Why did the effectiveness of the Sinovac vaccine in Brazil drop from 78% to only 50.4%?

[關乎老師]

Someone provided the answer below: (my previous post on postulation on compilation of vaccine efficacy statistics concur with below)

Vaccine in Brazil drop from 78% to only 50.4%?
It just didn’t. Percentages always refer to something: it is the proportion of a certain number over a certain total quantity of something, averaged to a 100 basis. 50.4% is the so-called global effectiveness of the Sinovac vaccine, a data that was only known to the wider public a couple of days after it had been leaked that a 78% effectiveness against the development of symptoms had been observed in the tests.

Those are two distinct things, and they are calculated by comparing the proportion of volunteers who took the vaccine and tested positive for covid-19 (global effectiveness), developed any kind of symptoms, however mild they might’ve been, and so on to the proportion of those who only took a placebo in the same situations .

Let me put it this way. The scientists’s conclusion was basically that, if 1,000 people take the Sinovac vaccine and are exposed to the Sars-CoV-2 virus in a situation that would otherwise get them infected, then:

504 won’t be contamined by the virus
496 will be contamined by the virus
276 will be asymptomatic cases, capable of spreading the virus to other potential hosts, but feeling no particularly noticeable symptoms
220 will be symptomatic cases, but all those studied in the tests for the Sinovac vaccine had exclusively mild symptoms, not even moderate ones, let alone really serious symptoms.

From the above, it is possible that none of them will develop further symptoms beyond the typical mild symptoms of a usual flu, though that is not a statistically reliable figure to use, because the number of volunteers who had serious Covid-19 cases was very low in the control group (those who took only a placebo), if I’m not mistaken only 7 individuals, whereas absolutely 0 individuals who took the vaccine had more severe symptoms and need to hospitalized.

Therefore, compared to the control group (which represents those who weren’t immunized), those who took the Sinovac vaccine had a 78% reduction in the odds of evolving Covid-19 symptoms from mild to severe ones. But those who took the Sinovac vaccine had an overall 50.4% lower chance to get infected by Sars-CoV-2, though they could evolve to become asymptomatic or symptomatic.