A malaria vaccine tested on infants in seven African countries has shown a protective effect that is small but possibly enough to prove useful in areas where the infection is a serious threat to children.

The vaccine, known officially as RTS,S/AS01, reduced by 33 percent the number of cases of malaria suffered by infants in the year after they were immunized, according to a study presented in South Africa on Friday and published online by the New England Journal of Medicine.

The actual number of infections was small. About 2.3 percent of babies not getting the vaccine suffered a case of severe malaria in their first year. The vaccine reduced that by one-third.

Preventive measures and better treatment have driven down malaria mortality over the past 20 years. Nevertheless, the mosquito-borne infection still causes about 216 million cases of illness and 655,000 deaths — almost all of them of African children — each year.

“If you broadly implement this across sub-Saharan Africa, it is going to prevent millions of cases and save thousands of lives,” said David Kaslow, director of the malaria vaccine initiative at the Program for Appropriate Technology in Health (PATH), a nonprofit group in Seattle that is helping run the vaccine trials.

One of the more successful tools against malaria is the insecticide-treated bed net. Studies have shown that consistently sleeping under one reduces cases of malaria by 25 to 75 percent. About 85 percent of the children in the vaccine study slept under a net, so they were substantially protected. In addition, indoor spraying with long-lasting pesticides — another preventive tool — was common in four of the 11 study areas.

The study enrolled infants in Kenya, Tanzania, Mozambique, Malawi, Ghana, Burkina Faso and Gabon. Malaria transmission differed greatly from place to place. In the worst area, babies averaged two bouts of malaria a year. In the site with the least transmission, the figure was one-hundredth that rate.

About 6,500 babies were randomly assigned to get the malaria vaccine or an unrelated one that helps prevent bacterial meningitis. By design, twice as many were assigned to the malaria vaccine as to the other one. The malaria vaccine was given in three shots a month a part, starting at two months of age.

At least one episode of severe malaria occurred in 2.3 percent of the babies getting the meningitis vaccine, compared with 1.5 percent in those getting the malaria vaccine.

Death from any cause, including malaria, was rare. It occurred in 1.5 percent of the babies who got the malaria vaccine and 1.3 percent in those who got the meningitis vaccine.

Meningitis was twice as likely in the malaria-vaccine babies. Whether that was because they didn’t get the meningitis vaccine or happened by chance is unknown.

A study of the same vaccine in children 5 to 17 months of age was reported last year. It found a bigger protective effect — about 50 percent.

The vaccine is made by GlaxoSmithKline. The company has said the price of the vaccine will cover the cost of its production and a 5 percent margin, which will be reinvested in research on other vaccines for tropical diseases.