In April 2009 During the 2009 influenza pandemic The Institu to Mexicano del Seguro Social (IMSS) provides medical care to O38 million registered users throughout Mexico. Its role in the pandemic was crucial in determining how the pandemic developed A total of 316,067 health care workers (HCWs) were employed by the IMSS (3). A high proportion A(H1N1) pdm09 vaccine arrived in Mexico in two shipments: during November 2009 and January 2010. IMSS HCWs vaccination began on November 27 Safety and hygiene methods were the only way to prevent persons from contracting the virus before the HCWs were vaccinated The aim of this study was to determine seroprevalence of antibodies to pandemic influenza A(H1N1)pdm09 virus in Mexican health care workers before and after vaccination