Everyone’s talking about it.

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Some US officials and physicians have said the Case Fatality Rate (CFR) for COVID-19 is similar to seasonal flu (0.1%). The World Health Organization says it is 3.4%.

First, mortality rate is not the same as case-fatality rate. The mortality rate uses the entire population as the denominator. The CFR is how many people out of the confirmed cases don’t survive. The mortality rate will therefore be lower than the CFR. Make sure you know which one is being discussed.

As seen in the table, the CFR varies dramatically by country. In China, that has to do somewhat with being the epicenter of this outbreak. It’s “normal” when a new disease emerges to see more deaths initially because health care providers don’t know what they’re dealing with. They look for all the usual suspects before they finally identify the culprit. Meanwhile, health care workers (HCWs) may be exposed. And when HCWs are exposed, it’s not like the rest of us. HCWs come into contact with the sickest patients who are most likely to be shedding more virus. They do procedures which can amplify exposure. They are more likely to become infected and with a bigger viral dose.

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edit: as of 3/10/2020, Italy’s CFR has jumped to 5%

So apart from China, why the huge variation in CFR by country? No, the US doesn’t have a more severe strain than South Korea, as speculated by some. The simple answer is testing. South Korea is doing extensive testing. The US has only been testing people with documented association with confirmed cases or a travel history to an affected region, so our denominator is lower. There are likely thousands more infected with mild symptoms, like in South Korea. We won’t know until we test more. At that point, the CFR in the US is likely to decrease.

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Source: Chinese Center for Disease Control

The second answer is the population affected. We know this disease mostly attacks the elderly, and unfortunately the epicenter of COVID-19 in the US is in a nursing home in Washington. The population exposed is not just elderly, but also medically fragile.

So what about Italy? Similar story. Italy’s population is the oldest in Europe. All of the patients who died were over 60, with most in their 80s and 90s with significant co-morbidities. I don’t have access to enough information about the situation in Italy, but I gather their health care system was quickly overwhelmed by the large number of cases coupled with a particularly high risk population.

Edit to add the age distribution of various countries

How does this compare to other diseases? SARS had a CFR of about 11%, MERS’ CFR was 34.4%.

With CFRs that high, typically the virus dies before it can be passed on to another host. Higher survival rates with mild illness, as it appears to be with COVID-19, means the virus will persist in the population longer.

Back to our original question. Will the CFR be close to seasonal flu’s 0.1% or will it stay around 3.4%? That 0.1% figure, in my opinion, is hope rather than reality. Based on some predictions, once we start testing, we’ll find many more mild cases and the CFR will decrease. But even in South Korea where broad-scale testing is happening, the CFR is 0.6%. When this outbreak runs its course, and maybe when we have a vaccine in 18 months or longer, the CFR will have gone down. But for now, we need to work with the data we have.

Public Health Response Plans need to address how we will protect those most at risk: nursing homes, elderly, and those with co-morbidities.

Dr. Rohr-Allegrini is an epidemiologist and tropical disease scientist currently working to prevent diseases through immunizations.

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