Defining the transmissible dose 50%, the donor inoculation dose that results in airborne transmission to 50% of contacts, for two pandemic influenza viruses in ferrets

Ferrets are widely used to model airborne transmission of influenza viruses in humans. Airborne transmission is evaluated by infecting donor ferrets with a high virus dose (106 infectious units) and monitoring transmission to contact animals sharing the same airspace. However, humans can be infected with a broad range of influenza virus doses. Therefore, we evaluated the relationship between virus inoculation dose and transmission for two pandemic influenza viruses in ferrets. Donor ferrets were inoculated with 100 to 106 tissue culture infectious dose 50 (TCID50) of the 2009 pandemic H1N1 or 1968 H3N2 pandemic virus, and were then paired with respiratory contacts. Using the proportion of donors that became infected across virus doses, we calculated the infectious dose 50 (ID50). Subsequently, by comparing the proportion of respiratory contacts that became infected, we calculated the transmissible dose 50% (TD50): the donor inoculation dose that resulted in transmission to 50% of contacts. For the 2009 H1N1 virus, the ID50 and TD50 were equivalent at <1 TCID50. However, for the 1968 H3N2 virus, the ID50 and TD50 were <4 and 103.49 TCID50, respectively. The increased TD50 for the H3N2 virus was associated with reductions in peak viral titers and viral shedding in donors over decreasing virus inoculation doses. Collectively, these studies define a new measure of transmission that permits comparisons of transmissibility over a log scale. Using this metric, we show the 1968 pandemic H3N2 virus has reduced transmissibility in ferrets relative to the 2009 pandemic H1N1 virus. ImportanceFerrets 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%, which is the donor inoculation dose at which a virus is transmitted to 50% of contacts. Importantly, this metric permits the evaluation of transmissibility over a 10-100 fold scale, and using this metric, we demonstrate the 1968 pandemic H3N2 virus has reduced transmissibility compared to the 2009 pandemic H1N1 virus in ferrets.