Estimating case fatality rates of COVID-19
Authors: David Dongkyung Kim, Akash Goel
Journal: The Lancet Infectious Diseases (2020)

In their model, David Baud and colleagues1Baud D Qi X Nielsen-Saines K Musso D Pomar L Favre G Real estimates of mortality following COVID-19 infection.Lancet Infect Dis. 2020; (published online March 12.)https://doi.org/10.1016/S1473-3099(20)30195-XSummary Full Text Full Text PDF Scopus (830) Google Scholar exclude individuals who die within 14 days of testing positive for severe acute respiratory syndrome coronavirus 2. If an individual contracts symptoms on March 1, tests positive on March 10, and dies on March 11, they would not be included in the denominator for case fatality rate (CFR) on March 11. In addition, patients might test positive up to 13 days after recovery.2Lan L Xu D Ye G et al.Positive RT-PCR test results in patients recovered from COVID-19.JAMA. 2020; (published online Feb 27, 2020)DOI:10.1001/jama.2020.2783Crossref PubMed Scopus (898) Google Scholar As testing is expanded, an asymptomatic patient infected 1 week before testing positive on March 3 should, but will not, be included in calculations for March 16. For the stated purpose of the authors, it might be useful to also include everyone who was symptomatic for 14 days before the calculated date. Consider a CFR calculation for March 15. If an individual was symptomatic on March 1 and tested positive on March 10, they would not be included in the denominator, even though the patient probably contracted the disease before the 14-day lag time. Moreover, patients with mild symptoms might not undergo testing and so might not be included in the overall denominator. A further consideration is the delay between testing and receipt of results. Consequently, individuals might not test positive until after the suggested 14-day incubation period. With disease spread, indications for COVID-19 testing will expand, thereby increasing the denominator size. These factors might act as time-varying confounding variables in the authors' calculation of CFR. One other published lag-time calculation has included half the additional cumulative deaths in the numerator and time from illness to death as the lag-time (13 days).3Wilson N Kvalsvig A Barnard LT Baker MG Case-fatality risk estimates for COVID-19 calculated by using a lag time for fatality.Emerg Infect Dis. 2020; (published March 13, 2020.)DOI:10.3201/eid2606.200320Crossref Google Scholar For example, if calculating the CFR for March 15, the denominator would be cumulative cases until March 2, and the numerator would be cumulative deaths until March 2, in addition to half of the deaths recorded from March 2–15. This method assumes a normal distribution of time from illness to death. Although underestimation of CFRs risks the population not taking the threat seriously, overestimation might lead to unnecessary additional panic and concern. During a rapidly evolving pandemic, accurate measures of disease characterisation are important. Future estimates will probably involve patient-specific data for refined calculation. However, the provided CFR estimate of 15·2% for countries outside China might be a premature statistic owing to the limitations of their methods. We declare no competing interests. The authors contributed equally. Real estimates of mortality following COVID-19 infectionAs of March 1, 2020, 79 968 patients in China and 7169 outside of China had tested positive for coronavirus disease 2019 (COVID-19).1 Among Chinese patients, 2873 deaths had occurred, equivalent to a mortality rate of 3·6% (95% CI 3·5–3·7), while 104 deaths from COVID-19 had been reported outside of China (1·5% [1·2–1·7]). However, these mortality rate estimates are based on the number of deaths relative to the number of confirmed cases of infection, which is not representative of the actual death rate; patients who die on any given day were infected much earlier, and thus the denominator of the mortality rate should be the total number of patients infected at the same time as those who died. Full-Text PDF Authors' replyWe thank David Dongkyung Kim and Akash Goel,1 Piotr Spychalski and colleagues,2 and Marc Lipsitch3 for their critical reading of our Correspondence.4 In response to the points raised regarding our statistical methods, we agree that our model might not be appropriate for the early epidemic period because of the rapid increase in the number of cases in the 14 days preceding reported deaths. During this period, many patients were certainly diagnosed with coronavirus disease 2019 (COVID-19) at the time they developed critical illness or even at the time of death. Full-Text PDF Estimating case fatality rates of COVID-19We congratulate David Baud and colleagues1 for their apt observations regarding the burden of the coronavirus disease 2019 (COVID-19) epidemic and the possibly higher than expected proportion of cases that are fatal. Precision, however, is as necessary in calculations as in semantics. Full-Text PDF Estimating case fatality rates of COVID-19In their Correspondence, David Baud and colleagues1 suggest that case fatality rates (CFRs) for coronavirus disease 2019 have been underestimated and propose to divide deaths at time t by cases at time t minus 14 days to correct this underestimation and provide so-called real estimates. Many biases in both directions afflict CFR estimates during outbreaks,2 and experts have spent 2 decades (since the outbreak of severe acute respiratory syndrome coronavirus) finding ways to overcome these.3 The delay problem highlighted by Baud and colleagues produces falsely low estimates, whereas the under-ascertainment of mild cases produces falsely high estimates. Full-Text PDF