Serono saw its shares in demand on Wednesday following the European Union's decision to grant it 10-year exclusive marketing rights in Europe for its AIDS drug, Serostim.
The exclusivity arrangement, known as orphan status, is a new EU policy that came into effect at the beginning of the year. Its aim is to encourage the development of drugs that treat rare diseases.
Serostim is used to treat AIDS-related weight loss, a condition that is suffered by up to 30 per cent of AIDS patients.
The drug is already on the market in the United States, Japan, Hong Kong and some Latin American countries.
swissinfo with agencies
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