The findings from the first year of the implementation of the active surveillance showed that most Lower Respiratory Tract-Related Post-Neonatal (LRTI) Mortality cases could not be included into the study, because most of the grieving parents refused to participate in the study. The European Medicines Agency Marketing Authorization Holders (EMEA MAH) also generated a substantial amount of additional clinical and post-marketing safety data throughout the world, which clearly indicated the absence of any safety signal when considering the LTRI-related fatalities. Therefore surveillance for LRTI mortality cases was removed from the study design. The number of sites was reduced from approximately 230 to about 60 to 90 in order to reflect the above mentioned change that reduced the need for so many sites. Additionally, passive surveillance for IS cases through the IMSS database was not feasible, due to restrictions of the IMSS internal policies and to the current limitations in the databases linkage and structure abilities.