Report 6: Relative sensitivity of international surveillance
Summary Report 6 – translation will be available shortly
Since the start of the COVID-19 epidemic in late 2019, there are now 29 affected countries with over 1000 confirmed cases outside of mainland China. In previous reports, we estimated the likely epidemic size in Wuhan City based on air traffic volumes and the number of detected cases internationally. Here we analysed COVID-19 cases exported from mainland China to different regions and countries, comparing the country-specific rates of detected and confirmed cases per flight volume to estimate the relative sensitivity of surveillance in different countries. Although travel restrictions from Wuhan City and other cities across China may have reduced the absolute number of travellers to and from China, we estimated that about two thirds of COVID-19 cases exported from mainland China have remained undetected worldwide, potentially resulting in multiple chains of as yet undetected human-to-human transmission outside mainland China.
Report 4: Severity of 2019-novel coronavirus (nCoV)
Summary Report 4
We present case fatality ratio (CFR) estimates for three strata of COVID-19 (previously termed 2019-nCoV) infections. For cases detected in Hubei, we estimate the CFR to be 18% (95% credible interval: 11%-81%). For cases detected in travellers outside mainland China, we obtain central estimates of the CFR in the range 1.2-5.6% depending on the statistical methods, with substantial uncertainty around these central values. Using estimates of underlying infection prevalence in Wuhan at the end of January derived from testing of passengers on repatriation flights to Japan and Germany, we adjusted the estimates of CFR from either the early epidemic in Hubei Province, or from cases reported outside mainland China, to obtain estimates of the overall CFR in all infections (asymptomatic or symptomatic) of approximately 1% (95% confidence interval 0.5%-4%). It is important to note that the differences in these estimates does not reflect underlying differences in disease severity between countries. CFRs seen in individual countries will vary depending on the sensitivity of different surveillance systems to detect cases of differing levels of severity and the clinical care offered to severely ill cases. All CFR estimates should be viewed cautiously at the current time as the sensitivity of surveillance of both deaths and cases in mainland China is unclear. Furthermore, all estimates rely on limited data on the typical time intervals from symptom onset to death or recovery which influences the CFR estimates.
Report 3: Transmissibility of 2019-nCoV
Summary Report 3
Self-sustaining human-to-human transmission of the novel coronavirus COVID-19 (previously termed 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 18th January 2020, based on an analysis combining our past estimates of the size of the outbreak in Wuhan with computational modelling of potential epidemic trajectories. This implies that control measures need to block well over 60% of transmission to be effective in controlling the outbreak. It is likely, based on the experience of SARS and MERS-CoV, that the number of secondary cases caused by a case of COVID-19 is highly variable – with many cases causing no secondary infections, and a few causing many. Whether transmission is continuing at the same rate currently depends on the effectiveness of current control measures implemented in China and the extent to which the populations of affected areas have adopted risk-reducing behaviours. In the absence of antiviral drugs or vaccines, control relies upon the prompt detection and isolation of symptomatic cases. It is unclear at the current time whether this outbreak can be contained within China; uncertainties include the severity spectrum of the disease caused by this virus and whether cases with relatively mild symptoms are able to transmit the virus efficiently. Identification and testing of potential cases need to be as extensive as is permitted by healthcare and diagnostic testing capacity – including the identification, testing and isolation of suspected cases with only mild to moderate disease (e.g. influenza-like illness), when logistically feasible.
Report 2: Estimating the potential total number of novel Coronavirus cases in Wuhan City, China
Summary Report 2
On January 16th we released estimates of the scale of the COVID-19 (previously termed 2019-nCoV) outbreak in China based on an analysis of the number of cases detected outside mainland China. Since then, cumulative confirmed cases reported by the Chinese authorities have increased 10-fold, to 440 by January 22nd. The number of detected outside China with symptom onset by 18th January had increased to 7 in the same time. Here we report updated estimates of the scale of the epidemic in Wuhan, based on an analysis of flight and population data from that city. Our estimate of the number of cases in Wuhan with symptoms onset by January 18th is now 4,000. The uncertainty range is 1,000-9,700, reflecting the many continuing unknowns involved in deriving these estimates. Our central estimate of 4,000 is more than double our past estimates, a result of the increase of the number of cases detected outside mainland China from 3 to 7. Our estimates should not be interpreted as implying the outbreak has suddenly doubled in size in the period 12th January to 18th January – delays in confirming and reporting exported cases and incomplete information about dates of symptom onset together with the still very small numbers of exported cases mean we are unable to estimate the epidemic growth rate at the current time.
Our analysis suggests that the COVID-19 outbreak has caused substantially more cases of moderate or severe respiratory illness in Wuhan than have currently been detected. However, recent rapid increases in officially reported confirmed case numbers in China suggest that case detection and reporting has been substantially enhanced in recent days. With further refinements and expansion of surveillance (for instance, to primary care providers) it is to be hoped that the differences between our estimates and official case numbers will lessen further. Given the increasing evidence for human-to-human transmission, enhancing rapid case detection will be essential if the outbreak is to be controlled.
Report 1: Estimating the potential total number of novel Coronavirus (2019-nCoV) cases in Wuhan City, China
Summary Report 1
Many aspects of the COVID-19 (previously termed 2019-nCoV) outbreak are highly uncertain. However, the detection of three cases outside China (two in Thailand, one in Japan) is worrying. We calculate, based on flight and population data, that there is only a 1 in 574 chance that a person infected in Wuhan would travel overseas before they sought medical care. This implies there might have been over 1700 (3 x 574) cases in Wuhan so far. There are many unknowns, meaning the uncertainty range around this estimate goes from 190 cases to over 4000. But the magnitude of these numbers suggests that substantial human to human transmission cannot be ruled out. Heightened surveillance, prompt information sharing and enhanced preparedness are recommended.