Deaths are 20 per cent higher than normal in England and Wales, with experts warning that more than half are not related to Covid and may be due to waning immunity to other infectious diseases.
The latest figures from the Office for National Statistics (ONS) showed there were 1,853 excess deaths in the week ending Sep 10 compared to the pre-pandemic five year average, but of those only 857 – 46 per cent – were Covid.
Health experts said that although influenza cases were currently low, there had been a rise in other infectious diseases which could be driving the increase.
There have been fears that restrictions put in place to fight Covid such as lockdowns, masks and social distancing, have lowered the population's immunity to other diseases.
Paul Hunter, professor in medicine at the Norwich School of Medicine, University of East Anglia, said: "Why we are seeing so many excess deaths at present that cannot be explained by Covid deaths is a real concern. Looking at causes of deaths it does look like respiratory deaths are particularly high this year compared to the average for the years 2015 to 2019.
"It could be that normal non-Covid, non-influenza infections are causing more severe disease now that these infections are spreading again after a long period where they were not common and when our immunity was not being boosted by repeat exposure.
"There is certainly quite a bit of both RSV and Rhinovirus infections around. However, it is not possible to be certain with the available data, though this is an issue that needs to be monitored."
In the week ending Sep 10, there were 11,035 deaths registered – 2,238 more than the previous week – although the ONS said that the August Bank Holiday would have caused a delay to registrations, meaning more would have been added in the most recent week.
The latest ONS monthly mortality bulletin also shows that in August there were 9.9 per cent more deaths than usual registered in England compared with the pre-pandemic average for the month, and 4.8 per cent more in Wales.
There were 3,769 excess deaths for the month, with Covid accounting for 2,232, leaving 1,537 unaccounted for. It is the highest percentage of extra deaths in a month since February.
Prof Kevin McConway, emeritus professor of applied statistics at the Open University, said: "It's still not clear, to me at least, what is causing these excess deaths.
"In England, the death rates for eight of the top 10 causes are actually lower than they were, on average, in 2015-19. The exceptions, where deaths were higher than average, are Covid-19 and the rather vague set of causes called symptoms, signs and ill-defined conditions."
Covid was the third leading cause of death in England last month, the highest ranking since March, new figures show. The leading cause was dementia and Alzheimer's disease (4,417 deaths), followed by ischaemic heart diseases (3,982 deaths).
Covid was the leading cause of death in England every month from November to February. In March it dropped to third place and then fell to ninth in April, 24th in May and 26th in June. But it rose to ninth in July before returning to third last month.