The analysis suggests overall levels of outsourcing in England increased consistently from 2013, rising from less than 4% to more than 6% of total regional health board spend by 2020. In total, £11.5bn was spent outsourcing health services to the private sector over the period, with the amount varying considerably by individual health board. The largest increases in outsourcing spend were on business and IT support, with steady increases in spending on healthcare businesses, social work, and transport companies.
The statistical analysis showed that an annual increase in outsource spending of 1% is associated with a rise in treatable mortality of 0.38% (0.29 deaths per 100,000 population) the following year. Based on observed changes in outsourcing spend and treatable deaths for each health board, the analysis shows 557 additional deaths between 2014 and 2020 might be attributed to changes in outsourcing.
There was no association between outsourcing and rates of preventable mortality. This further suggests that the relationship between outsourcing and treatable deaths is linked to quality of care, rather than a consequence of general trends in health outcomes in the population.
Study author Dr Aaron Reeves, of the University of Oxford, says: "These results clearly have implications for the NHS privatisation debate, suggesting that increased outsourcing to the private sector could lead to a decline in the quality of care provided to patients. While more research is needed to determine the precise causes of the declining quality of healthcare in England, our findings suggest that further increases in NHS privatisation would be a mistake.
"The findings of this research are timely as the way England's health boards are organised is set to be overhauled. This creates a key moment where the role of the private sector within the NHS must be closely scrutinised." 
The authors acknowledge some limitations to their study. Major changes to regional health boards since 2013 limited the ability to precisely measure outsourcing before then, or to conduct before-and-after analyses. The findings are not evidence of a causal relationship between outsourcing and mortality rates, so it cannot be ruled out that other factors may also be involved. Files on outsourcing do not contain details of specific services provided by suppliers, meaning more research is needed to determine if certain services are the main cause of the trends observe.