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The coronavirus and surface tension

Srinagar Municipal Corporation employees spray disinfectant in Dal Gate area.  (Photo: Waseem Andrabi/Hindustan Times)Premium
Srinagar Municipal Corporation employees spray disinfectant in Dal Gate area. (Photo: Waseem Andrabi/Hindustan Times)

  • Can you touch home-delivered packages and newspapers without fear of infection? Here’s what science says
  • Viral load is crucial. Presence of a few viral particles is not enough to cause infection, which is why the possibility of catching the disease from a home-delivered package is extremely low

WASHINGTON D.C. : 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 disease. The new virus is about three times as infectious as influenza. More importantly, as many as 25% of the people who are infected don’t seem to show any symptoms. Unfortunately, even though these people get off easy, they can infect others who can get ill. Some people can be infectious for a few days before they even know they’re sick.

Naturally, there has been a lot of panic over how the new disease spreads from the surfaces of various objects. Many people have stopped subscribing to print newspapers. Others have stopped having meals delivered home by restaurants. People are even washing currency notes.

When the lockdown is 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.

After all, 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. Science can answer many of these questions to a reasonable extent.

How it spreads

The most common mode by which the new virus is thought to 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.

Another mechanism of spread is by airborne transmission when an infected person speaks or breathes, but the number of viruses released this way is much lower than through droplets produced by coughing and sneezing. SARS-CoV-2 has also been found in the faeces of infected people, but currently it is not known if faecal contamination is a major mechanism of transmission.

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 up 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.

First, someone who is infected has to handle the object or surface just 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 specific conditions of light, humidity and temperature. From the moment the surface is infected, the amount of virus on it starts decreasing. This decay is rapid and exponential. A recent study found that half of the virus particles applied to paper or cloth had decayed in just 30 minutes.

Finally, you have to touch the infected surface and then touch your nose, eyes or mouth for the virus to 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 act of cleaning the surfaces of objects with disinfectants, washing hands with soap and water, and washing and cooking meat and vegetables before eating them, the risk of infection can be greatly reduced even if surfaces may have trace amounts of virus particles.

In any case, transmission from surfaces is currently not thought to be the main way covid-19 is spread. However, the exact extent to which surfaces contribute to the spread isn’t known, and there is reason for uncertainty.

As of now, researchers can’t observe the transmission of the virus from an object to a person while it happens. This would have been 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.

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 UK, for over 40 years, volunteers were deliberately infected with viruses, so that researchers could find out how it spread. For a more serious disease like covid-19, exposing healthy people would not be an ethical practice, so a direct experiment to observe transmission is not currently possible.

Instead of infecting people, scientists are planning to 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 the novel virus, and can display varying degrees of symptoms similar to humans. By exposing rhesus macaque monkeys to various amounts of SARS-CoV-2 in laboratory settings, it will be possible to get a clearer picture of transmission.

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 taken samples at intervals to see how long it takes to degrade below detectable levels. A broad conclusion that seems to hold currently is that the virus is more likely to persist for a longer period of time on a hard surface like a doorknob or an elevator button, than on a soft one like clothing. But there are also specific details that are now known.

Viral decay is rapid

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 virus. 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 the survival of the virus from direct droplets created by coughing or sneezing, which is probably the most common way it spreads.

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 relief to those worried about reading 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 was very low. The virus begins to degrade as soon as it settles on any surface. Even on stainless steel, a hard surface on which SARS-CoV-2 is stable, half of all virus particles decay in just six hours.

Since a critical amount of virus particles is required to cause infection, the clock starts ticking as soon as the virus settles on any surface. It is this crucial aspect of viral load which is missing in the popular understanding of surface transmission. Presence of a few viral particles is not enough to cause an infection, which is why the possibility of catching the disease from a home-delivered package is extremely low.

In addition, laboratory conditions are not like those in the real-world. Scientific experiments are usually conducted at 21 degrees Celsius (°C) and 65% humidity in closed chambers. In the real-world, an increase in humidity, light exposure, airflow from a breeze, and temperatures might result in less virus sticking around.

The summer puzzle

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 yet.

What we do 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 the virus was found to decrease as temperatures got higher. At 4°C, the virus degraded a little bit after 14 days, but was reasonably stable. At 22°C, the virus degraded greatly after seven days and couldn’t be detected after two weeks. At 37°C, almost all of the virus degraded after one day.

Ramping temperatures up speeds up the process of degradation. At 56°C, 30 minutes was all it took to get rid of most of the virus, while at 70°C, 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 hospital wards and 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 due to these contaminated surfaces, it does indicate that surface cleaning is necessary to remove the virus. Both studies found viruses on surfaces and there’s a great deal of work on other similar viruses that indicate that airflow decreases viruses’s 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. But the crucial point for disease transmission is the viral load, or how many stable virus particles enter 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 and high viral loads. We don’t yet know details of just how many SARS-CoV-2 particles need to enter the body to cause an infection, but surfaces will likely have much lower viral loads 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. 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 maybe what we should be doing anyway—cleaning objects with soap and water or disinfectant wipes.

Anirban Mahapatra is a microbiologist and assistant director at the American Chemical Society. These are his personal views

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