Coronavirus patients at West Hertfordshire Hospitals Trust (WHHT) spent an average of 13 days in hospital at the beginning of last summer, figures suggest.
The Nuffield Trust said Covid-19 patients require intense care from stretched staff and that despite positive signs, the NHS is still under "significant pressure" across England.
NHS Digital data reveals how long patients with suspected or confirmed coronavirus spent in West Hertfordshire Hospitals NHS Trust hospitals between March and November last year – during which time there were 1,685 admissions.
The median stay peaked for patients admitted in June – 13 days.
By contrast, patients admitted in March spent just three days in hospital on average.
The Nuffield Trust said national figures up to December showed the median hospital stay – used to exclude extreme values which could skew the average – was seven days for Covid-19 patients.
This is compared to five days for an average flu patient.
The WHHT is urging people to take the Government guidelines seriously and get vaccinated when invited to. The trust also wants to emphasise that with restrictions lifting, it is even more important to follow ‘hands, space, face’ guidance as coronavirus has not gone away.
Across England, coronavirus patients discharged in June had the highest median length of stay of 11 days.
Sarah Scobie, deputy director of research at the think tank, added: “Pressure on the NHS is not only from the hundreds of people still being admitted a day, it is also the length of time that Covid patients tend to stay in hospital.
"Longer stays do not only lead to pressure on the NHS, they are also more distressing for patients.
“Covid patients require more intense care, such as help with breathing, and recovery takes more time, so they will occupy hospital beds for longer, requiring more time from stretched care staff."
She added that treatment has improved and the number of cases and deaths have fallen amid the vaccine roll-out.
But she said the NHS is still under "significant pressure" and the number of Covid-19 patients will remain high for some time due to the large numbers of admissions during the second peak.
Edge Health, which provides data to NHS trusts, said there are many factors that could cause a rise in the average hospital stay.
George Batchelor, co-founder of the organisation, said: "It became both harder to discharge into the community – an attempt to avoid further spreading in care homes – and less urgent to discharge people from hospital that had many available beds.
"It is also important to remember that there has been improvement in treatments that mean some people who would have died quickly are now staying in hospital longer before going home."