Similar to salt, the effect of protein on MS2 was also RH-dependent (Fig 4A and 4C). The relative viability of MS2 decreased as the concentration of BSA increased in droplets at 20% RH. However, at RHs of 50% and 80%, the relative viability was higher in the presence of BSA. At 50% RH, the viability of MS2 was reduced by only 7% in droplets containing 100 μg/mL BSA after 1 h, a significantly lower loss than the >80% reduction in droplets that did not contain any BSA. At 80% RH, there was no decay in droplets containing BSA, regardless of its concentration, suggesting that BSA has a protective effect on the viability of MS2.