Among the questions scientists are trying to answer about the new coronavirus is how many virus particles an infected person can pass on to others. According to a new study by Chinese researchers, the answer could be less than 10.
That would be considered a narrow transmission bottleneck – meaning the number of virus particles that can be transmitted from someone who is infected to other people.
“Only a few virions [entire virus particles] successfully enter host cells and eventually cause infection,” the researchers, led by Xu Yonghao, from the National Clinical Research Centre for Respiratory Disease in Guangzhou, wrote in a non-peer reviewed paper posted on bioRxiv.org on Friday.
This would suggest that hand hygiene and wearing face masks were effective ways to stop transmission of the virus, said the paper, co-authored by top Chinese respiratory expert Zhong Nanshan.
Sars-CoV-2, the new coronavirus that causes Covid-19, uses ribonucleic acid, or RNA, produced by the body to store its genetic information. RNA can produce proteins at a rapid rate, but its single-stranded structure makes it less stable than double-stranded DNA, or deoxyribonucleic acid.
That means the virus can mutate at a relatively fast pace – so as the coronavirus spreads in a host from organ to organ, the viral genes are constantly changing.
For the study, Xu and his team worked with Shenzhen-based genome research company BGI. They conducted in-depth sequencing on viral strains found in 13 patients, including four from two families, in Guangdong province, analysing their genetic diversity.
By comparing the difference in the genetic data, the scientists were able to reconstruct how the virus had spread from one person to another within the same household. Based on their modelling, the transmission bottleneck was estimated to be six virions in one of the families, and eight in the other.
The scientists also found that most of the mutations were lost when the virus was passed on.
About six months after the coronavirus was first reported in central China, more than 10 million people have been infected worldwide, while the death toll has passed 500,000. The virus is highly contagious and is spread through close contact with an infected person, through droplets from a cough or sneeze, or touching objects or surfaces where there are droplets. It has been found to survive on surfaces – say, a door handle or the dinner table – for up to 48 hours. So if a family member is infected, the rest of the household could be easily exposed to the virus even if they do not have contact with that person.
But according to the study, most of the viral strains found in the first host of the two families were not passed on to the next person. Some could also have been fought off by the immune system of the infected person, they said.
This may point to a potential weakness in how the virus is transmitted, according to the researchers, and that is where hand sanitiser and protective face masks could help.
“Instant hand hygiene and mask-wearing might be particularly effective in blocking the transmission chain,” they said.
Zhang Shuye, a principal investigator with the Shanghai Public Health Clinical Centre at Fudan University, who was not involved in the study, said there was strong evidence to support the use of face masks to control the spread of the virus.
But he said this would be less effective in a household where someone was confirmed or suspected to be infected, saying it would be better for them to be isolated elsewhere.
He also cautioned that it was still not certain whether the transmission bottleneck was as narrow as the study suggested, but said “there is little doubt that the virus is highly contagious”.
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