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Funding: BH was funded by the UK Medical Research Council through a Special Training Fellowship in Health of the Public Research, and BB was supported by the Swedish Council for Working Life and Social Research. The funders had no influence on the content or publication of findings
Competing interests: None.
Ethical permission for the study was granted by the regional ethics committee in Stockholm (ref 04-521/5). Statistics Sweden provided the data.
All authors contributed to study design and data interpretation. BH and AW analysed the data, BH wrote the first draft and all authors approved the final draft. BH and BB will act as guarantors.