The failure to provide an effective flu vaccine last year may have contributed to the largest rise in deaths in England and Wales for 12 years, the government’s public health agency has admitted.

An additional 28,189 people died in 2015 compared with 2014, the largest percentage increase since 1968, according to figures published by the Office for National Statistics. The majority (24,201) of the excess deaths were people aged 75 and over, many of whom died from flu or dementia.

The flu vaccine issued last winter only provided low protection against the main strain of the virus then affecting the UK, Public Health England (PHE) said.

The vaccine was estimated to work in just 34% of cases in lab tests, compared with a more typical past effectiveness of 50%, due to genetic “drift” in the flu virus.

Dr Richard Pebody, head of flu surveillance for PHE, said the problem was caused by a mismatch between the A(H3N2) influenza strain used to make the vaccine and the main strain that spread in the UK last winter.

He said: “Although in most winters, the vaccine is well matched, winter 2014-15 saw the circulation of a drifted H3 flu strain, making the vaccine less effective than the typical 50% we had seen in recent previous years. It is possible that this contributed to the increase in excess mortality.”

The data from the ONS showed a rise of 28,189 deaths in 2015 (5.6%), from 501,424 in 2014 to 529,613 in 2015. This is the highest number since 2003, when there were 539,151 deaths. The percentage increase in 2015 is the largest year-on-year rise since 1967 to 1968 (6.3%).

The rise in the number of deaths lowered life expectancy for boys born in 2014-2015 by 0.2 years to 79.3 and for girls by 0.3 years to 82.9.

The report showed there was also a rise in the number of people whose death certificate said the underlying cause was dementia or Alzheimer’s, accounting for 41% of extra deaths among over-75s, although a third of these also had a respiratory disease, such as flu.

Jeremy Hughes, chief executive of the Alzheimer’s Society, said: “With one in six deaths in England and Wales of people over the age of 75 now attributed to dementia, these findings serve as a stark reminder of the need for good community care to support the most vulnerable.

“People living with dementia often have a lowered immune system and so are at a greater risk of contracting flu viruses. The condition also makes it harder for people to look after themselves and in the cold winter months this can become a real danger.”

Overall, 31% of death certificates said there was an underlying cause of respiratory disease, although the ONS said flu was generally underreported and so the figure was likely to be higher.

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.”

Professor John Newton, chief knowledge officer at Public Health England, which worked on the analysis, said: “An outbreak of flu can have a big impact, especially on those who are most vulnerable or experiencing other illnesses, such as dementia.”

PHE said at this stage the flu vaccine for this season was well matched to the circulating strains of the virus.