The study has generated lots of questions from our community. We sat down with some of our lead researchers to bring you the answers to your burning questions.
Answering your questions about SARS-CoV-2 on surfaces
Why did you want to study virus survival on surfaces?
We already know the main way the SARS-CoV-2 virus is transmitted is via respiratory droplets and aerosols. However, it is still important to understand other ways the virus may be spread.
Studies on other viruses, like influenza, have shown that high-contact surfaces can be contaminated. And that these contaminated surfaces can play a role in virus transmission.
This is why it is important to understand how long SARS-CoV-2 can remain viable (infectious) on a surface at different temperatures. Knowing more about how the virus behaves helps us to evaluate the risks of surface transmission. It helps inform mitigation strategies (such as cleaning) and should be considered when there are cases with unknown sources of infection.
What was the aim of this study?
The aim of the study was to look at how temperature affects SARS-CoV-2 on different surfaces, when dried down in an artificial mucous.
The surfaces chosen for the study are examples of high-contact surfaces. Mobile touchscreens, public transport furnishings, food preparation areas and bed linen are all examples of these common surfaces.
What did you test?
Our study was testing how long SARS-CoV-2 remains viable at different temperatures (20°C, 30°C, 40°C). This was done with controlled humidity (50 per cent relative humidity), for a range of common surfaces. These include: Australian polymer banknotes, de-monetised paper banknotes, brushed stainless steel, glass, vinyl, and cotton cloth.
Why did you conduct the experiment in the dark?
The purpose of this study was to measure the survival of SARS-CoV-2 at different temperatures, on different surfaces, in the mucous-type matrix present in people's lungs and upper respiratory tracts. To ensure our study specifically looked at how temperatures affected the virus' survival, it was important to reduce the number of variables, which might have inactivated the virus. Other studies have indicated sunlight can rapidly inactivate it, so we conducted the experiment in the dark.
Is the amount of virus used in testing similar to what we would come across in real life?
In this experiment, the viral load in the artificial mucous solution was similar to levels observed in infected patients. It was reflective of mucous at the peak of their infection and before they started to mount an immune response.
In people with healthy immune systems, different molecules and cells release into mucous and can rapidly destroy viruses. The impact of these elements on virus survival is highly variable. This depends on the individual's immune competence, the phase of the infection, temperature and humidity, and the surface on which the contaminated mucous exists. In our study the viral load therefore represented the 'worst case scenario' of contamination by someone newly infected.
The time it takes for viruses to be inactivated in the environment depends on many factors such as the type of surface it is on and whether the virus is liquid or dried. Environmental conditions such as temperature, exposure to sunlight and humidity also play a part in virus survival.
Why didn't you use human mucous in testing?
Human mucous from infected individuals would be a great substance to use in testing. However, with scientific study there is a need to make the data reproducible. Human mucous contains many different components and varies in makeup. Not only does it vary from person to person but also over time from the same person. This leaves researchers with too many 'uncontrolled variables'.
The standard artificial mucous we used is described in an international standard for testing how well disinfectants work against viruses. The formulation is the best representation it can be while also being reproducible. It has also been used for inactivation studies of other viruses in the past.
If this study is true why haven't more people caught COVID-19?
While researchers did find that some infectious virus was still present at 28 days, the detection system they used was very sensitive and the amount of virus remaining at that point would be unlikely to be enough to infect a person.
In real life, exposure to UV light, variable humidity and changing temperatures all affect how long a virus can survive in the environment. Many things would have to line up for this pathway of infection to occur. But while respiratory droplets and aerosols are the main route of transmission, transmission via surfaces can also occur. This study shows that the virus may last on some surfaces, in some conditions, for longer than previously understood.
The results of this study are different from other studies. Why is that?
When considering other studies of SARS-CoV-2 inactivation, the most common difference is the liquid the virus is suspended in prior to being dried down. In our study, this seems to be contributing significantly to the stability. There is not an agreed standard for this sort of study and there are many variables to be considered.
Artificial mucous has also been used for studies of other virus, such as influenza, where it was shown to be detectable for up to 17 days on banknotes. An understanding of the impact of these different variables will be important for contributing to our ongoing knowledge about the characteristics of this virus and its survival in different conditions.
Should we stop using cash?
The virus is certainly stable on polymer notes in the right conditions. For transmission to occur, a note would need to have significant virus present and for that contamination to be transferred to your nose or mouth. We suggest after using cash you wash your hands with soap and water or sanitise with an alcohol-based hand rub (with at least 70 per cent ethanol). Avoid touching your face, especially your eyes, mouth and nose, until you have done this.
How much virus has to be on a surface to catch it?
That's something researchers are still investigating. We really don't know exactly what an 'infectious dose' of SARS-CoV-2 is yet. The amount of 'contamination' that is transferred to a surface from an infected person and then from a surface to a susceptible person is uncertain.
Could the virus survive on packaging arriving from other countries?
Even though this virus can survive many hours or up to many days (depending on the surface type and the temperature), it is very unlikely the virus will persist on a surface after exposure to different conditions and temperatures.
However, the storage of packages in consistently cold and dark environments may pose a greater risk.
Does this mean the warmer weather will help reduce the spread of the virus?
While we have demonstrated that temperature reduces the time that infectious virus survives on various surfaces, we don't know yet if warmer weather and Australia's coming summer will have an impact on the spread of COVID-19.
Some viruses, including the common cold and flu, spread more during cold weather months. There may be several reasons for this, such as people being indoors more with less ventilation and in closer proximity to others.
How do we kill/inactivate this virus?
We know heat and humidity are factors as well as UV light. Our research found that as the temperature increased, the length of time that infectious material remained on the surface decreased.
However, one of the best ways to protect yourself is to avoid touching your eyes, nose and mouth and keep washing your hands. It's important to be careful when removing facemasks as the virus can survive on the outside where you could transfer it to your hands. And we should make sure to clean surfaces regularly.
What can we do with this information to stay safe?
This study emphasises the ongoing importance of washing your hands and avoiding touching your face. Be alert to the potential for surfaces to be potentially contaminated. You should also avoid touching common surfaces whenever possible and don’t put your fingers in your mouth, nose or rub your eyes after doing so.
Further research about SARS-CoV-2 on surfaces
Looking to find out more? We suggest exploring these studies:
- Hirose, R., Ikegaya, H., Naito, Y., Watanabe, N., Yoshida, T., Bandou, R., Daidoji, T., Itoh, Y., Nakaya, T., 2020. Survival of SARS-CoV-2 and influenza virus on the human skin: Importance of hand hygiene in COVID-19. Clin Infect Dis.
- Hirose, R., Nakaya, T., Naito, Y., Daidoji, T., Watanabe, Y., Yasuda, H., Konishi, H., Itoh, Y., 2017. Mechanism of Human Influenza Virus RNA Persistence and Virion Survival in Feces: Mucus Protects Virions From Acid and Digestive Juices. J Infect Dis 216, 105-109.
- Otter, J.A., Donskey, C., Yezli, S., Douthwaite, S., Goldenberg, S.D., Weber, D.J., 2016. Transmission of SARS and MERS coronaviruses and influenza virus in healthcare settings: the possible role of dry surface contamination. J Hosp Infect 92, 235-250.
- Thomas, Y., Vogel, G., Wunderli, W., Suter, P., Witschi, M., Koch, D., Tapparel, C., Kaiser, L., 2008. Survival of influenza virus on banknotes. Appl Environ Microbiol 74, 3002-3007.