Source: coronaviral.wordpress.com
(First published as a Long Story in Livemint. Expanded and linked to references).
It has been a little over three months since the novel coronavirus, also known as SARS-CoV-2, was identified as the cause of a new coronavirus disease, COVID-19. Since then SARS-CoV-2 has infected over a million people all across the world, and has caused tens of thousands of deaths. COVID-19 is a serious illness that is thought to be at least ten times as deadly as the seasonal influenza. SARS-CoV-2 is about three times as infectious as influenza. As many as 25% of people who are infected don’t show any symptoms of COVID-19. Unfortunately, even though these people get off easy, they can infect others who can get seriously ill. Even people who do suffer from full-blown COVID-19, can be infectious for a few days before they know they’re even sick.
Globally, there is panic over how SARS-CoV-2 is spread from the surfaces of objects. Many people have stopped subscribing to print newspapers. Others have stopped having meals delivered home from restaurants. People are washing currency notes. When lockdowns are eased, we will still have to deal with the virus. To protect ourselves, our families, and our communities, it is important to know how likely we are to get COVID-19 from surfaces.
A person can remain isolated during a lockdown and can wear masks and other protective gear while venturing outside. However, it is impossible to stop interacting with objects for an extended period of time. It is natural to wonder how likely it is to get infected from surfaces in public transportation and in buildings, and from everyday objects like newspapers, packages, groceries, and clothes. Here, I will cover what is currently known about survival and transmission of SARS-CoV-2 on surfaces.
The most common mode by which SARS-CoV-2 is thought to be spread is by direct human-to-human transmission of droplets loaded with the virus. An infected person releases a massive number of SARS-CoV-2 particles when he or she sneezes, coughs and sheds tears. These viruses can enter the body of a healthy person through the nose, mouth, and eyes, and cause an infection.
Another way that SARS-CoV-2 can be spread is by airborne transmission when an infected person speaks or breathes, but the number of viruses released this way is much lower than by droplets from coughing and sneezing.
SARS-CoV-2 has also been found in faeces of infected people, but currently it is not known if faecal contamination is a major way that others can get COVID-19.
Finally, an infected person can contaminate the surfaces of objects by sneezing, coughing, or touching them with infected hands. The virus lingers on these surfaces until a healthy person picks it and touches his or her face.
Let me first reassure you that the risk of getting COVID-19 from surfaces is probably not as high as you might think it is. There are a number of steps that have to happen in order for you to get infected by SARS-CoV-2 from a surface or object. First, someone who is infected has to handle the object or surface before you, and has to contaminate it with a high load of virus by coughing, sneezing or touching it.
Second, the virus has to remain viable on the surface in those specific conditions of light, humidity, and temperature. From the moment the surface is infected, the amount of virus on it decreases over time. This decay is rapid and exponential. A study found that half of SARS-CoV-2 particles applied to paper or cloth had decayed in thirty minutes.
Finally, you have to touch the infected surface and touch your nose, eyes, or mouth to let the virus enter your respiratory tract. It is important to remember that SARS-CoV-2 has no way of infecting you if it cannot get inside your body.
By the simple acts of cleaning surfaces of objects with disinfectants, washing hands with soap and water, and washing and cooking meats and vegetables before eating them, even if surfaces are infected with SARS-CoV-2, the risk of infection can be greatly reduced.
Transmission from surfaces is currently not thought to be the main way COVID-19 is spread. However, the exact extent by which surfaces contribute to the spread isn’t known, and there a reason for this uncertainty.
Right now researchers can’t observe the transmission of the virus from an object to a person while it happens. This would be possible if healthy people could be exposed to surfaces contaminated with different amounts of SARS-CoV-2, and researchers could track them getting sick in real time. In this way they could know more about the actual transmission process.
Indeed, in the case of a relatively minor disease like the common cold, that is what scientists did. At the Common Cold Research Unit in the United Kingdom, for over forty years, volunteers were deliberately infected with viruses that cause the common cold, so that researchers could find out just how it was spread. For a more serious disease like COVID-19, exposing healthy people to SARS-CoV-2 would not be an ethical practice, so a direct experiment to observe transmission is not currently possible.
Instead of infecting people, scientists will establish a direct link of transmission by infecting certain animals which can also suffer from COVID-19. In the past few weeks, scientists have worked out that rhesus macaque monkeys can be infected by SARS-CoV-2, and can display varying degrees of symptoms similar to those in humans who have COVID-19. By exposing rhesus macaque monkeys to various amounts and degrees of SARS-CoV-2 in laboratory settings, it will be possible to get a clearer picture of transmission of SARS-CoV-2.
Until the time that transmission can directly be established, we can look at how long the virus survives on various surfaces to get a rough estimate of its stability. In the last month, scientists around the world have grown SARS-CoV-2 in labs, spread it on surfaces, and take samples at intervals to see how long it takes to degrade below detectable levels. A broad conclusion that seems to hold currently is that SARS-CoV-2 is more likely to persist for a longer period of time on a hard surface like a door-knob or an elevator button, than it is on a soft one like clothing. But there are also specific details that are now known.
