Yet vaccines aren't a silver bullet, and they don't work for everyone. For instance, the immune systems of the elderly may be so weak that they can't muster a sufficient reaction. And some pathogens are particularly adept at mutating and avoiding the defense system created by the vaccine. Take, for instance, malaria, tuberculosis, and HIV-all have eluded attempts to create effective vaccines.
然而，疫苗並不是所有疾病的萬靈丹，也不是對所有人都有效，例如老年人的免疫系統可能因太虛弱，無法對疫苗做出足夠反應；而某些病原則因特別狡猾善變，可以躲避疫苗引發的免疫防禦機制，例如瘧疾、結核病與愛滋病等，仍是目前疫苗技術無法克服的疾症。