A new coronavirus carrier in town could pass the virus to eight other people, significantly more than previous estimates, according to a new study by Chinese scientists.

Such a high reproduction number occurred in the central Chinese city of Wuhan early this year when people were caught by surprise by an unknown virus, and it is still happening in the United States “in the absence of control measures”, the researchers said.

Modelling from a team led by data scientist Zhu Hongjun from the Chongqing University of Posts and Telecommunications suggests that the basic reproduction number (R0) of the Sars-CoV-2 virus in these two areas – Wuhan at the beginning of the outbreak and the US recently – is quite similar at around 8. The study was published in a non-peer-reviewed paper posted in medRxiv.org on Monday.

R0 is the number of secondary infections caused by a carrier in an unexposed population. Governments take the R0 number into account when planning coronavirus response measures. Most previous estimates on the R0 ranged from 2 to 4 for the Covid-19 pandemic, a similar measure to the Sars outbreak.

“These previous results are controversial, and even misleading,” Zhu said.
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Sars, or severe acute respiratory syndrome, was caused by another strain of deadly coronavirus in 2003, spreading to 29 countries and infecting more than 8,000 people worldwide.

Covid-19 has infected more than 33.5 million people, caused more than 1 million deaths and is still spreading.

Zhu and colleagues recalculated the basic reproduction number in Wuhan with a new model. After correcting for underestimation caused by delays in diagnosis and laboratory confirmation in the early stage of the outbreak in Wuhan, they found the actual value should be 7.9.

The R0 varies from one place to another in a global pandemic. The reproduction rate is not only determined by the transmission capability of the virus, but also human behaviour and the environment.

A separate study in July by a research team from the Shanghai Jiao Tong University school of medicine found the R0 in the US reached 8.2, one of the highest in the world. Zhu’s modelling agreed with the Shanghai study.

These results raised the question of whether the infectivity of the virus had increased over time.

The reproduction rate in the US was higher than that in Wuhan. The dominant strains circulating around the globe today carried D614G, a mutated gene that could increase the number of spike proteins used by the virus to bind with the host cell, according to computer modelling.

Some researchers believed the virus had become more adaptive to human bodies.

Zhu’s team said their analysis suggested that the virus transmission capability remained roughly the same. The lower R0 in Wuhan was caused by a greater proportion of residents who were admitted to hospital and isolated after they got sick, while more patients in the US tended to stay at home, increasing the risk of transmission to their families, the researchers said.

An epidemiologist in Shanghai said the new Chongqing study could explain the rapid growth of infections around the globe.

“It supports the mainstream opinion in the research community that the virus has not become more infectious,” said the government scientist who asked not to be named.

Zhu’s team said transmission in enclosed settings such as hospitals was quite different from those in normal communities. Ignoring such differences could easily lead to underestimating the reproduction number.

Their model divided the population into two areas: in hospital and in the community. The new model not only showed that the R0 of the novel coronavirus was much higher than previously thought, but also predicted the end of the epidemic in Wuhan by April.

The R0 of the 1918 pandemic caused by the H1N1 virus was 2.2-2.9 while measles, one of the most infectious diseases, can be 12 to 18, according to some estimates.

Stephen Chen in Beijing