About the Authors
Influenza Division, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America
The authors have declared that no competing interests exist.
Conceived and designed the experiments: HY LMC PJC ROD JS. Performed the experiments: HY LMC PJC JS. Analyzed the data: HY LMC PJC ROD JS. Wrote the paper: HY LMC PJC ROD JS.