This is a claim from the submitted article with no evidence:
> In all likelihood, however, given the one-off infections that are being detected in disparate locations, thousands if not millions have already had mild or asymptomatic SARS-CoV2.
An assistance director general and epidemiologist for the WHO, Bruce Aylward, makes the opposite argument in a recent Vox interview (https://www.vox.com/2020/3/2/21161067/coronavirus-covid19-ch...):
> Bruce Aylward: ... More of a surprise, and this is something we still don’t understand, is how little virus there was in the much broader community. Everywhere we went, we tried to find and understand how many tests had been done, how many people were tested, and who were they.
> In Guangdong province, for example, there were 320,000 tests done in people coming to fever clinics, outpatient clinics. And at the peak of the outbreak, 0.47 percent of those tests were positive. People keep saying [the cases are the] tip of the iceberg. But we couldn’t find that. We found there’s a lot of people who are cases, a lot of close contacts — but not a lot of asymptomatic circulation of this virus in the bigger population. And that’s different from flu. In flu, you’ll find this virus right through the child population, right through blood samples of 20 to 40 percent of the population.
> Julia Belluz: If you didn’t find the “iceberg” of mild cases in China, what does it say about how deadly the virus is — the case fatality rate?
> Bruce Aylward: It says you’re probably not way off. The average case fatality rate is 3.8 percent in China, but a lot of that is driven by the early epidemic in Wuhan where numbers were higher. ... This suggests the Chinese are really good at keeping people alive with this disease, and just because it’s 1 percent in the general population outside of Wuhan doesn’t mean it [will be the same in other countries].
I find Aylward's arguments cogent and backed with evidence from the ground. I also don't see "mass hysteria." The evidence points to COVID-19 being more severe than the seasonal flu, we have an opportunity to contain it, and we're trying to do so. Canceling large events and unnecessary travel seems reasonable.