SINGAPORE - Dengvaxia, a dengue vaccine available here since March, could prove detrimental to people who have never been infected by dengue.

Sanofi, which produces the vaccine, sent out an alert saying: "For those not previously infected by dengue virus, the analysis found that in the longer term, more cases of severe disease could occur following vaccination upon a subsequent dengue infection."

The French pharmaceutical giant said it will ask the health authorities in countries where it has been approved for use, such as in Singapore, to update their information on the vaccine.

A spokesman for the Health Sciences Authority (HSA) said on Thursday (Nov 30) that it is working with Sanofi Pasteur to "strengthen the package insert" to include the warning of an increased risk of hospitalisation for dengue and clinically severe dengue in vaccinated individuals not previously infected by dengue.

A package insert is a document or leaflet provided along with the medication with information about the product, including instructions and risks.

HSA spokesman said the package insert will also contain advisories on an assessment of prior dengue infection in individuals before vaccination, and that vaccination should not be recommended in individuals who have not been previously infected with dengue.

The new finding comes from six years of clinical data, it said in a press statement from Paris.

However, for those who had been previously infected, it "provides persistent benefit against dengue fever".

Dr Ng Su Peing, global medical head of Sanofi Pasteur said: "These findings highlight the complex nature of dengue infection. We are working with the health authorities to ensure that prescribers, vaccinators and patients are fully informed of the new findings."

Dengvaxia is the world's first dengue vaccine. It was approved by HSA for use in patients aged between 12 and 45, and requires three doses to be administered over 12 months by injection.

Sanofi expects the new information to impact its business to the tune of €100 million (S$160 million) after tax.