Millions of children's lives could be saved by a new vaccine shown to halve the risk of malaria in the first large-scale trials across seven African countries.

The long-awaited results of the largest-ever malaria vaccine study, involving 15,460 babies and small children, show that it could massively reduce the impact of the much-feared killer disease. Malaria takes nearly 800,000 lives a year – mostly children under five. It damages many more.

The vaccine has been in development for two decades – the brainchild of scientists at the UK drug company GlaxoSmithKline, which has promised to sell it at no more than a fraction over cost-price, with the excess being ploughed back into further tropical disease research.

"This data brings us to the cusp of having the world's first malaria vaccine, which has the potential to significantly improve the outlook for children living in malaria endemic regions across Africa," said GSK's chief executive, Andrew Witty.

"The addition of a malaria vaccine to existing control interventions, such as bed nets and insecticide spraying, could potentially help prevent millions of cases of this debilitating disease. It could also reduce the burden on hospital services, freeing up much-needed beds to treat other patients who often live in remote villages, with little or no access to healthcare."

Witty told the Guardian he was thrilled for the scientists, who were thought by many of their peers to be attempting the impossible when they started work on a vaccine 25 years ago. "When the team was first shown the data, quite a number of them broke down in tears," he said. "It was the emotion of what they had achieved – the first vaccine against a parasitic form of infection. They were overwhelmed. It says something about the amount of heart that has gone into this project."

In an indication of the weight of expectation around this vaccine, still known only as RTS,S, the results were announced at a malaria forum in Seattle called by Bill and Melinda Gates, attended by the World Health Organisation director general, Margaret Chan, and the UK development secretary, Andrew Mitchell. The results were published at the same time online by the New England Journal of Medicine.

Mitchell said a vaccine "offers real hope for the future", adding: "An effective, long-lasting and cost-effective vaccine would make a major contribution to malaria control … but we must not lose sight of the fact that over 2,000 people die from malaria every day and they need help now. Britain's focus remains on driving down this terrible loss of life by preventing and treating malaria with the tools we have now and tackling resistance."

Small-scale studies, in a few hundred children, have shown promising results in the past, but a trial of this size is needed to prove the vaccine's usefulness across populations. It is being carried out in Burkina Faso, Gabon, Ghana, Kenya, Malawi, Mozambique and Tanzania.

The early data from five- to 17-month-old children is the first of three important results; the outcome from the vaccination of newborn babies will be published next year. These figures are crucial, because the malaria vaccine needs to be incorporated into the infant immunisation schedule, alongside the usual diphtheria and measles jabs. Earlier small-scale trials suggest the results in six- to 12-week-old babies will also show around 50% protection.

The third important outcome, on how well the protection lasts, will not be known until 2014. The data so far, over 22 months, suggests there may be a drop in the numbers protected from severe malaria.

The WHO has said that if the results are satisfactory, it will recommend its use and the vaccine may begin to be rolled out as early as 2015, but it will need to be used in conjunction with all the other existing tools of malaria prevention, such as bed nets and insecticide spraying on the inside of homes.

Questions remain over the price of the vaccine and whether donors will be willing to pay. Dr Regina Rabinovitch, from the Bill and Melinda Gates Foundation, declined to say if they would fund it, saying they would want to look at the final data on efficacy, duration and safety. "Would I prefer to see a 100% effective vaccine? Certainly," she told a press conference.

Witty says he is exhorting everybody involved in the vaccine's production to pare their costs to the bone. "We are absolutely dedicated to making it as low as possible," he said.

Christopher Elias, president and chief executive of Path, a non-profit organisation that has helped fund the study, with the assistance of the Gates Foundation, said such high-quality science was moving the fight against malaria on.

"The Path malaria vaccine initiative's mission is to deliver a vaccine to the children of Africa so that instead of carrying near lifeless babies to crowded paediatric wards, mothers will carry their infants past noisy school playgrounds to bustling immunisation clinics. Today, we are an important step closer to realising that vision, and we look forward to continuing our drive, together with our partners, to bring this vaccine home to the children of Africa."

Bill Gates said a vaccine is the simplest, most cost-effective way to save lives. "These results demonstrate the power of working with partners to create a malaria vaccine that has the potential to protect millions of children from this devastating disease," he said. All the children in the trial received three doses either of vaccine or an ineffective placebo. The analysis published in the journal relates to the first 6,000 children, aged five to 17 months, to be immunised. Over the 12 months after immunisation, the vaccine reduced their risk of developing clinical malaria – meaning the high fevers and chills that need medical treatment – by 56%, and of developing severe malaria by 47%.

Severe malaria affects the brain, kidneys and blood and can kill. Most children still suffered malaria, but fewer and less serious bouts. For every 1,000 children who received the vaccine there were 750 cases of malaria over a year, compared with 1,500 per 1,000 children who were given a dummy jab. Side-effects were roughly the same in both the vaccine and placebo groups and relatively high, at around 20%, but investigators say this has to do with other health problems among rural African children.