What caused the spike in mortality in England and Wales in January 2015?

In the accompanying paper, we described the exceptionally large increase in mortality in England and Wales in 2015, showing how it was driven by deaths among older people, and pointed to a need to look at the functioning of the health and social care system. While that paper examined the overall increase in deaths in 2015, it also noted a very large spike in deaths in January. So far, consensus is lacking on its causes, with some suggesting influenza and cold weather, and others, the cuts to health and social care.1,2 In this essay, we examine the January spike in more detail.