Use of Highly Pathogenic Avian Influenza A(H5N1) Gain-Of-Function Studies for Molecular-Based Surveillance and Pandemic Preparedness

Davis, C. Todd; Chen, Li-Mei; Pappas, Claudia; Stevens, James; Tumpey, Terrence M.; Gubareva, Larisa V.; Katz, Jacqueline M.; Villanueva, Julie M.; Donis, Ruben O.; Cox, Nancy J.

Within hours of the posting of the H7N9 sequence data, CDC and other investigators using the H5N1 Genetic Changes Inventory identified several of the mutations described previously in H5 viruses shown to possess GOF phenotypes, such as increased respiratory droplet transmission in ferrets (using H5 numbering: HA T156A and HA Q222L), mammalian host adaptation (PB2 E627K and PB2 D701N), and enhanced virulence in mice (M1 N30D and M1 T215A) (48, 50) . Early detection of these molecular markers in H7N9 viruses isolated from humans gave public health authorities evidence that these viruses posed an immediate pandemic threat. Based on H7N9 sequence data alone, development of a candidate vaccine virus began within a day using synthetic biology (51) . H7N9 viruses obtained from human cases in China were subsequently shared with international partners within 2 weeks so that additional virologic characterization could be performed. Over a period of approximately 8 to 10 weeks from the initial reports of human infection, numerous laboratories identified H7N9 viruses (and the responsible mutations) with binding affinity to ␣2,6 host cell receptors (48, 52) , replication without prior adaptation in mouse and ferret models (52) , limited respiratory droplet transmission in ferrets (52) , and reduced susceptibility to antiviral drugs (53) , all substantiating inferences obtained from molecularly based surveillance using knowledge gained from GOF studies. The IRAT was used, as data accumulated, to assess and reassess the risk posed by H7N9 viruses compared with other zoonotic influenza viruses, and it was determined that the risk was greater than for H5N1 subtype viruses. More recently, GOF studies have focused specifically on H7N9 viruses and have identified other mutations (i.e., PB2 K526R) associated with enhanced mammalian replication (54) . Researchers also assessed the antigenic relationships of the novel viruses with existing H7 prepandemic vaccine candidates. These collective virologic findings, along with the molecular markers identified, led WHO Collaborating Centers for Influenza and vaccine manufacturers to rapidly develop candidate vaccine viruses (55) and then led the Department of Health and Human Services to perform human clinical trials (56) and to stockpile H7N9 vaccine in the United States (57) for pandemic preparedness, shaving months off the time required to deploy this vaccine, should it be needed.