We also evaluated the interactions of double combinations of neuraminidase inhibitors (NAIs) against 2009 H1N1. As shown in Table 1 , the synergy volumes for the zanamivir/oseltamivir carboxylate were 2246116 mg/mL 2 %, 2155689 mg/mL 2 %, and 2197698 mg/mL 2 % against CA04, CA05, and CA10, respectively. These values suggest that the zanamivir/oseltamivir carboxylate combination was additive against these viruses. For the zanamivir/peramivir combination, synergy volumes were 2356112 mg/mL 2 %, 21976108 mg/mL 2 % and 2239693 mg/ mL 2 % against CA04, CA05, and CA10, respectively, indicative of additivity to moderate antagonism. Consistent with these values, the synergy plots for the NAI double combinations against CA05 revealed regions of antagonism (red areas), which occurred at higher concentrations of zanamivir (0.01-0.1 mg/mL) and at variable concentrations of the second NAI (oseltamivir or peramivir, Figure S2 ). Similar results were found with the other two 2009 H1N1 strains. Furthermore, evaluation of the EC 50 of each NAI in combination with a fixed concentration of a second NAI revealed that the antiviral activity of each drug was not enhanced in combination with a second drug (data not shown). The observation that double combinations of neuraminidase inhibitors were not synergistic is consistent with the fact that all three drugs are known to target the same enzyme, and all bind in the same substrate binding pocket in a similar manner [19] .