WHO recommendations concerning the distribution, handling and synthesis of variola virus DNA: based upon recommendations made to WHO by the WHO Ad Hoc Committee on Orthopoxvirus Infections (1990 and 1994) and the WHO Advisory Committee on Variola Virus Research (2003, 2004 and 2007): revised 13 January 2016 (by the WHO Advisory Committee on Variola Virus Research at its seventeenth meeting)