Defining the transmissible dose 50% for two pandemic influenza viruses in ferrets

Ferrets are the gold standard animal model used to assess the transmissibility of influenza viruses. Airborne transmission is evaluated by infecting donor ferrets with a high virus dose and monitoring transmission to contact animals sharing the same airspace. However, the relationship between inoculation dose and transmission has not been evaluated in ferrets. Therefore, we performed studies evaluating airborne transmission of the 2009 pandemic H1N1 and 1968 pandemic H3N2 viruses over log scale reductions in donor inoculation doses. Using the results of these studies, we define a new measure of transmission, the transmissible dose 50%: the donor inoculation dose at which a virus is transmitted to 50% of contacts. Importantly, this metric permits the evaluation of transmissibility over a log scale. We demonstrate that the 1968 pandemic H3N2 virus has reduced transmissibility compared to the 2009 pandemic H1N1 virus in ferrets.