The 50x/100x numbers comes from the last 30 years of data from the VAERS database set up by the FDA to track vaccines adverse reactions.
I have been using the Influenza vaccines as the control. Influenza has the same kind of pathogen, pathology and epidemiology. And deaths rates. Except for those under 70. Who have much higher mortality risk from Influenza.
Basically every year there are about 150 million flu shots in the US. With about 30 to 50 adverse reaction deaths reported. A very safe vaccine. So far with under 100 million full vaccinations starting to enter the recording system (the recording lag can be up to 3 to 6 months) there are more than 5000 adverse reaction deaths recorded for the various SARs CoV 2 vaccines in the US. Same recording methodology , same system, as with the influenza vaccines.
What should be also factored in is that one of the most well know problems will all adverse reaction database is an under counting of actual cases. Studies have show the the under-reporting rate could be as high as 90%. So the eventual final vaccination death toll could be in the 40,000 to 50,000 range. Which would make it about the same or greater than the total number of viral pneumonia deaths which were not substitute case deaths. The actual annual death rate did not budge that much because most of the people who died FROM SARs 2 were very likely to die of some respiratory infection / pneumonia anyway. The non substitute case deaths, true excess deaths, is unlikely to be greater than 10% / 15% of all SARS viral pneumonia deaths, when the final numbers are crunched.
But the 50x/100x greater risk still holds. For general population adult mass vaccination.