What does a 95% effective vaccine mean?

A  Pfizer COVID-19 (C19) vaccine said to be 95% effective in a recently-completed clinical trial is being widely welcomed, particularly by the governments that have already bought it. However, what does 95% effective mean? Effective against what – C19, the virus that causes C19 (SARS-CoV-2), hospitalisation, severity, death? Further, does 95% effective mean that if

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