1. Department of Virology, Swedish Institute for Infectious Disease Control and Microbiology and Tumorbiology Center, Karolinska Institutet, SE-171 82 Solna, Sweden.
2. Department of Medicine, Unit of Infectious Diseases, Karolinska Institutet, Karolinska Hospital, SE-171 76 Stockholm, Sweden.
3. Department of Environmental Science, ETH Zurich, CH-8092 Zurich, Switzerland.