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Contributors TM contributed to acquisition of data. KK conceived the letter, analysed the data and wrote the first draft. CV, SJ, LA, HB and TM contributed with editing of the content and specifics of the letter. All authors reviewed and approved the final product.
Competing interests None.
Ethics approval Regionala etikprövningsnämnden i Stockholm (Dnr:2009/587-32).
Provenance and peer review Not commissioned; externally peer reviewed.