with a minimum interval of one month between each dose, followed by the fourth dose 15 to 18 months after the third dose. The first dose will be administered at about five months of age and the third dose has to be completed by nine months of age. While the drop-out rate increases as the number of doses increases, the biggest challenge is the fourth dose, which warrants a new immunisation contact to be made 15 to 18 months after the last dose. In Phase III trials, the efficacy of the vaccine was around 30% when children received all the four doses; the vaccine also reduced the most severe cases by a third. But there was a significant drop in these benefits when children did not receive the fourth dose. Given the low protection efficacy of the vaccine even in tightly controlled clinical settings, the pilot tests will be useful in evaluating the likelihood of replicating the immunisation schedule in the context of routine health-care settings. Also, the extent to which the vaccine reduces the all-cause mortality has to be evaluated as this was not “adequately addressed” during the trial. There is, specifically, a need to ascertain if excess cases of meningitis and cerebral malaria seen during the trials are causally related to the vaccination. Unlike other vaccines, the less-than-optimum protection offered by this vaccine would mean that existing malaria intervention measures will have to be used in conjunction to reduce the incidence of the disease.

Related News