Unions representing nurses and doctors say more should be done to encourage experienced staff to stay in their jobs, as they warn that tens of thousands of vacancies are still unfilled across England.
NHS Digital figures show that 2,656 professionally qualified clinical staff were working at Northern Lincolnshire and Goole NHS Foundation Trust in January – 35 fewer than in January 2020.
The number of doctors working for the trust fell by 21 to 559 and the number of nurses and health visitors dropped by two to 1,319.
Other clinical workers include midwives, ambulance and technical staff.
The numbers refer to full time posts, rather than individual staff members.
Across England, the number of professionally qualified clinical staff make up more than half of the hospital and community health service workforce.
In January, the number of such workers increased by 4.2 per cent, including an extra 9,800 nurses and health visitors and 6,400 doctors.
However, separate figures show that there were 36,200 vacant nursing jobs and 7,000 vacant medical posts in the three months to December.
Dr Helena McKeown, chairman of the British Medical Association, said: "Despite there being some minor increases in staff numbers, this does not go anywhere near far enough to fill the known, as well as unknown, vacancy gaps that currently exist within the NHS. We are in the midst of a serious staffing crisis.
“Given the demands of Covid and the enormous backlog of care the NHS faces – the largest ever – with patient demand outstripping staffing levels, the situation has become even more acute."
Dr McKeown added that a survey by the BMA revealed thousands of doctors are already planning to leave the NHS, as they are struggling to cope with demands.
The union wants the Government to implement measures to retain staff, as well as expanding the medical workforce.
"The BMA believes that the number of medical school places must double over the next decade to ensure workforce supply can match growing patient demand,” said Dr McKeown.
The Royal College for Nursing warned it was a false economy to recruit more nurses, without keeping the experienced staff to train them.
Pat Cullen, RCN acting chief executive, said: "Every unfilled nurse job piles the pressure on other staff and jeopardises safe care for patients.
“If ministers want to not only retain those experienced nurses as well as recruit the new nurses needed, they must consider the level of reward and recognition.
“Failure to act now will make it even harder for services to recover after the pressures of the pandemic."
An NHS spokesman said the excellence of existing staff had inspired a 35 per cent increase in nursing degree applications as well as the increase in number of nurses, doctors and healthcare support workers.
“NHS staff have faced a year like no other and their response to the pandemic, including caring for 400,000 people with Covid in hospital as well as keeping other routine services going for patients, has been extraordinary.
“The NHS continues to increase support for staff including a 24/7 health and wellbeing text support line, rapid access to mental health services, and more opportunities for flexible working."