F2 and diphtheria: The high immunogenicity of ap1gen, a low-dose stand-alone vaccine, is compatible with its use as a booster with an acceptable safety profile in pregnant women where the benefit of the additional tetanus and diphtheria toxoids is not required, analogous to the current use of aP5gen (Pertagen®, a stand-alone recombinant acellular pertussis vaccine containing genetically inactivated PT and FHA).