NHS staff responded to a life-threatening ambulance call out every 33 seconds in December – the highest number on record.
In total, more than 66,000 NHS staff were off work each day in December amid the rapid spread of the Omicron variant.
Separate data published on Thursday showed around 46,000 staff were off sick each day last week as a result of Covid – up from 12,000 per day at the start of December.
The average response time in December for ambulances dealing with the most urgent incidents – defined as calls from people with life-threatening illnesses or injuries – was nine minutes and 13 seconds.
This is just under the nine minutes and 20 seconds in October which was the longest average response time since current records began in August 2017.
Meanwhile, nearly one in four patients arriving at hospitals in England by ambulance last week waited at least 30 minutes to be handed over to A&E departments.
Around 18,307 delays of half an hour or more were recorded across all hospital trusts in the seven days to January 9.
Health Secretary Sajid Javid on Thursday announced the Covid self-isolation period would be cut to five full days to help ease staff shortages in the health service.
From Monday, people can test twice before they go – leaving isolation at the start of day six if they return a negative result.
Matthew Taylor, chief executive of the NHS Confederation, hailed the move as “pragmatic” and said it would help NHS workers return to the frontline.
Responding to the figures, NHS National Medical Director Professor Stephen Powis said: “Omicron has increased the number of people in hospital with COVID at the same time as drastically reducing the number of staff who are able to work.
“Despite this, once again, NHS staff pulled out all the stops to keep services going for patients – there have been record numbers of life-threatening ambulance call outs, we have vaccinated thousands of people each day and that is on top of delivering routine care and continuing to recover the backlog.”