Open Access This is an open access publication, available online and distributed under the terms of the Creative Commons Attribution-Non Commercial 3.0 IGO licence (CC BY-NC 3.0 IGO), a copy of which is available at http://creativecommons.org/licenses/by-nc/3.0/igo/

Enquiries concerning use outside the scope of the licence terms should be sent to Springer Nature Switzerland AG.

The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of WHO concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Dotted and dashed lines on maps represent approximate border lines for which there may not yet be full agreement.

The mention of specific companies or of certain manufacturers’ products does not imply that they are endorsed or recommended by WHO in preference to others of a similar nature that are not mentioned. Errors and omissions excepted, the names of proprietary products are distinguished by initial capital letters.