This paper presents independent research funded by the National Institute for Health
Research (NIHR). The views expressed are those of the author(s) and not necessarily
those of the NHS, the NIHR or the Department of Health. The NIHR had no role in
study design; the collection, analysis and interpretation of data; and the writing of the
article or the decision to submit it for publication. The Centre for Paediatric
Epidemiology and Biostatistics was supported in part by the Medical Research
Council in its capacity as the MRC Centre of Epidemiology for Child Health (award