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Effect of Environmental Conditions on SARS-CoV-2 Stability in Human Nasal Mucus and Sputum

Our take —

This study assessed the degradation of SARS-CoV-2 as both a liquid and on a plastic surface across 3 temperature/humidity combinations. They found that at the highest temperature/humidity, 27C/85%, there was a significantly shorter half-life in the virus on surfaces compared to at the either lower temperature/humidity condition. There was also no statistical difference between temperature/humidity conditions when the virus was in a liquid setting. This study did not use specimens sampled from actual participants, but rather created their own samples for the purpose of this experiment. They also assessed temperature/humidity in a lab setting that may be different from what might occur outdoors and in the real world. However, this work has implications for potential transmission, and suggests that the virus is less stable at higher temperature/humidity levels.

Study design

Other

Study population and setting

The researchers used commercial human nasal mucus and sputum, and combined it with SARS-CoV-2 to create a 50% infective dose per mL, which was placed in sealed tubes to simulate a liquid setting or on a plastic disk to simulate a surface setting. The liquid and surface settings were them exposed to 3 conditions with different temperatures and humidity levels: 4 degrees Celsius (39 degrees Fahrenheit) and 40% humidity, 21 degrees Celsius (70 degrees Fahrenheit) and 40% humidity, and 27 degrees Celsius (81 degrees Fahrenheit) and 85% humidity. Samples were then collected between 1 to 72 hours afterwards and assessed for stability defined as half-life of viral titer level and viral RNA level.

Summary of Main Findings

There was no statistically significant difference between the effects of environmental conditions on liquid nasal mucus in the sealed tubes, while the half-life of the virus on plastic surface nasal mucus was much shorter at 27C/85% humidity, compared to lower temperature/humidity levels. Plastic surface samples had half-lives at 27C/85% humidity significantly shorter than the half-life in liquid form. SARS-CoV-2 RNA could be detected for more than 7 days on all surface samples.

Study Strengths

The main study strength is that it tested both liquid and surface settings for SARS-CoV-2 across different temperature and humidity conditions. They also had numerous follow-up points, which allowed for greater granularity in assessing when, in particular, the half-life occurred.

Limitations

This study was conducted in an experimental setting, which may not reflect real-life conditions, such as a different type of surface than polypropylene or different levels of virus within a given sample. The study also used specific temperature-humidity combinations, rather than testing every combination of their 3 temperature and 3 humidity levels, therefore it is not possible to tell if the temperature or the humidity had the greatest effect on degradation. Also, it is not clear whether, after degradation, to what degree infection could still occur. For instance, while RNA was detectable for many days afterward, it is not known whether these particles could result in later infection.

Value added

This study shows that at their highest level of temperature/humidity tested, SARS-CoV-2 degraded significantly more quickly than at lower levels, which has important implications for transmissibility in different settings and seasons.

This review was posted on: 18 July 2020