Widely reported in the media on Monday this past week were the results from an Australian study which showed that in the laboratory the Covid-19 virus can survive for 28 days on a wide range of surfaces from bank notes to mobile phone screens, in an amount considered sufficient to cause infection. How worried should we be and what is the best estimate about the role of surfaces in increasing infection risk?
What was known about the risk of surfaces before Covid-19?
- For over 40 years it has been known that the flu virus can survive on non-porous surfaces such as stainless steel for perhaps 24-48 hours Many other viruses have since been shown to survive on a variety of surfaces and be a potential source of the transmission of infection
- More relevant is that for most corona viruses that have been studied, including the ones responsible for the SARS pandemic in 2002-04 and the MERS epidemic in 2012, it has been shown in the laboratory that they could survive for more than a few hours on a variety of surfaces
- The early advice about surfaces with Covid-19 was based on that laboratory evidence from other viruses
- Thus, it is reasonable to ask specifically about Covid-19: “can it survive on surfaces and, if so:
- which surfaces?
- for how long?”
What had we learnt in the first 6 months?
- Given the findings from other corona viruses, from the start of the current pandemic there was a considerable amount of research on the survival of Covid-19 on different surfaces.
- The studies are tricky undertake and there are lots of different ways to do such research
- Perhaps not surprisingly, the results varied but there was broad conclusion that this virus can survive on a wide range of surfaces for a reasonable amount of time after say a sufferer has coughed
- In general, the virus survived much shorter times on porous surfaces such as clothing than shiny surfaces
- Somewhat surprisingly the virus also survived for a few days on non-shiny surfaces such as wood, paper and cardboard
- Given results such as these, many public health authorities recommended stringent approaches to cleaning
- Businesses and members of the public then to varying extents adopted mitigation practices which ranged from stringent wiping of surfaces in health care settings to some households removing the front pages of a newspaper prior to reading.
Are there limitations to the conclusions that can be drawn from such studies?
- The studies have to be undertaken in laboratory conditions, but these do not necessarily replicate daily life
- Temperature, humidity, UV light all can alter virus survival
- The nature of the sample of virus spread on these surfaces was not the same that would be spread from an infected individual, for example from a patient coughing
- The fact that the virus could be cultured did not necessarily mean there was sufficient virus on the surface to be harmful to health
- Furthermore, taking a sample ‘scrape’ from the surface is very different for example from what a person can pick up by touching the surface.
- Thus, my conclusion was that the virus can survive on surfaces but that was not proof that in the form it survives it is capable to causing infection
What does this new Australian study add?
- I thought the study was impressive!
- The research ‘infected’ the surfaces with the concentration and form of the virus that would be typical of (say) that infected patient cough
- They allowed the liquid preparation of the virus to dry
- They studied a large range of materials (see table below). The items listed are those directly studied or referred to as relevant in the research report
|Polymer bank note|
|Paper bank note|
|100% pure cotton|
|Brushed stainless steel|
- They studied the virus survival at ‘real life’ humidity and temperatures
- They measured how much of the virus they could grow from 1-28 days
- This is what they found:
- On some surfaces such as cotton there was an immediate absorption of virus
- Even on cotton virus could still be grown after 14 days
- On all the other surfaces virus could still be grown 28 days later
- The rate that the virus disappeared was calculated by how many days would be needed to lose (i) 50% and (ii)90% of the dose that was present after the initial application.
- The results (which I have drawn below) show that for all the materials half of the virus that had been applied was still present at 2 days
- Further although 90% had been lost by a week, that meant there was still 10% of the original amount of virus left, which was sufficient to cause infection
- The authors argued, based on their findings, that common objects such as mobile telephones, ATM machines and even modern and old bank notes could still be a risk for transmission after gaps of several days
- One major criticism of the study is that all objects were kept in the dark, given that UV light exposure would have reduced the amount of the virus (although from a research perspective it made sense to control that aspect)
Should these findings change behaviour?
- Studies such as this are undertaken by virologists (indeed the results appeared in a publication with the not very surprising title Virology Journal!)
- Virologists in research of this type are addressing questions on “can the virus survive on surfaces?” and even (as in this article) “following a typical exposure to an infected person, can the virus survive for so many days that it could be an important source of transmission?”
- An epidemiologist asks as different set of questions: we know that surfaces can be a source of transmission for these periods but how likely is this to happen in the ‘real world’?
- As examples there are several potential health hazards that can happen following common health related activities (with no prior history)
- Haemorrhage after tooth extraction
- Severe allergic reaction after paracetamol (USA acetaminophen)
- Similar reactions after certain foods
- And in the non-health sector:
- Tyre blow-out whilst travelling at speed
- Domestic kitchen appliance catching fire
- These are all rare and we take the view that the risks are acceptably low and we don’t change our behaviour
Do we know how likely is it that transmission can occur from surfaces?
- The short answer is no!
- It is very challenging, outside a local outbreak with a single source, to know what was the route of transmission for any individual Covid-19 infected patient
- There are anecdotal reports in the media of a person who contracted the infection after touching a door handle or whatever, but the existence of one case does not allow us to calculate the numerical level of risk
- Logically it is impossible to test all the surfaces that such an individual may have been in contact with over the previous 10 days
- It was ‘easy’ with Novichok and the Salisbury poisoning, but that entailed closing down a whole town and a military style exercise!
- For sure transmission in hospitals in the early phase of the pandemic could be traced to high concentrations of the virus on surfaces, exhaled from patients and probably staff
- Outside health care settings the likelihood of transmission from surfaces, I would conclude, is lower, probably substantially lower, than most other risks I would accept – including travel on a flight
- The good news is that infection cannot occur by absorption through the skin, so if I am worried from any surface, I just wash my hands or use hand gel and I’m happy reading the front page of news
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