The Philippines is prepared for a "worst-case scenario" after warnings that a vaccine against dengue fever could cause symptoms of the disease to become more severe.

French pharmaceutical giant Sanofi Pasteur on Wednesday admitted clinical data showed Dengvaxia – the first licensed dengue vaccine – could make the disease worse in people who had not previously been infected with the virus.

The Government suspended a school-based immunisation programme following the revelations, which came after more than 733,000 children had been immunised.Sanofi Pasteur said Dengvaxia did provide persistent protective benefit in those who had previously had dengue.

"The Department of Health is prepared for a worst-case scenario," Department of Health spokesman Eric Tayag told ABS-CBN television.Mr Tayag said the vaccine was only given to children aged nine or older and that the immunisation scheme only operated in areas where dengue was already widespread.Those who had been vaccinated "are being followed up for adverse effects", he said.

He added that the health department was checking hospital records for acute cases of dengue.

But Sanofi had said it could take around five years for such severe dengue cases to become evident, Mr Tayag added.

Out of more than 211,000 suspected cases of dengue fever in the Philippines last year, at least 1,000 people had died, according to government figures.