A single person infected with the Wuhan coronavirus could spread it to another two or three people on an average at current transmission rates, making it difficult to control the contagion. Accordingly, control measures must block over 60% of transmission to be effective in controlling the outbreak, say researchers from Imperial College London, UK.

Self-sustaining human-to-human transmission of the novel coronavirus (2019-nCov) is the only plausible explanation of the scale of the outbreak in Wuhan. We estimate that, on average, each case infected 2.6 (uncertainty range: 1.5-3.5) other people up to January 18, 2020, based on an analysis combining our past estimates of the size of the outbreak in Wuhan with computational modeling of potential epidemic trajectories. This implies that control measures need to block well over 60% of transmission to be effective in controlling the outbreak, says the research team in their findings.

They add, It is likely, based on the experience of SARS and MERS-CoV, that the number of secondary cases caused by a case of 2019-nCoV is highly variable with many cases causing no secondary infections, and a few causing many.

The researchers from the MRC Centre for Global Infectious Disease Analysis say despite the recent decision of the World Health Organization (WHO) emergency committee to not declare this a Public Health Emergency of International Concern at this time, this outbreak represents a clear and ongoing global health threat.

According to the scientists, whether transmission continues at the same rate now critically depends on the effectiveness of the intense control effort now underway in Wuhan and across China. It also depends on the extent to which the populations of affected areas have adopted risk-reducing behaviors. They emphasize that in the absence of antiviral drugs or vaccines, control relies upon the prompt detection and isolation of symptomatic cases.

It is uncertain at the current time whether it is possible to contain the continuing epidemic within China. In addition to monitoring how the epidemic evolves, it is critical that the magnitude of the threat is better understood, says the study.

In their report published on January 22, the research team used an estimate of the frequency of international travel from Wuhan to estimate that 4,000 cases (uncertainty range: 1000-9700) had occurred there with the onset of symptoms up to January 18. The new report estimates human-to-human transmissibility of 2019-nCoV.

The transmissibility of a virus is measured by the reproduction number, which measures the average number of new infections generated by each infected person.

The team explains that for their baseline estimates, they assume that two key characteristics of 2019-nCoV are similar to those observed for SARS: that there is high level of variability in the number of new infections generated by each infectious individual, and that the generation time (the average time between generations of infection) is the same as was estimated for SARS (mean of 8.4 days).

We conclude that self-sustaining human-to-human transmission of the virus must have occurred, with a reproduction number estimate of 2.6 to explain our previous central estimate of the scale of outbreak (namely 4000 cases by January 18). Even assuming our lowest estimate of 1000 cases by January 18, it is highly likely that sustained human-to-human transmission was occurring, says the analysis.

Our analysis indicates that it is highly likely that the human-to-human transmissibility of 2019- nCoV is sufficient to support sustained human transmission unless effective control measures are implemented, the study cautions.

Based on their results, the researchers emphasize the need to track transmission rates over the next few weeks, especially in Wuhan. If a clear downward trend is observed in the numbers of new cases, that would indicate that control measures and behavioral changes can substantially reduce the transmissibility of 2019-nCoV. Genetic data from Wuhan after the implementation of strong public health measures may also provide valuable insight into the patterns and rate of transmission, says the study.

A second study estimates the basic reproduction of the number of infection to be significantly greater than one. We estimate it to be between 3.6 and 4.0, indicating that 72-75% of transmissions must be prevented by control measures for infections to stop increasing, say researchers from Lancaster University, UK, University of Florida, US, and University of Glasgow Centre for Virus Research, UK.

The study estimates that only 5.1% of infections in Wuhan are identified, indicating a large number of infections in the community, and also reflecting the difficulty in detecting cases of this new disease.

The team explains that the rapidity of the growth of cases since the recognition of the outbreak is much greater than that observed in outbreaks of either SARS or MERS-CoV.

Our estimates of the reproductive number for this novel coronavirus are higher than most estimates reported for SARS and MERS-CoV, but similar to some estimates from subsets of data in the early period of SARS. For the SARS coronavirus, estimates ranged from 1.1 to 4.2 with most estimates between 2 and 3, says the team in the study.

They say: The rapidity of the growth of cases since the recognition of the outbreak is much greater than that observed in outbreaks of either SARS or MERS-CoV. This is consistent with our higher estimates of the reproductive number for this outbreak compared to these other emergent coronaviruses, suggesting that containment or control of this pathogen may be substantially more difficult.

The team cautions that if no change in control or transmission happens, they expect further outbreaks to occur in other Chinese cities and that infections will continue to be exported to international destinations at an increasing rate.

Our model suggests that travel restrictions from and to Wuhan city are unlikely to be effective in halting transmission across China. With a 99% effective reduction intravel, the size of the epidemic outside of Wuhan may only be reduced by 24.9% on February 4, say experts.

Read more:
Wuhan coronavirus: Rate of infection 'accelerating' as scientists cite 'clear and ongoing' global health threa - MEAWW