NHS Lanarkshire has taken on extra staff to deal with its largest-ever flu programme after some patients were only informed of their appointment after it was due to take place.
The health board said it is working to rectify issues with the national booking system and postal system, which has added to the backlog.
Some patients received their letter to attend an appointment after it had taken place, with extra staff now taken on board to deal with the high volume of calls that has caused.
Dr Mark Russell, associate medical director at NHS Lanarkshire, said: “We know there has been a number of challenges with delivering the programme and we are working to rectify these.
“The programme will take longer than other years to deliver, however our plan is to deliver all vaccinations before the end of November and we are on track to achieve this.
“We know a number of patients have received their letter after their scheduled appointment.
“We would like to apologise for the frustration this has caused.
“There have been issues with the national booking system and postal system and these are being resolved.”
The health board added it is looking into different ways of reaching patients other than through letters.
It said it cannot provide details of individual appointments to those who call in, as this is arranged nationally.
Information is to be issued in “the coming weeks” on what patients should do if they have not received their letter.
Drop-in clinics have been cancelled as a way to manage the number of patients wishing to receive vaccinations.
More than 43,000 people across Lanarkshire have been vaccinated since the start of the flu programme four weeks ago.
The programme is due to take longer than other years to deliver, however NHS Lanarkshire believes all vaccinations will be carried before the end of November.
It comes as the health board said it is currently dealing with more Covid-19 hospital admissions than it did at the peak of the first wave of the pandemic.
Hospitals are now at 90% capacity and the NHS Lanarkshire area has the highest rate of infection across Scotland at 307.1 cases per 100,000 people.