"Although vaccines are strictly monitored before authorization, the possibility of AEs due to annual changes in vaccine formulations, vaccine administration patterns, environmental factors or genetic factors of the host cannot be totally eliminated. Consequently, annual post-licensure vaccine safety surveillance is fundamental.10 With regard to extremely rare events (1 case every 10,000 vaccinations),11 the relatively low number of cases available for analysis constitutes a study limitation. If no event of concern is registered in a study (i.e. a zero numerator is reported), it is necessary to conduct further investigations. Indeed, a zero numerator does not mean the absence of risk.12 In the case of a vaccine, AEs can only be reliably identified after larger populations have been vaccinated. For this reason, post-marketing surveillance is fundamental. "
