Persistence of SARS-CoV-2 on Surfaces, Fomites, All Depends on Conditions and Initial Amount

A recent carefully done laboratory experiment has been published https://virologyj.biomedcentral.com/articles/10.1186/s12985-020-01418-7 . The information needs to be put in context to assess real field environmental survival of SARS-CoV-2. Viruses, in some sense, survive better environmentally than vegetative bacteria but less well than bacterial spores. The rule of thumb is the “dirtier the virus, the longer it survives inactivation”. The cleaner it is, the less it survives. However, as stated in my earlier post, the initial amount can lead to even longer or shorter apparent survival due to exponential decay and the threshold of the detection methods: viral culture in vitro or true infectious dose in a susceptible host. The paper to be discussed used the in vitro method to assess virus viability and vigorous extraction methods (worst case transmission) not touch transfer or re-aerosolization (which are more hazardous to do in the lab). They also used a standard biological matrix (mimic of human expelled biological material) and maximum inoculum dose (based on assumed human expelled dose) which is appropriate to produce optimal survival data in the laboratory. Real life conditions are messier and UV light (all light) was excluded from the experimental conditions. They observed half lives of between 1.7 and 2.7 days at 20 °C, decreasing to a few hours when temperature was elevated to 40 °C. Using initial viral loads approximately equivalent to the highest titers excreted by infectious patients and in a simulated biological matrix, viable virus was isolated for up to 28 days at 20 °C from common surfaces such as glass, stainless steel and both paper and polymer banknotes. In contrast, infectious virus survived less than 24 h at 40 °C (104 F) on some surfaces. Fomite transmission is highly efficient for transmission (efficiencies of 33%) for both fomite to hand and fingertip to mouth transfer for bacteria and phages (no data for CoV). The inoculum was 10 microliters of the suspension (final concentration of 3.38 × 10⁵/10 µL). A single relative humidity set point (50%) was maintained for experiments at 20 °C, 30 °C, 40 °C, respectively, although dew point would have been a more accurate measure since relative humidity changes with temperature. Three replicates of each surface type were inoculated and sampled at the following time points; 1 h, 1 day, 3 days, 7 days, 14 days, 21 days and 28 days post inoculation. For the 40 °C experiment, triplicate samples were inoculated for the following time points; 1 h, 1 day, 2 days, 3 days, 4 days, and 7 days. For non-porous surfaces, for each replicate, virus was eluted in 2 × 115 µL volumes of DMEM (cell culture medium) with repeated pipetting then titrated individually, in quadruplicate wells on a 96-well plate. The recovery from porous surfaces, such as cotton cloth, which are likely to inhibit transfer and recovery under field conditions, was accomplished by soaking swatches of the cloth in 500 µL DMEM and pipetting repeatedly for at least 1 min before 230 µL of the recovered eluent from each swatch was titrated separately, in quadruplicate. Suspensions of Vero E6 cells (3 × 10⁵/mL) were added to the wells and the plates were incubated for 3 days at 37 °C with 5% CO2 to look for cell damage (cytopathic effect) from viral replication in the cells. The majority of virus reduction on cotton occurred very soon after application of virus, suggesting an immediate adsorption effect. The calculated 90%, 1 log, inactivation of virus for surfaces at 20 °C (room temperature = 25 degrees C) ranged from 5.5 days for cotton to 9.1 days for paper notes. At 30 °C, infectious virus was recoverable for 7 days from stainless steel, polymer notes and glass, and 3 days for vinyl and cotton cloth. For paper notes, infectious virus was detected for 21 days, although there was less than 1 log of virus recovered for both 14 day and 21 day time points. At 40 °C, virus recovery was reduced compared to both 20 °C and 30 °C experiments. Infectious SARS-CoV-2 was not recovered past 24 h for cotton cloth and 48 h for all remaining surfaces tested. Greater than 4-log reduction (99.99% reduction from starting titer) was observed in less than 24 h at 40 °C on all surfaces. All calculations were based on linear decay fits, not exponential decay. Again, as stated in an earlier post, the latter model is what is observed under field conditions for other biological agents. Also, CoV is less durable than other viruses ( non-enveloped) such as adenovirus, picornaviruses (can persist for up to a year in the environment) and Norovirus (diverse group of single-stranded positive-sense RNA, non-enveloped viruses belonging to the family Caliciviridae). What needs to be kept in mind here is that incident surface contamination does not equal transferable host absorbed infectious dose. Smooth non-porous surfaces are more likely to transfer virus than porous surfaces that adsorb and bind virus. This is borne out by the low transmission rate (approximately 10% infected) for most exposed populations. However, the paper indicates that we should err on the side of caution and rigorously disinfect potentially contaminated surfaces.