1 Garvan Institute of Medical Research, 384 Victoria Street, Darlinghurst, New South Wales 2010, Australia. Electronic address: d.burnett@garvan.org.au.
2 Institute of Virology, Freie Universität Berlin, Robert von Ostertag-Straße 7-13, 14163 Berlin, Germany. Electronic address: luca.bertzbach@fu-berlin.de.