You welcome to this MIT mastery coronavirus update, I’m princess Bella, I’m an internist trained in epidemiology and public health at Johns Hopkins and the founder of mat mastery where we teach important clinical skills to doctors and other health care providers around the world. Today, we’re going to talk about case fatality rates and how to estimate and calculate them. Lets go to the Johns Hopkins coronavirus map there. We have the total number of confirmed cases, total deaths and total cases recovered. Please note that these are the data as of March 3rd 2020, so here you can find the total confirmed cases and the total number of deaths. Now you could calculate a case fatality rate with these numbers. You could divide the number of deaths by the number of total confirmed cases, multiply it by a hundred to receive a case. Fatality rate in this present situation of 3.4 %. But this way of doing it is slightly erroneous. I have constructed a fictitious epidemic showing daily new cases and new daily deaths of a certain disease in green and red respectively. Now, as you can see from the graph and probably from your own reasoning, is that someone needs to be sick or classified as a case for some time before they can die in this fictitious population. The time from being identified as a case to the time of death is two days on average.

You can see that the peak of cases was reached on day, seven to nine, whereas the peak of deaths occurred between days 9 and 11. Here you can see the same two curves of daily deaths and daily new cases in light red and light green on the very bottom. These are the daily new deaths. These are the daily new cases. In addition, you can now also see the cumulative cases and deaths in dark red and dark green, as you can appreciate when we are in the a sending limb of the epidemic. The current number of cumulative deaths is also delayed by two days when compared to the number of cumulative cases, so deaths that occurred over here correspond to cases that occurred over here. So one would have to compare the cumulative death from day ten to the number of cumulative cases from they ate in order to arrive at an accurate comparison or case fatality rate. So, what’s that time delay in Kovach 19 so over here you have the time of infection, the time of symptom onset, then the time when the case is reported and the time of death. Now the time between infection and symptom onset is called the incubation period. The incubation period of Kovach 19 has been estimated to be around 5.2 days on average. However, there are cases with a lower incubation period, but there are also patients with a much longer incubation period of up to 21 days. Possibly even 24 days have been described. So this interval is five days and what’s the time from symptom onset to the reporting of a case, the authors of this study found a mean duration of 7.1 days. Obviously this will be quite a bit different from country to country. So, let’s say that this period is 7 days and what’s the time from symptom onset to eventual death in the analyses done at the Imperial College, London authors found that the time from disease onset to eventual death was twenty-two point three days very similar to the Time of recovery, so this interval is 22 days long according to this paper, so 22 minus seven days equals a time delay between reporting and death of 17 days and that’s the interval.

One should use to determine where to take the current case number from so. Instead of comparing the current total deaths to the current total number of confirmed cases, one should compare the current total deaths to the total number of confirmed cases from 17 days prior. This number can be retrieved from enlarging this inset down here. So we said these numbers were taken from March 3, going back, seven day’s means we need to retrieve the number of total cases from February 15th. So on that day, the number of total cases in mainland China was 68 thousand three hundred and the total number of confirmed cases in other locations was 685, so 68 thousand three hundred plus six hundred eighty-five equal 68 thousand nine hundred eighty-five total cases On February 15. So what’s the new case, fatality rate using this lag time of 17 days, remember this was the calculation from the beginning of the video which did not account for the lag time, and now we have three thousand one hundred seventeen divided by sixty-eight thousand nine hundred. Eighty-five times, one hundred, which equals four points: five percent. Now you should not only use a late time of 17 days but also see what a shorter lag time of say. Eight days will bring up. This is called doing a sensitivity, analyses or testing your numbers. Now the problem in all of this, as described by the researchers from Imperial College London, is these calculations are likely skewed, because we don’t know how many asymptomatic cases or mild cases are out there whom we don’t know of. They argue that mostly the very severe cases have been detected in mainland China, whereas it appears that internationally, all symptomatic cases, meaning deaths, severe cases and symptomatic cases are picked up. If there truly is a substantial number of cases in the dark. Blue bottom part of the pyramid, then the true case, the fatality rate – will be much lower than what we have just calculated. The authors of this paper found that we might be underestimating the true number of cases by a factor of 19, which would mean that we would have to divide our calculated case, fatality rate by 19. In order to arrive at the true rate, which would obviously make the mortality rate seems much less scary, one argument in favor of that view can be seen here. This graph from nucleus wealth shows that there might be significant underreporting of cases in most Chinese, but also in places like Iran. The bars depict the reporter at kovat 19 cases per million inhabitants, South Korea, which has done the most extensive testing up until now. Reports around 80 cases per million the Chinese provinces are way lower, when they should be really higher, especially those provinces, next to Hubei where the outbreak started. So this should be an indicator that the true case, fatality rate, is lower. In reality, we will only be able to tell once we measure antibodies in the general population in order to tell what the true number of cases was. That’s it for now, if you want to improve your understanding of epidemiology, make sure to register for Fremont master a trial account and attend our epidemiology central scores. We’Ve just opened it up to trial users due to the huge demand so stay safe and talk soon.