Unlike older vaccines (such as polio vaccine) which generally used weakened or killed whole virus particles (and which occasionally produced infectious cases, due to the virus not being as weak or dead as intended), this vaccine contains no whole virus particles and thus cannot result in an infectious case.

It contains RNA that cause the patient's cells to emit proteins normally produced by the viral infection (which can then be recognized by the immune system), but contains none of the genetic material required to produce standalone infectious virus particles.

This is a new thing; these will the first vaccines using this technology to be approved.

The chairman likely isn't worried about the vaccine itself producing infections, but rather does not know whether a vaccinated patient might also develop an active infection before the vaccine takes effect.