Acknowledgements
The authors thank Joe Orlando and Brandon Medeiros for generating feedstocks, Elizabeth Goodrich, Akshat Gupta, Santosh Rahane for helping prepare feedstocks, David Casaperalta, Dominic Smith, Alvaro Botero for performing infectivity assays, Robert Kelly, Thomas Jefferies, David Bohonak, Ushma Mehta, Herb Lutz and Tom Elich for valuable discussions. We are grateful to Kate Smith for her careful review of the manuscript.