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The number of people who died in Scotland last winter hit an 18-year high, new statistics have revealed.

There were 23,137 deaths between December 2017 and March 2018, according to the National Records of Scotland - the highest figure since 1999/2000.

It also revealed that the seasonal increase in mortality - the number of "additional" deaths in winter - was 75% greater than in 2016/17.

The main underlying causes of the deaths were influenza and pneumonia.

Last winter saw the highest rate of flu-like illness is Scotland for seven years, leaving the NHS facing "exceptional pressures".

Age Scotland said the National Records of Scotland (NRS) figures were "staggering".

It joined the Royal College of Physicians and Scotland's chief medical officer in urging at-risk groups to get the flu vaccine before winter sets in.

The report was published as the Scottish government admitted that there was a shortage of one of two flu vaccines being offered this winter.

It said some GP practices have experienced higher than anticipated demand for the aTiv vaccine, which is given to the over 75s.

However Scotland's chief medical officer Dr Catherine Calderwood told BBC Scotland more vaccine will be made available.

"People may have to return to have this but we will be able to vaccinate all those in the eligible groups, in particular the elderly, pregnant women and those with chronic diseases," she said.

Image copyright Getty Images Image caption There has been higher than anticipated demand for a flu vaccine which is given to the over 75s

The NRS report found that the total number of deaths in 2017/18 increased by 10.5% on the previous winter, when 20,946 deaths were recorded.

Last winter's death total was the largest number since 23,379 deaths were recorded in 1999/2000.

Anne Slater, the chief executive of the NRS, said there was a long-term downward trend of winter deaths since the early 1950s.

"However, the average value for the latest five years (which smoothes out much of the year-to-year fluctuation) is now above the level that had applied since the early 2000s," she said.

"It is too soon to say whether there has been a change in the long-term trend: it could just be a short-term rise, like that seen roughly 20 years ago, after which the average fell for several years."

Image copyright Crown/ National Records of Scotland

It also found that there were 4,797 additional deaths last winter, compared to the average number of deaths in the four months before December 2017 and after March 2018.

It was the largest increase in seasonal mortality since 1999/2000 when the figure stood at 5,190, and a significant increase on the 2,730 recorded in 2016/17.

The NRS said the seasonal increase was larger than in most of the previous 66 winters and exceeded the level seen in 19 of the previous 20 winters.

Around 80% of additional deaths (3,860) last winter were among people aged 75 and older.

Image copyright Crown copyright/ National Records of Scotland

The underlying causes of most of the "additional" deaths included respiratory diseases, heart disease, stroke and dementia.

Very few deaths were caused by hypothermia and only a small proportion directly by influenza.

The Scottish Liberal Democrats' health spokesman, Alex Cole-Hamilton, said the new statistics were a "cause for concern" and he called on the Scottish government to ensure the NHS was ready for winter.

Miles Briggs, Scottish Conservative shadow health secretary said the figures were a "stark warning" for the SNP that they must prepare the NHS properly for seasonal pressures.

Image copyright Crown copyright/ National Records of Scotland

Age Scotland urged older people to reduce their risks of illness by keeping their houses warm and getting a flu vaccine.

The charity's head of policy, Adam Stachura, said: "These figures are staggering and a real shock to the system. The large increase in deaths due to flu and pneumonia should be setting alarm bells ringing.

"We know that during winter months the homes of many older people are insufficiently heated, as a result of high fuel costs and poor heating systems, and can lead to a greater risk of ill health and even death."

He said older people should check to see if they are receiving the benefits they are entitled to, which could help them heat their homes.

The Scottish government said it was investing an extra £10m to support winter resilience planning, and it has commissioned work to investigate the link between winter deaths and flu, and to explore potential factors behind the rise in mortality.

Dr Calderwood, the chief medical officer, said: "Every year in Scotland, around two thirds of people who get severe flu and need intensive care treatment have a health condition such as chronic lung or heart disease.

"Last winter, Scotland had its highest flu rates for seven years, a pattern seen across Europe and the United States, and it is likely that flu was a significant factor in many deaths, particularly among older people and those with long-term conditions.

"Flu vaccines are available free to all eligible adults, including everybody aged 65 and older, and protects against a number of different flu strains. Vaccination remains our best defence against flu, and I urge people to take up the offer of a free vaccine."