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

Sanofi Pasteur, 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 authorities in countries where it has been approved for use, such as here, to update their information on the vaccine, and to disseminate it to doctors and patients.

A spokesman for the Health Sciences Authority (HSA) said yesterday 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.

The 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 findings come from six years of clinical data, said Sanofi Pasteur in a statement in 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."

Dengvaxia, the world's first dengue vaccine, was approved by HSA for use among 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.

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