Headline numbers: Why are winter death rates so high?

The Office for National Statistics (ONS) says that in the first six weeks of 2015, just over 82,000 deaths were registered, which is 23% higher than the average from the previous five years. The ONS recently published the graph above, which compares the last week in December and first three weeks in January for the last 16 years.
 
First of all, while we are currently well above the average for the past five years, we are at about the same level as we were in winter 2008-09. Secondly, the spike for this year is exaggerated somewhat by last year, when there was the smallest number of Excess Winter Deaths since records began in 1950.
 
Excess Winter Deaths are the number of people whose deaths were registered between December and March, compared with the numbers for the previous four months and the following four months. Thirdly, while the figures are high by recent standards, they are dwarfed by the levels in 1999-2000, which was the last year classified as a flu epidemic. An epidemic year is one in which more than 200 people per 100,000 go to see their GPs with flu-like symptoms.
 
We know that flu has played a part in this year’s high numbers. Public Health England’s analysis says that the high number of deaths, "coincides with circulating influenza and cold snaps", with the over-65s particularly hard hit by the strains of the virus spreading this year. It also appears that this winter’s flu vaccine has been less effective than usual, which is suspected to be contributing.