Low temperature refrigeration, a shaker for coronavirus

Recently, the epidemic in Beijing, which was close to zero COVID-19, has become serious again. The news that the imported salmon cutting board was tested positive for COVID-19 has turned salmon from a mouth-watering delicacy into a possible source of contamination that people are afraid to avoid. Supermarkets and restaurants in many parts of the country removed salmon from their shelves overnight, and many consumers were worried about this. In the hot summer, how do the temperature and humidity of refrigerated salmon affect the survival and spread of the COVID-19 outside the body?

Written by | Shi Jun

On March 17, 2020, The New England Journal of Medicine published a correspondence from American scientists, which studied the survival time of the new coronavirus (SARS-CoV-2) in aerosols and on various environmental surfaces, and compared it with the SARS coronavirus (SARS-CoV-1) [1]. The Bayesian regression model was also used to estimate the decay rate of viral activity.

The data in this article include the survival time of the two viruses in five environments with a temperature of 21°C to 23°C and a relative humidity of 40%. The five environments are: aerosols with a diameter of less than 5 microns, plastic surfaces, stainless steel surfaces, copper surfaces, and cardboard surfaces.

The titer of the virus directly determines the virus's ability to infect and how easily the virus loses its activity. The higher the concentration, the easier it is to cause infection and the harder it is to inactivate. If the nucleic acid test method is used for quantification, the starting titer of the virus selected in this article is similar to that observed in the patient's respiratory tract.

The results are summarized in one picture:

Figure 1: Stability of SARS-CoV-1 (SARS coronavirus) and SARS-CoV-2 (new coronavirus) in aerosols and on various environmental surfaces (from reference [1]). (Click to see larger image)

To sum up:

In aerosols, the observations for the new coronavirus were similar to those for the SARS virus: infectious viruses were detected throughout the entire experimental process (3 hours), and the infectious titer was slightly reduced (by about 1log10).

The stability ranking of the new coronavirus on different environmental surfaces is: plastic > stainless steel > cardboard > copper surface.

On plastic surfaces, infectious coronavirus can exist for 72 hours. On stainless steel surfaces, it can exist for 48 hours. However, the titer of infectious virus is significantly reduced.

On copper, no infectious coronavirus was detected after 4 hours. On cardboard, no infectious coronavirus was detected after 24 hours.

A more intuitive diagram is shown below:

Figure 2: Survival time of the new coronavirus (SARS-CoV-2) on environmental surfaces (modified original image)
https://www.medscape.com/viewarticle/926929).

Another new study, using a higher starting titer of the virus, showed similar results [2], at room temperature (22°C) and a relative humidity of approximately 65%:

There is currently no direct data on the impact of temperature and humidity on the survival of the new coronavirus in vitro, but we can learn about the situation of other coronaviruses.

One study provides a very detailed analysis [3].

The original purpose of this study was to look at the survival period of the SARS coronavirus on the surface of objects, but due to the high infectivity of SARS, two animal coronaviruses belonging to different categories were used instead. The experiment measured the survival time of the two viruses on hard, non-porous stainless steel surfaces under different temperature and humidity combinations.

The starting amount of the virus in this experiment is 10^4-10^5 (MPN, most probable number) infectious virus particles. In real life, the starting amount of the virus can be high or low. The higher the starting amount, the more difficult it is to completely inactivate the virus.

Remark

Virus titer is often seen in the literature. Virus titer refers to the amount of virus in a certain volume. Depending on the type of test, there can be two values:

One is the total number of virus particles in a certain volume (including infectious and non-infectious viruses). For example, the RT-PCR technology used in the new coronavirus test kit detects the nucleic acid content in the virus. These virus particles are not necessarily all infectious. Some may have been "dead".

The other is the number of infectious virus particles in a certain volume. To quantify the infectious virus, the virus and host cells are usually placed together and then the number of cells that become diseased is calculated.

The units used to express virus titers vary depending on the type of test used in the experiment, such as PFU/ml, MPN/ml, etc. For ease of understanding, this article will directly use (number of viruses) to replace these professional terms.

