1 Janssen Pharmaceutical Companies of Johnson and Johnson, Beerse.
2 Laboratory of Virology and Chemotherapy, Rega Institute for Medical Research, Department of Immunology and Microbiology, University of Leuven, Leuven, Belgium.
3 Laboratory of Virology and Chemotherapy, Rega Institute for Medical Research, Department of Immunology and Microbiology, University of Leuven, Leuven, Belgium. Electronic address: johan.neyts@kuleuven.be.