The effect of heat on SARS-CoV-2 viability and RNA integrity as determined by plaque assay, virus culture and real-time RT-PCR

The effect of heat on SARS-CoV-2/England/2/2020 viability was assessed by plaque assay and virus culture. Heating to 56°C and 60°C for 15, 30 and 60 minutes led to a reduction in titre of between 2.1 and 4.9 log 10 pfu/ml but complete inactivation was not observed. At 80°C plaques were observed after 15 and 30 minutes of heating, however after 60 minutes viable virus was only detected following virus culture. Heating to 80°C for 90 minutes and 95°C for 1 and 5 minutes resulted in no viable virus being detected. At 56°C and 60°C significant variability between replicates was observed and the titre often increased with heat-treatment time. Nucleic acids were extracted and tested by RT-PCR. Sensitivity of the RT-PCR was not compromised by heating to 56°C and 60°C. Heating to 80°C for 30 minutes or more and 95°C for 1 or 5 minutes however, resulted in an increase of at least three Ct values. This increase remained constant when different dilutions of virus underwent heat treatment. This indicates that high temperature heat inactivation of clinical samples prior to nucleic acid extraction could significantly affect the ability to detect virus in clinical samples from patients with lower viral loads by RT-PCR.