Have you read up on this? There have been population screening blood testing for antibodies, indicating exposure, mild or not, after the outbreak has passed through in parts of China. 320,000 screened in Guangdong. That is helping to zone in on the actual case fatality rate for that location at least.
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From ARS Technica:
How many people die from the infection?
This is a difficult question to answer. The bottom line is that we don’t really know.
Case fatality rates (CFR)—that is, the number of infected people who will die from the infection—are simply calculated by dividing the number of dead by the number of recovered plus dead. The CFRs you’ve probably seen so far have likely been a crude version of this: deaths divided by total cases.
One problem with these crude calculations is that the cases we’re counting aren’t all resolved. Some of the patients who are currently sick may later go on to die. In that situation, the patients' cases are counted, but their deaths are not (yet). This skews the current calculation to make the CFR look artificially low.
But a much larger concern is that we are undercounting the number of cases overall. Because most of the COVID-19 cases that we know about are mild, health experts suspect that many more infected people have not presented themselves to healthcare providers to be tested. They may have mistaken their COVID-19 case for a common cold or didn’t notice it at all. In areas hard hit by COVID-19, there may not have been enough testing capacity to detect all of the mild cases. If a large number of mild cases are being missed in the total case count, it could make the CFR look artificially high.
The best way to clear up this uncertainty is to wait until one of the local outbreaks is completely over and then to do blood tests on the general population to see how many people were infected. Those blood tests would look for antibodies that target SARS-CoV-2. (Antibodies are Y-shaped proteins that the immune system makes to help identify and attack pathogens and other unfriendly invaders.) The presence of antibodies against a specific germ in a person’s blood indicates that the person has been exposed to that germ, either through infection or immunization. Screening the general population for SARS-CoV-2 antibodies will give a clearer picture of how many people were actually infected—regardless of whether they were symptomatic or diagnosed while sick. That number can then be used to calculate an accurate CFR.
So far, some preliminary population screening for COVID-19 infections has been done in China, specifically in Guangdong province. Screening of 320,000 people who went to a fever clinic suggested that we may not be missing a vast number of mild cases. This in turn suggests that the CFRs we are calculating now are not wildly higher than they should be. However, experts still suspect that many mild cases are going unreported, and many still anticipate that the true CFR will be lower than what we are calculating now.
Beyond getting the basic number of cases and deaths right, CFRs are also tricky because they can vary by population, time, and place. We’ve already noted above that the CFR increases in patient populations based on age, gender, and underlying health. But as time goes on, healthcare providers will get collectively better at identifying and treating patients, thereby lowering the CFR.
Complicating these statistics further, the quality of healthcare differs from place to place. The CFR in a resource-poor hospital may be higher than that in a resource-rich hospital. Additionally, health systems overwhelmed in an outbreak may not be able to provide optimal care for every patient, artificially increasing the CFR in those places.
This seems to be what we’ve seen in China so far. In the WHO-China Joint Mission report, the experts noted that in Wuhan—where the outbreak began and where health systems have been crushed by the number of cases—the CFR was a whopping 5.8 percent. The rest of China at the time had a CFR of 0.7 percent.
As of March 5, there were about 13,000 cases and 400 deaths reported outside of China’s Hubei Province (where Wuhan is located). A crude calculation puts the CFR around 3 percent, but this calculation will likely drift throughout the outbreak. We will update the current crude CFR periodically.
https://arstechnica.com/science/2020/03/dont-panic-the-comprehensive-ars-technica-guide-to-the-coronavirus/I highly recommend that everyone read the article above.