One of the first studies to look at the surface stability of SARS-CoV-2 was published in The New England Journal of Medicine. In this study, scientists tested the survival of SARS-CoV-2 on surfaces, and compared it to the stability of the original SARS. Since both are structurally similar coronaviruses, as expected, scientists found that both had similar stability too.
SARS-CoV-2 could be detected for up to four hours on copper, up to a day on cardboard, and up to two to three days on plastic and stainless steel. Scientists also created an aerosol, a fine mist of SARS-CoV-2 in the lab, and were able to detect it in a small contained environment up to three hours later. However, in this study they didn’t test survival of the virus by direct droplet from coughing or sneezing, which is probably the most common way it is spread.
A second study which has not been peer-reviewed by other scientists yet found that SARS-CoV-2 couldn’t be detected on paper after three hours, on cloth after two days, and on stainless steel after seven days. The relatively short life of the virus on paper should be a matter of some relief to those worried about reading paper newspapers.
The survival of SARS-CoV-2 for reasonably long periods of time was quoted extensively by the media, but was not put into suitable context. In each case, even though the virus could be detected hours later, the amount left was very low. SARS-CoV-2 began to degrade just as soon as it had been applied to each surface. Even on stainless steel, a hard surface on which SARS-CoV-2 is stable, half of all virus particles has decayed in six hours.
In addition, laboratory conditions are not like those in the real-world. These experiments are usually conducted at 21℃ and 65% humidity in closed chambers. In the real-world an increase in humidity, light exposure, air flow from a breeze, and temperatures might result in less virus sticking around.
Will the transmission of SARS-CoV-2 go down in the hot, humid months of summer? This question has been the subject of immense speculation. A number of other respiratory viruses do decline in transmission during the summer months in the Northern Hemisphere, a phenomenon known as “seasonality”. We don’t definitely know if that will be the case with SARS-CoV-2. We will have an answer to this burning question in the next few months.
What we do already know from a research study is that there is an impact of temperature on the stability of SARS-CoV-2 on surfaces. In a study submitted as a preprint (which hasn’t been reviewed by other scientists yet) the stability of SARS-CoV-2 was found to decrease as temperatures got higher. At 4℃, the virus degraded a little bit after 14 days, but was reasonably stable. At 22℃, the virus degraded greatly after seven days and couldn’t be detected after two weeks. At 37℃, almost all of the virus degraded after one day.
Ramping temperatures up speeds up the process of degradation. At 56℃, 30 minutes was enough to get rid of most of the virus, while at 70℃, it only took 5 minutes. This is preliminary work that needs to be verified, but it is in line with what has been seen with other similar viruses. SARS-CoV-2 seems to be heat-sensitive, and this has great practical implications for disinfection. For example, by washing clothes with detergent in warm water, even if clothes are infected, we can expect to get rid of most of the virus.
Apart from deliberately dosing surfaces with a heavy load of a virus, there’s another way to see how long it can survive. Scientists can study the natural environment after an outbreak much like a forensic detective. They can go to places where there are a lot of infected people such as inside hospital wards and on cruise ships, and see how much of SARS-CoV-2 is left on various surfaces. The advantage of this approach is that it happens outside the lab in the real-world, so the conditions are likely what we might encounter in day to day conditions.
On 23 March 2020, 601 samples were collected from the infamous Diamond Princess cruise-ship, where there was a major outbreak. Around 10% of these samples tested positive for viral RNA, a molecular signature of the virus days after cabins had been vacated, but before proper disinfection.
In another study, air and surface samples from 11 hospital rooms of the University of Nebraska Medical Center in the US where SARS-CoV-2 infected patients stayed were tested. Around 80% of personal items and room surfaces tested positive for the coronavirus.
Although it isn’t clear how many people on board the ship or in the hospital were infected by these contaminated surfaces, it does indicate that surface cleaning is necessary to remove the virus. Both studies found viruses on surfaces indoors and there’s a great deal of work on other similar viruses that indicate that airflow decreases the concentration. In addition, bright light reduces the stability of viruses.
One detail to remember about these research studies is that often they don’t actually check for active SARS-CoV-2 capable of infection. They chemically treat samples to try to detect pieces of the virus, their RNA signatures, and there might not actually be a direct correlation to infection.
When we tie all of these studies together, we see that there are still huge gaps in our knowledge. We still need to establish a direct link between various surfaces infected with viruses and actual infections.
Another crucial point is the viral load, or how much virus is in the body. One reason we are seeing doctors and nurses get infected is because they are in environments where there are a lot of COVID-19 patients with high viral loads. We don’t yet know details of just how much SARS-CoV-2 needs to enter the body to cause an infection, but surfaces will likely have fewer virus particles than droplets caused by coughing or sneezing. That’s why direct human to human transmission by droplets is thought to be the main way SARS-CoV-2 is spread.
But that doesn’t mean we should be complacent either. The scientific data gives us a good yardstick to compare the stability of SARS-CoV-2 to that of other viruses. For SARS, the World Health Organization had recommended cleaning surfaces that were frequently touched by many people, such as door handles every two hours. SARS-CoV-2 has similar stability, so this could be a good approach. Application of heat, wherever feasible, does seem to degrade the virus as well. But the simplest approach may be what we should be doing anyway- cleaning objects with soap and water or disinfectant wipes.