SINGAPORE – 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 are picking 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 on Monday (March 23).

In the study, the CDC said that 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.

Prof Wang is not the only expert to weigh in on the study.

Dr Michael Mina, a physician and Assistant Professor in Immunology and Infectious Diseases at the Harvard T.H. Chan School of Public Health, said on Twitter that the CDC findings did not did not show that 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 (the) person is alive.”

HOW VIRUSES WORK

Under a microscope, coronaviruses like the one causing the Covid-19 disease appear spherical, with a “crown” of spikes on their 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 ultra-violet rays. When these spikes are destroyed, viruses lose their ability to latch on to a human host cell.

Similarly, disinfectants and alcohol-based sanitisers 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.”

Source: Read Full Article