Science on data so far
Novel Coronavirus (2019-nCoV) Mortality Rate
Presented on this page:
How to correctly calculate the mortality rate during an outbreak
Mortality Rate (2.1% Nationwide, 4.9% Wuhan, 3.1% Hubei, and 0.16% other provinces) by the NHC of China
Mortality Rate comments by the WHO (2% estimate, but too early to tell)
Study providing a tentative mortality rate of 3%
Death rate among patients admitted to hospital (HFR)
Days from first symptom to death
Comparison with other viruses
How to correctly calculate the mortality rate during an outbreak
The case fatality rate (CFR) represents the proportion of cases who eventually die from a disease.
Once an epidemic has ended, it is calculated with the formula: deaths / cases.
But while an epidemic is still ongoing, as it is the case with the current novel coronavirus outbreak, this formula is, at the very least, "naïve" and can be "misleading if, at the time of analysis, the outcome is unknown for a non negligible proportion of patients." [8]
(Methods for Estimating the Case Fatality Ratio for a Novel, Emerging Infectious Disease - Ghani et al, American Journal of Epidemiology).
In other words, current deaths belong to a total case figure of the past, not to the current case figure in which the outcome (recovery or death) of a proportion (the most recent cases) hasn't yet been determined.
The correct formula, therefore, would appear to be:
CFR = deaths at day.x / cases at day.x-{T}
(where T = average time period from case confirmation to death)
This would constitute a fair attempt to use values for cases and deaths belonging to the same group of patients.
One issue can be that of determining whether there is enough data to estimate T with any precision, but it is certainly not T = 0 (what is implicitly used when applying the formula current deaths / current cases to determine CFR during an ongoing outbreak).
Let's take, for example, the data at the end of February 8, 2020: 813 deaths (cumulative total) and 37,552 cases (cumulative total) worldwide.
If we use the flawed formula (deaths / cases) we get:
813 / 37,552 = 2.2% CFR (flawed formula).
Instead, even with a conservative estimate of T = 7 days as the average period from case confirmation to death, we would correct the above formula by using February 1 cumulative cases, which were 14,381, in the denominator:
Feb. 8 deaths / Feb. 1 cases = 813 / 14,381 = 5.7% CFR (correct formula, and estimating T=7).
An alternative method, which has the advantage of not having to estimate a variable, and that is mentioned in the American Journal of Epidemiology study cited previously as a simple method that nevertheless could work reasonably well if the hazards of death and recovery at any time t measured from admission to the hospital, conditional on an event occurring at time t, are proportional, would be to use the formula:
CFR = deaths / (deaths + recovered)
which, with the latest data available, would be equal to:
815 / (815 + 2,990) = 21% CFR (worldwide)
If we now exclude cases in mainland China, using current data on deaths and recovered cases, we get:
2 / (2 + 42) = 4.5% CFR (outside of mainland China)
The sample size above is extremely limited, but this initial discrepancy in mortality rates, if confirmed as the sample grows in size, could be explained with a higher case detection rate outside of China, especially with respect to Wuhan, where priority had to be initially placed on severe and critical cases, given the ongoing emergency.
As the days go by and the city organizes its efforts and builds the infrastructure, the ability to detect and confirm cases should improve. As of February 3, for example, the novel coronavirus nucleic acid testing capability of Wuhan had increased to 4,196 samples per day from an initial 200 samples.[10]
A similar discrepancy in case mortality rate can be observed when comparing mortality rates, as calculated and reported by China NHC: a CFR of 3.1% in the Hubei province (where Wuhan, with the vast majority of deaths is situated), and a CFR of 0.16% in other provinces (19 times less).
Finally, we shall remember that while the 2003 SARS epidemic was still ongoing, the World Health Organization (WHO) reported a fatality rate of 4% (or as low as 3%), whereas the final case fatality rate ended up being 9.6%.
Novel Coronavirus Mortality Rate, as discussed by the National Health Commission (NHC) of China
Asked at a press conference on February 4 what the current mortality rate (or case fatality rate, CFR) is, an official with China NHC said that [7]:
The formula they are using is: cumulative current total deaths / current confirmed cases. Therefore, as of 24:00 on Feb. 3, the formula used was 425/20,438.
Based on this figure, the national mortality rate to date was 2.1% of confirmed cases.
There might be mild cases and other cases not reported.
97% of the country's total deaths (414) were in the Hubei Province.
Mortality rate in Wuhan was 4.9%.
Mortality rate in the Hubei Province was 3.1%.
Mortality rate nationwide was 2.1%.
Fatality rate in other provinces was 0.16%.
Deaths in Wuhan were 313, accounting for 74% of China's total.
Most of the cases were still mild cases, therefore there was no need to panic.
Asked why Wuhan was so much higher than the national level, the NHC official replied that it was for lack of resources, citing as an example that there were only 110 critical care beds in the three designated hospitals where most of the cases were sent.
National mortality rate was basically stable, as of Feb. 4 at 2.1%, and it was 2.3% at the beginning of the epidemic, which can be seen as a slight decline.
Front the analysis of death cases, it emerged that the demographic profile was mainly male, accounting for 2/3, females accounting for 1/3, and is mainly elderly, more than 80% are elderly over 60 years old, and more than 75% had underlying diseases present such as cardiovascular and cardiovascular diseases, diabetes and, in some cases, tumor.
Elderly people with basic diseases, as long as they have pneumonia, were clinically a high-risk factor regardless of whether it is a coronavirus or not, and the case fatality rate was also very high, so it is not that the case fatality rate of pneumonia is high because of the infection with the new coronavirus. "This point must be explained to everyone," concluded the NHC official.[7]
World Health Organization: too early to make conclusive statements
The World Health Organization (WHO) had mentioned 2% as a mortality rate estimate in a press conference on Wednesday, January 29, 2020[1][2]. However, they specified that this is a very early and provisional estimate that may change. Surveillance is increasing, within China but also globally, but at the moment:
We don't know how many were infected ("When you look at how many people have died, you need to look at how many people where infected, and right now we don't know that number. So it is early to put a percentage on that."[1][2]).
The only number currently known is how many people have died out of those who have been reported to the WHO.
It is therefore very early to make any conclusive statements about what the overall mortality rate will be for the novel coronavirus, according to the World Health Organization [1][2].
Preliminary study providing a tentative 3% estimate for case fatality rate
A preliminary study published on The Lancet on January 24 [3] provides an early estimation of 3% for the overall case fatality rate. Below we show an extract (highlights added for the relevant data and observations):
Of the 41 patients in this cohort, 22 (55%) developed severe dyspnoea and 13 (32%) required admission to an intensive care unit, and six died.
Hence, the case-fatality proportion in this cohort is approximately 14.6%, and the overall case fatality proportion appears to be closer to 3%.
However, both of these estimates should be treated with great caution because not all patients have concluded their illness (ie, recovered or died) and the true number of infections and full disease spectrum are unknown.
Importantly, in emerging viral infection outbreaks the case-fatality ratio is often overestimated in the early stages because case detection is highly biased towards the more severe cases.
As further data on the spectrum of mild or asymptomatic infection becomes available, one case of which was documented by Chan and colleagues, the case-fatality ratio is likely to decrease.
Nevertheless, the 1918 influenza pandemic is estimated to have had a case-fatality ratio of less than 5% but had an enormous impact due to widespread transmission, so there is no room for complacency.
A novel coronavirus outbreak of global health concern - Chen Wang et al., The Lancet. January 24, 2020
Fatality rate can also change as a virus can mutate, according to epidemiologists.
