Giving vitamin A supplements to children under the age of five in developing countries could save 600,000 lives a year, researchers claim.
Writing in the British Medical Journal, UK and Pakistani experts assessed 43 studies involving 200,000 children, and found deaths were cut by 24% if children were given the vitamin.
And they say taking it would also cut rates of measles and diarrhoea.
The body needs vitamin A for the visual and immune systems to work properly.
It is found in foods including cheese, eggs, liver and oily fish.
The World Health Organization (WHO) estimates that, around the world, 190 million children under the age of five may have a vitamin A deficiency.
But despite widespread efforts, supplementation programmes do not reach all the children who could benefit.
Capsules are now distributed twice a year in at least 60 countries, with average annual coverage rates nearing 80%.
However, University of Oxford and Aga Khan University researchers who carried out this work say the effectiveness of vitamin A is so well-established that policy-makers should provide supplements to all children at risk.'Effective and cheap'
They evaluated studies that involved children aged six months to five years, and compared rates of illness and death among those who were given vitamin A and those who were not.
They found vitamin A supplements reduced child mortality by 24% in low- and middle-income countries.
They calculate that, considering the estimated 190m children who are vitamin A deficient, reducing deaths by 24% would save more than 600,000 lives each year.
Dr Evan Mayo-Wilson from the University of Oxford, who worked on the study, said: "Until other sources are available, supplements should be given to all children who are at risk of vitamin A deficiency.
"After just one year, children who had taken supplements were less likely to have died than children who received a placebo [dummy version].
Vitamin A supplements are highly effective and cheap to produce and administer."
He said there had been recent criticism of vitamin A programmes - with some saying there were risks that respiratory infection rates could increase, particularly in children who were not vitamin A deficient - but he added there was "little doubt" that vitamin A prevented young children from dying.
In an editorial for BMJ Online, Professor Wafaie Fawzi, from the department of nutrition, epidemiology, and global health at Harvard School of Public Health, added: "Effort should now focus on finding ways to sustain this important child survival initiative and fine tune it to maximise the number of lives saved."