Thousands of children have been given a vaccine that may worsen the symptoms of the disease it is designed to treat.

More than 733,000 youngsters in the Philippines have been administered Dengvaxia - the world's first anti-dengue vaccine.

But the country's department of health is now heeding a warning from French pharmaceutical giant Sanofi that it may trigger more severe symptoms in people who have not previously been infected, following a new study.

Sanofi - the developer of the drug - had previously described it as "critical" in the fight against the disease.

It has now cautioned: "For those not previously infected by dengue virus, however, the analysis found that in the longer term, more cases of severe disease could occur following vaccination upon a subsequent dengue infection."


Image: The Philippines was the first country in the world to use the landmark vaccine

Spokesman Eric Tayag said the agency was "prepared for a worst-case scenario" and that children who have received it are being "followed up for adverse effects following immunisation".

For now its mass vaccination programme - which was focused on areas where dengue was already widespread - has been suspended and hospital records are being checked for severe cases of the disease.

Spread by mosquitoes, dengue can cause a fever, a severe headache, muscle and joint pain, but more severe and ailments include a swollen stomach, persistent vomiting and vomiting blood, bleeding gums and breathing difficulties.

The Philippine government says there were more than 1,000 deaths caused by the disease last year, out of around 211,000 suspected cases.