1. Chiron Vaccines, Siena, Italy; These authors contributed equally to this work.
2. National Institutes of Health, Bethesda, Maryland, USA;These authors contributed equally to this work.
3. Institut für Virologie, Marburg, GermanyThese authors contributed equally to this work.
4. National Institutes of Health, Bethesda, Maryland, USA;
5. Institut für Virologie, Marburg, Germany
6. Chiron Vaccines, Siena, Italy;