Why Pfizer's or Moderna's CoViD-19 vaccine cannot be 95% effective?

The phase III clinical trial results of the two frontrunner CoViD-19 vaccines developed by Pfizer [1] and Moderna [2] has claimed ~95% efficacy in their interim analysis. There are many reasons why such efficacy claims are premature and are likely to be proven incorrect in the long term.
This publicity, no doubt, has given the world great hope but can be disappointing to the public when the long-term efficacy was not as good as claimed in the interim assessment. Albeit not a surprise to many clinicians and scientists, the public may react badly. This may also fuel anti-vaccine thoughts and is not in the greater interest of public safety and may jeopardise their trust in science and medicine. Moreover, this is going to be the first mRNA-based vaccine being developed for the mass public use and a longer-term safety and efficacy assessment is inevitable. The mass publicity and celebration of the interim results, therefore, was a very bad idea and likely to have adverse implications.