Heat Inactivation of Different Types of SARS-CoV-2 Samples: What Protocols for Biosafety, Molecular Detection and Serological Diagnostics?

Abstract

Standard precautions to minimize the risk of SARS-CoV-2 transmission implies that infected cell cultures and clinical specimens may undergo some sort of inactivation to reduce or abolish infectivity. We evaluated three heat inactivation protocols (56 °C-30 min, 60 °C-60 min and 92 °C-15 min) on SARS-CoV-2 using (i) infected cell culture supernatant, (ii) virus-spiked human sera (iii) and nasopharyngeal samples according to the recommendations of the European norm NF EN 14476-A2. Regardless of the protocol and the type of samples, a 4 Log₁₀ TCID50 reduction was observed. However, samples containing viral loads > 6 Log₁₀ TCID₅₀ were still infectious after 56 °C-30 min and 60 °C-60 min, although infectivity was < 10 TCID₅₀. The protocols 56 °C-30 min and 60 °C-60 min had little influence on the RNA copies detection, whereas 92 °C-15 min drastically reduced the limit of detection, which suggests that this protocol should be avoided for inactivation ahead of molecular diagnostics. Lastly, 56 °C-30 min treatment of serum specimens had a negligible influence on the results of IgG detection using a commercial ELISA test, whereas a drastic decrease in neutralizing titers was observed.

Keywords: COVID-19; ELISA; SARS-CoV-2; coronavirus; heat inactivation; neutralization; serology; virus neutralization test.

Figure 1

Impact of heat treatment at 56 °C-30 min on the SARS-CoV-2 ELISA EuroImmun IgG assay compared with the same sera maintained at ambient temperature (20 °C). Panel (A), histogram representation. (B): point cloud representation. Positivity threshold is represented by the discontinuous line.

Figure 2

Impact of heat treatment at 56 °C-30 min on the SARS-CoV-2 neutralization assay compared with the same sera maintained at ambient temperature (20 °C). Panel (A), histogram representation. (B): point cloud representation. * Serum specimens with a titer ≥ 20 were considered positive.