The World Health Organization says Malawi has become the first country to begin immunizing children against malaria, using the only licensed vaccine to protect against the mosquito-spread disease.

Although the vaccine only protects about one-third of children who are immunized, those who get the shots are likely to have less severe cases of malaria. The parasitic disease kills about 435,000 people every year, the majority of them children under 5 in Africa.

“It’s an imperfect vaccine but it still has the potential to save tens of thousands of lives,” said Alister Craig, dean of biological sciences at the Liverpool School of Tropical Medicine, who is not linked to WHO or vaccine. Craig said immunizing the most vulnerable children during peak malaria seasons could spare many thousands from falling ill or even dying.

The vaccine, known as Mosquirix, was developed by GlaxoSmithKline and was approved by the European Medicines Agency in 2015. A previous trial showed the vaccine was about 30% effective in children who got four doses, but that protection waned over time. Reported side effects include pain, fever and convulsions.

Pedro Alonso, director of WHO’s malaria program, said similar vaccination programs would begin in the coming weeks in Kenya and Ghana, with the aim of reaching about 360,000 children per year across the three countries.

Alonso called the vaccination rollout a “historical moment,” noting that it was significantly more difficult to design a vaccine against a parasite as opposed to a bacterium or virus.

He acknowledged the vaccine was flawed, but said the world could not afford to wait for a better option. “We don’t know how long it will take to develop the next-generation vaccine,” he said. “It may be many, many years away.”