Those born before 1991 had probably been repeatedly exposed to the virus, due to the frequent epidemics which occurred in these years, including the pandemic in the 1960s, which are not found in local records.
Es probable que los nacidos antes de 1991 se hayan expuesto repetidamente al virus, dadas las frecuentes epidemias que ocurrían en esos años, incluyendo la pandemia de los años 60, de la cual no se encontraron registros locales.