COVID-19 SPECIAL

Presence of virus on surfaces may not mean it is infectious: Scientists

Learn why washing your hands with soap can prevent the spread of viruses.
Cleaner Ling Li, 70, cleaning a playground in Tampines last month. Disinfectants and alcohol-based sanitiser break the lipid membrane encasing a coronavirus' genetic material, rendering the virus impotent and incapable of infecting its host.
Cleaner Ling Li, 70, cleaning a playground in Tampines last month. Disinfectants and alcohol-based sanitiser break the lipid membrane encasing a coronavirus' genetic material, rendering the virus impotent and incapable of infecting its host.ST PHOTO: LIM YAOHUI

A virus may be detectable on a surface days after a patient leaves behind traces, but this does not necessarily mean it is still infectious, experts say.

This is because most detection methods pick up the presence of the viral genetic material, which may linger for days even after the structure of the virus itself is broken.

An intact structure is needed for a virus to infect a host, such as a human, said Professor Wang Linfa, director of the emerging infectious diseases programme at Duke-NUS Medical School.

"Currently, the only way we can detect (the presence of the virus) is through its nucleic acid material, and this may or may not mean that we still have a live virus," he told The Straits Times. "This is why hygiene is very important, because disinfectants and soap can get rid of live viruses that are transmissible."

His points clarify a study by the United States Centres for Disease Control and Prevention (CDC), which was published earlier this week.

In the study, the CDC said the genetic material of the coronavirus could be detected on surfaces in the cabins of infected passengers on board a cruise ship for up to 17 days after cabins were vacated, but before disinfection procedures had been conducted.

Dr Michael Mina, a physician and assistant professor in immunology and infectious diseases at the Harvard T.H. Chan School of Public Health, remarked on Twitter that the CDC findings did not show the virus causing Covid-19 could survive for more than 17 days on a surface.

"(It just shows) that the viral RNA is detectable for at least 17 days," he said. "That's like saying that detecting DNA on a piece of hair post-mortem means a person is alive."

Under a microscope, a coronavirus like the one causing the Covid-19 disease appears spherical, with a "crown" of spikes on its surface. In its centre, encased in a lipid membrane, is its genetic material: a single-strand RNA genome.

For it to infect its host, this structure has to be intact, said Prof Wang. Various measures, such as exposing the virus to heat or through the use of disinfectants, break this structure, rendering the virus impotent and incapable of infecting its host.

 
 
 
 

For instance, the spikes on the virus' surface are susceptible to being destroyed, or denatured, by heat or the sun's ultraviolet rays. When these spikes are destroyed, viruses lose their ability to latch on to a human host cell. Similarly, disinfectants and alcohol-based sanitiser help to make a virus unviable by breaking the membrane.

Prof Wang said current tests for the virus make use of a molecular biology process known as the polymerase chain reaction (PCR), which helps to determine if the sample contains any detectable genetic material of the coronavirus.

He said: "The PCR test zooms into the genetic material. So we have to differentiate between a live virus, which has an intact structure, with its lipid membrane intact, that can transmit and cause disease, versus a situation where you have only a little bit of genetic material left."

He added: "In the latter, we can still get a PCR positive, but the virus is gone."

 
A version of this article appeared in the print edition of The Straits Times on March 28, 2020, with the headline 'Can't touch this...'. Print Edition | Subscribe