The Philippines is prepared for a "worst-case scenario" following warnings that an anti-dengue vaccine administered to thousands of children may worsen the disease in some cases, a health official said Saturday.

Department of Health spokesman Eric Tayag said the country had already taken precautions against potential mishaps when it became the first country to use the landmark vaccine in 2016.

The developer of the world's first vaccine for the potentially deadly virus, French pharmaceutical giant Sanofi, revealed earlier this week that it could trigger more severe symptoms in people who had not been previously infected with dengue.

More than 733,000 children have already received Dengvaxia, raising fears that many could develop the harsher form of the disease.

"The Department of Health is prepared for a worst-case scenario," Tayag told ABS-CBN television, a day after the agency announced it was suspending its mass vaccination program.

Tayag said the government had been careful to only implement the scheme in areas where dengue was already widespread and had only given it to children aged 9 or older.

"They are being followed up for adverse effects following immunization," he said.

He added that the department, which had previously said there were no reported cases of worsened infection among those who received the vaccine, was also checking hospital records for severe cases of dengue.

Sanofi had said such acute dengue cases would not become apparent till about 5 years after vaccination, Tayag added.

The developer initially said its Dengvaxia vaccine was "critical" in the fight against dengue, the world's most common mosquito-borne virus.

It said Wednesday that a new study has confirmed Dengvaxia's benefits for "those who had prior infection" with the potentially-lethal disease.

"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," Sanofi said.

More than 1,000 people in the Philippines died from dengue last year, out of more than 211,000 suspected cases, according to the government.