Undiagnosed Covid-19 is a “likely explanation” for some of the excess deaths not linked to coronavirus of elderly people with other health conditions in England and Wales, analysis suggests.

Dementia and Alzheimer’s disease and symptoms mainly indicating old age account for two thirds of the total number of non-Covid-19 excess deaths in England and Wales from March 7 to May 1, the Office for National Statistics (ONS) said.

There were 5,404 more deaths than expected due to dementia and Alzheimer’s disease – a rise of 52% compared with the average number of deaths for this period over five years.

And 1,567 excess deaths occurred due to “symptoms signs and ill-defined conditions” – a 77.8% rise from the five-year average.

The ONS said it was “implausible” that the sharp rise in deaths linked to dementia and Alzheimer’s disease could have happened by chance.

Undiagnosed Covid-19 could “help explain the rise” in the deaths of frail elderly people with underlying conditions, particularly women and those in care homes, the ONS added.

While there is no reason to believe that Covid-19 was knowingly omitted from death registrations, it is possible that symptoms were not apparent or the virus was mistaken for an illness with similar symptoms.

It said: “The absence of large rises in deaths due to this cause that mention conditions that could exhibit similar symptoms to Covid-19 suggests that if Covid-19 is involved in the increase in deaths due to dementia and Alzheimer disease, the usual symptoms of Covid-19 were not apparent.

“This could fit with recent clinical observations, where atypical hypoxia has been observed in some Covid-19 patients.

“In someone with advanced dementia and Alzheimer disease, the symptoms of Covid-19 might be difficult to distinguish from their underlying illness, especially with the possibility of communication difficulties.

“Care home residents have experienced changes to their usual routine as a result of measures to tackle the coronavirus pandemic. Adverse effects of such changes cannot be discounted as another possible explanation of the increase in the number of deaths in care homes.”

Friday’s release is the first detailed analysis from the ONS looking at the increased number of deaths during the pandemic where coronavirus was not mentioned on the death certificate.

It found that 28% of excess deaths registered between March 7 and May 1 in England and Wales did not involve coronavirus – a total of 12,900 deaths out of 46,380.

Deaths related to asthma and diabetes are occurring at a “significantly higher rate compared to the five-year average” and could indicate that some people with these conditions are not receiving care fast enough to prevent death taking place, due to extra pressures on the healthcare system.

It is also “plausible” that some of these deaths are because undiagnosed Covid-19 “exacerbated the pre-existing condition”.

Deaths from ischaemic heart disease and other forms of circulatory disease, stroke, meningitis and appendicitis have all risen, suggesting people “are not receiving care fast enough”.

The highest number of excess non-Covid-19 deaths have taken place in care homes, with a weekly maximum of 2,975 of these deaths being registered in the seven days to April 17.

Non-Covid-19 deaths in private homes saw a separate peak in the week ending April 24, when 1,760 were registered.

The ONS said that if patients have been discharged from hospital sooner than they may have been typically, because of pressure on the NHS’s resources, this “could have resulted in some deaths occurring in care homes or private homes that would have otherwise occurred in hospital”.

It added that the reported lower rates of testing in all settings outside hospitals “could lead to some deaths in other locations involving Covid-19 not having Covid-19 listed on the death certificate as a contributory factor, leading to apparently higher non-Covid-19 excess deaths”.

A rise in deaths due to cancer and renal failure occurring outside hospitals “could indicate that people who are very ill with these conditions are remaining at home when previously they may have been admitted to hospital or a hospice for near end-of-life care”.

The ONS said it was unclear whether stress-related causes linked to social distancing and isolation were a factor in the overall excess non Covid-19 deaths.

Of those aged 10-39, registered deaths were generally under the five-year average, but because deaths in this age group often require an inquest, for example suspected suicides, registration is typically delayed and so the true toll is not yet clear.

London and the West Midlands, which have the highest proportion of black and minority ethnic (BAME) people in their populations, had particularly large differences between the five-year average and number of excess deaths not due to coronavirus, it was noted.