Life expectancy in the US has dropped for the first time since the most fatal point of the AIDS crisis, according to government statistics.

The US population had a life expectancy of 78.8 years in 2015, down from 78.9 the previous year – a reversal of the slow upward trend seen in the last two decades and most years since World War Two.

Americans were more likely to die from almost every major cause of mortality, including heart disease, Alzheimer’s and accidental injuries last year than in 2014, found a new report from the National Centre for Health Statistics (NCHS).

The report’s lead author Jiaquan Xu told AFP news agency the results, based mainly on 2015 death certificates, were “unusual”.

There had been “much more death than we have seen in the last few years”, he said.

Demographer Philip Morgan from the University of North Carolina called the drop “a big deal”, adding “there’s no better indicator of well-being than life expectancy,” reported NPR.

The obesity epidemic, a rise in the abuse of heroin and prescription opioid painkillers, an ageing population and a limit to the effectiveness of current treatments for heart disease could all have contributed to the change.

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However, leading demographers have said they are reluctant to ascribe any one reason for the drop in life expectancy and would wait for further information to be released later this year before drawing conclusions.

In 1993, average life expectancy in the US dropped from 75.8 to 75.5 amid the AIDS epidemic caused by transmission of the HIV virus.

One year previously, AIDS became the number one cause of death for US men aged 25 to 44.

Men from that age group are now most likely to die from unintentional injuries, according to data from 2013.

HIV disease is the 6th leading cause of death among men aged 25 to 34 and the 9th most common among men aged 35 to 44.

Last year saw spikes in mortality rates for eight out of ten of the most frequent causes of death, including lung disease, stroke, diabetes and suicide.

Heart disease remained the most frequent cause of death in 2015, with 168.5 deaths per 100,000 people, up from 167 the year before.

But deaths from cancer, the second leading cause of mortality, dropped by 1.7 per cent from 151.2 deaths per 100,000 people in 2014 to 158.5 last year.

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The death rate for influenza and pneumonia was the only one in the ten leading causes of death not to change significantly.

A baby born in the UK between 2013 and 2015 can expect to live to 79.09 years old, according to the Office for National Statistics.