The conclusion of the study can be summarized in one sentence: The survival time of the virus is related to humidity and temperature (see Figure 3 below). Note that the indoor temperature is generally 20°C and the humidity is around 40-50%:

At 4°C and 20% relative humidity, both viruses can survive for up to 28 days. The higher the relative humidity, the faster the virus is inactivated.

At 20°C, inactivation of both viruses was accelerated. Inactivation was fastest at 50% relative humidity, rather than 80%. Infectious virus was detectable after 3-28 days.

At 40°C, the virus is inactivated more quickly. The higher the humidity, the faster the inactivation.

According to previous studies, SARS patients’ nasopharyngeal aspirates contain 10^5 - 10^8 viruses per milliliter (quantified using genomic templates, so it cannot be guaranteed that all of them are infectious) [4-6]. Assuming that most of the virus particles in the nasopharyngeal aspirates are infectious, if these aspirates fall on a stainless steel surface in an air-conditioned indoor environment (about 20°C, 50% relative humidity), using the data from this article, after five days, there will still be 1/1000 of the initial amount of infectious virus particles. In other words, in this environment, at least about 100 to 100,000 virus particles in 1 ml of nasopharyngeal aspirate will still be infectious.

Do the virus concentrations have to reach a certain level for them to “cooperate” with each other to cause infection? Can a single virus particle cause infection or disease?

A 2009 study using insect viruses demonstrated that a single virus particle could cause infection.[7] The lowest dose of the new coronavirus that can infect humans has not yet been determined.

Figure 3: Survival of two animal coronaviruses on stainless steel surfaces at different relative humidity and temperature. Each curve represents a virus (modified from the original image in reference [3]). (Click to see the larger image)

Another article directly looked at the SARS coronavirus [8], and the conclusion was very similar to the previous one:

The researchers dripped the SARS coronavirus (active virus titer of 10^7/ml, which is similar to the total virus titer in the nasopharyngeal aspirate of SARS-infected people) onto plastic (a non-porous hard surface). At a temperature of 22-25°C and a relative humidity of 40-50%, after 5 days, the dried virus on the plastic still had an infectivity of 10^6/ml, only one log10 less in titer. After 2 weeks, there was still a lot of infectious virus. After 4 weeks, infectious virus could still be detected.

They also found that the higher the temperature and humidity, the faster the virus was inactivated.

Figure 4: Inactivation rate of SARS coronavirus under different environmental conditions in vitro (from reference [3] Figure 1).

The red curve in the figure above is the inactivation curve of SARS coronavirus in liquid. It can be seen that SARS coronavirus can survive in a liquid environment at room temperature for at least 3 weeks. However, it is easily killed by heating to 56°C for 15 minutes[9]. For coronaviruses in liquid, the higher the temperature, the faster they are inactivated[10, 11].

Two more points about the coronavirus:

Effect of pH on coronaviruses: Most coronaviruses are more stable in weakly acidic conditions (pH = 6~6.5) than in alkaline conditions (pH = 8) [12-16].

The presence of the new coronavirus can be detected in feces. According to research on the SARS virus, the SARS virus cannot survive for more than 24 hours in normal feces of adults, and in the feces of newborns (acidic pH), it cannot survive for more than 3 hours. However, it can survive for a long time in diarrheal feces, which may have a pH of 9, up to 4 days[17]. At the same time, a new study found that 48.5% of COVID-19 patients had digestive system symptoms such as diarrhea, vomiting, and abdominal pain[18]. Therefore, for patients who may have COVID-19, their diarrheal feces must be handled promptly and carefully.

Although there is no conclusion yet on the exact virus concentration that will cause infection, these results suggest that the new coronavirus is likely to be transmitted through aerosols (a low-probability event under high virus concentration conditions) and contact.

How efficient is the transfer of the new coronavirus from contaminated environmental surfaces to hands? No data has been seen yet. However, studies on influenza A virus have shown that as long as you touch a contaminated environmental surface for 5 seconds, 31.6% of the viral load will be transferred to your hands[19].

Remind everyone again: hand hygiene is very important!

References

[1] N. van Dormalen et al., Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. New England Journal of Medicine, (2020).

