Health Security and Countering Weapons of Mass Destruction Act This bill makes changes to the Countering Weapons of Mass Destruction (CWMD) office of the Department of Homeland Security (DHS) and provides statutory authority for and expands DHS's Office of Health Security (OHS). Specifically, the bill establishes the new OHS to advise DHS on medical, public health, and workforce safety matters; sets forth the responsibilities of the Chief Medical Officer and a Privacy Officer in OHS; repeals the terminating date for the CWMD office and sets forth the responsibilities of that office, including efforts to counter chemical, biological, radiological, and nuclear (CBRN) threats, as well as other emerging terrorism threats; requires DHS to report to Congress regarding the Securing the Cities program (which seeks to reduce the risk of a successful deployment of radiological or nuclear weapons against major metropolitan areas in the United States); establishes a CWMD Advisory Council; and requires a biodefense review and issuance of a DHS biodefense strategy. The Government Accountability Office must report to Congress on efforts of the CWMD office to prioritize its programs and activities to safeguard against CBRN threats and on other activities of that office.