Author: Lipsitch, Marc; Inglesby, Thomas V.
Title: Reply to “Studies on Influenza Virus Transmission between Ferrets: the Public Health Risks Revisited” Document date: 2015_1_23
ID: x8yswoua_25
Snippet: To take a simplified example, suppose it were the case that 25% of the time, strains produced in ferret passage experiments were highly lethal and transmissible in humans, and 75% of the time they were attenuated. We would not know which instances are which, but suppose we knew these are the overall frequencies. In this case, it would be appropriate to multiply our pandemic risk calculations by about 25%, because 3 out of 4 ferret passage experim.....
Document: To take a simplified example, suppose it were the case that 25% of the time, strains produced in ferret passage experiments were highly lethal and transmissible in humans, and 75% of the time they were attenuated. We would not know which instances are which, but suppose we knew these are the overall frequencies. In this case, it would be appropriate to multiply our pandemic risk calculations by about 25%, because 3 out of 4 ferret passage experiments would produce strains not very harmful to humans. Twenty-five percent of the risk we estimated (5) is still exceptionally high. Yet now consider the use of this information by public health authorities. At best, three out of every four times they identified a veterinary or zoonotic isolate as high risk, they would actually be targeting a strain with features that make it attenuated in humans. They would be deploying resources to contain a strain that is, unbeknownst to them, human attenuated. One in four times, they might identify a strain with somewhat increased risk for humans, albeit not necessarily the strain most deserving of attention. In fact, because the prediction of mutational effects becomes more uncertain with changes in the genetic background, the predictive power of such targeting activities is even lower. In summary, while the possibility that ferret gain-of-transmission strains are attenuated in humans modestly reduces the risk estimate associated with producing and using them, it may nullify and even reverse the utility of such studies for public health.
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