[2] AWH Chin et al., Stability of SARS-CoV-2 in different environmental conditions. The Lancet Microbe.

[3] LM Casanova, S. Jeon, WA Rutala, DJ Weber, MD Sobsey, Effects of air temperature and relative humidity on coronavirus survival on surfaces. Appl Environ Microbiol 76, 2712-2717 (2010).

[4] C.-M. Chu et al., Viral load distribution in SARS outbreak. Emerg Infect Dis 11, 1882-1886 (2005).

[5] IFN Hung et al., Viral loads in clinical specimens and SARS manifestations. Emerg Infect Dis 10, 1550-1557 (2004).

[6] SCC Wong, JKC Chan, KC Lee, ESF Lo, DNC Tsang, Development of a quantitative assay for SARS coronavirus and correlation of GAPDH mRNA with SARS coronavirus in clinical specimens. J Clin Pathol 58, 276-280 (2005).

[7] MP Zwart et al., An experimental test of the independent action hypothesis in virus–insect pathosystems. Proceedings of the Royal Society B: Biological Sciences 276, 2233-2242 (2009).

[8] KH Chan et al., The Effects of Temperature and Relative Humidity on the Viability of the SARS Coronavirus. Adv Virol 2011, 734690 (2011).

[9] L. Casanova, WA Rutala, DJ Weber, MD Sobsey, Survival of surrogate coronaviruses in water. Water Research 43, 1893-1898 (2009).

[10] BJ Tennant, RM Gaskell, CJ Gaskell, Studies on the survival of canine coronavirus under different environmental conditions. Veterinary Microbiology 42, 255-259 (1994).

[11] GJ Harper, Airborne micro-organisms: survival tests with four viruses. J Hyg (Lond) 59, 479-486 (1961).

[12] A. Lamarre, PJ Talbot, Effect of pH and temperature on the infectivity of human coronavirus 229E. Canadian Journal of Microbiology 35, 972-974 (1989).

[13] BD Zelus, JH Schickli, DM Blau, SR Weiss, KV Holmes, Conformational Changes in the Spike Glycoprotein of Murine Coronavirus Are Induced at 37°C either by Soluble Murine CEACAM1 Receptors or by pH 8. Journal of Virology 77, 830-840 (2003).

[14] C. Daniel, PJ Talbot, Physico-chemical properties of murine hepatitis virus, strain A 59. Brief report. Arch Virol 96, 241-248 (1987).

[15] DH Pocock, DJ Garwes, The influence of pH on the growth and stability of transmissible gastroenteritis virus in vitro. Arch Virol 49, 239-247 (1975).

[16] A. Pratelli, Canine coronavirus inactivation with physical and chemical agents. The Veterinary Journal 177, 71-79 (2008).

[17] MYY Lai, PKC Cheng, WWL Lim, Survival of Severe Acute Respiratory Syndrome Coronavirus. Clinical Infectious Diseases 41, e67-e71 (2005).

[18] MM Lei Pan, Pengcheng Yang, Yu Sun, Runsheng Wang, Junhong Yan, Pibao Li, Baoguang Hu, Jing Wang, Chao Hu, Yuan Jin, Xun Niu, Rongyu Ping, Yingzhen Du, Tianzhi Li, Guogang Xu, Qinyong Hu, Lei Tu, Clinical characteristics of COVID-19 patients with digestive symptoms in Hubei, China: a descriptive, cross-sectional, multicenter study. The American Journal of Gastroenterology, (2020).

[19] B. Bean et al., Survival of Influenza Viruses on Environmental Surfaces. The Journal of Infectious Diseases 146, 47-51 (1982).

Shi Jun, pen name "Cat with Free Will", currently lives in Boston, USA. He graduated from the Department of Biological Sciences and Technology of Tsinghua University with a bachelor's degree. After obtaining a doctorate in the United States, he joined a well-known multinational pharmaceutical company to engage in drug research and development. For more than ten years, he led the team to fight against diabetes, muscular dystrophy, etc. In recent years, he has focused on the research and development of anti-aging drugs. His personal WeChat public account "Yi Ran Sui Xin" will chat with you about medical care.