The steep rise in the number of deaths in England and Wales last year was the result of an increase in dementia, related conditions and respiratory diseases among older people, according to analysts.
Provisional data for 2015 showed the highest number of deaths in a single year since 2003 and the highest year-on-year percentage increase since 1968.
“The majority of the increase in deaths in 2015 happened during the first few months of the year”
The ONS said last year there were 529,613 deaths registered in England and Wales, an increase of 28,189 (5.6%) compared with 2014.
It noted that 86% of the extra deaths occurred among the over 75s age group and 38% in the over 90s.
In comparison, deaths remained close to the five-year average in the first 12 weeks of 2016, and the large increase in deaths in the early part of 2015 was not repeated in the early part of 2016.
The ONS highlighted influenza as likely to be an “important factor” in contributing to the increase in deaths seen during 2015.
The predominant circulating flu virus in 2015 was influenza A(H3N2), a strain known to predominantly affect older people, it said.
“We have seen these annual fluctuations before and the overall trend has remained positive”
It noted there were many outbreaks in care homes, hospital admissions for flu were higher than recent seasons, and there was evidence that the flu vaccine was less effective than expected.
Claudia Wells, head of mortality analysis at ONS, said: “The majority of the increase in deaths in 2015 happened during the first few months of the year, coinciding with an increase in hospital admissions for flu and reports of numerous outbreaks of the virus in care homes.
“Respiratory diseases, such as flu, were also mentioned in a third of deaths from dementia and Alzheimer’s last year,” she said.
“The number of deaths where dementia and Alzheimer’s were listed as the underlying cause have been steadily increasing over the last 15 years, but were well above the five year average in 2015,” said Ms Wells.
Professor John Newton, chief knowledge officer at Public Health England, added: “A range of factors can push up the number of deaths in older people in a particular year.
“An outbreak of flu can have a big impact, especially on those who are most vulnerable or experiencing other illnesses, such as dementia,” he said.
He added: “An increase in deaths will generally lead to a decrease in life expectancy that year, but we have seen these annual fluctuations before and the overall trend has remained positive”.
The ONS said it would publish the final 2015 death registration figures later in the summer.