SARS-CoV-2 has been keeping us on tenterhooks for months and the infection numbers during winter have remained at a consistently high level for weeks. Already in spring 2020 it was suspected that the spread of the virus slows down or even disappears in the summer months, as this is the case with many other viruses which are sensitive to higher temperatures. A new study by Australian scientists published in the Virology Journal in early October 2020,(1) examined the resistance of coronavirus on different surfaces in relation to ambient temperature. The study shows that the virus survives for a significantly shorter time at higher temperatures.
The main route of transmission for corona remains droplet infection or aerosols. Little is known about the possibilities of infection via surfaces. To assess the risks of transmission from contaminated surfaces, we would need to know more about the effect of environmental factors, such as temperature or humidity, on the virus. This led the Australian scientists to investigate the survival rate of infectious coronavirus on different surfaces at different temperatures.
Usual surfaces under test
The surfaces tested were Australian polymer banknotes, paper banknotes and everyday materials such as brushed stainless steel, glass, vinyl and cotton fabric. Both polymer and paper banknotes were included in the study to gather information on the potential for transmission through banknotes. Stainless steel is often used in kitchen areas and public facilities, as well as being used as a standard test surface in studies on disinfectants. Glass was chosen because it is everywhere in our environment – in public areas such as hospitals or shopping centres, as contact surfaces in mobile phones, ATMs and self-service checkouts. Vinyl is a common substrate used in social settings, tables, floors, grab bars on public transport and as a protective material for mobile phone screens. Cotton was chosen as a porous surface often found in clothing, bedding and household textiles. A suspension with SARS-CoV-2 virus was applied to the different materials and stored in the incubator at 20° C, 30° C and 40° C and a humidity of 50 %. This was additionally done in absolute darkness to minimise interference of the results by UV radiation. The samples were examined after one hour, one day, three days and seven days.
Different temperatures – different survival
The results showed that the infectious SARS-CoV-2 virus was still detectable on all tested non-porous surfaces (glass, banknotes, stainless steel, vinyl and paper) at 20°C after 28 days. Significantly less active virus was detected on porous material (cotton fabric) and no virus was detectable after fourteen days. At 30 °C, the virus was detectable on stainless steel, polymer banknotes and glass for seven days and on vinyl and cotton fabric for three days. On the paper banknotes, the infectious virus could even still be detected for 21 days. Compared to the experiments at 20°C and 30°C, significantly fewer viruses could be detected at 40°C. The infectious SARS-CoV-2 was no longer found after 24 hours on cotton fabric and after 48 hours on all other surfaces tested. After less than 24 hours at 40°C, more than 99.99% of the viruses were no longer detectable on all surfaces tested.
Transmission through surfaces
These results raise the question of whether the potential for transmission of the coronavirus via surfaces was underestimated after all. Other studies carried out under different conditions show quite different results. For example, humidity or artificial sunlight can lead to shorter survival rates. But the data obtained in the study presented here are significantly higher than the scientists' previous assumptions. On the other hand, the studies also allow the conclusion that survival rates are many times higher at lower temperatures. According to calculations, the survival rate of the virus at a temperature of about 6° C would be more than 64 days. This could also explain the high infection rates in the meat industry. Another study published in bioRxiv in August 2020 confirms this assumption. The study showed that after 21 days, there was no decline in infectious virus in appropriately prepared chicken, salmon and pork cuts at 4°C (standard chilling) and -20°C (standard freezing).(2)
The studies show that coronavirus is detectable on non-porous surfaces for at least 28 days under appropriate conditions, but that survival drops dramatically to only 24 hours at 40°C as temperatures rise. In our temperate climate, this may well be significant for public areas, health care and the transport sector. Current hygiene concepts should therefore take these findings into account to minimise the risk of transmission of infectious agents during the current pandemic.
1) https://virologyj.biomedcentral.com/articles/10.1186/s12985-020-01418-7 (last accessed: 05.01.2021)
2) https://www.biorxiv.org/content/10.1101/2020.08.17.255166v1.full.pdf (last accessed: 05.01.2021)
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Published: 19. January 2021