Virus survival in evaporated saliva microdroplets deposited on inanimate surfaces

The novel coronavirus respiratory syndrome (COVID-19) has now spread worldwide. The relative contribution of viral transmission via fomites is still unclear. SARS-CoV-2 has been shown to survive on inanimate surfaces for several days, yet the factors that determine its survival on surfaces are not well understood. Here we combine microscopy imaging with virus viability assays to study survival of three bacteriophages suggested as good models for human respiratory pathogens: the enveloped Phi6 (a surrogate for SARS-CoV-2), and the non-enveloped PhiX174 and MS2. We measured virus viability in human saliva microdroplets, SM buffer, and water following deposition on glass surfaces at various relative humidities (RH). Although saliva microdroplets dried out rapidly at all tested RH levels (unlike SM that remained hydrated at RH ≥ 57%), survival of all three viruses in dry saliva microdroplets was significantly higher than in water or SM. Thus, RH and hydration conditions are not sufficient to explain virus survival, indicating that the suspended medium, and association with saliva components in particular, likely affect physicochemical properties that determine virus survival. The observed high virus survival in dry saliva deposited on surfaces, under a wide range of RH levels, can have profound implications for human public health, specifically the COVID-19 pandemic.