More than 5,000 Covid vaccination appointments have been booked online since people aged between 65 and 69 were included in the roll out programme.
The Department of Health confirmed the figure on Thursday after launching the service the previous evening.
Slots for people aged 65 and 69 were opened for one of Northern Ireland's seven regional vaccination centres which had only provided vaccinations for health care workers.
After the booking site became active on Wednesday evening, some users posted on social media that they were having difficulties making appointments.
The Department of Health tweeted: "Since going live last night over 5,000 people aged 65-69 have booked their #COVID19 vaccination appointment online.
"Demand for the online system remains high, please be patient."
Dan West, chief digital information officer for health and care, addressed the issues saying some had problems with the verification system due to the high level of people trying to access the site.
"Delays in SMS/email causing time-out of verification," he said.
"We’ve temporarily extended validity of codes and added extra capacity. Bear with us while we get this sorted."
The Department of Health has announced a dual approach to accelerating the vaccination programme, meaning GPs will be focusing on the 70 plus age group, while the seven regional vaccination centres will in tandem be offering offer vaccination appointments to 65-69 year-olds.
Confirming this, the DoH added: "Those aged 70 and over will be contacted by their GP about vaccination."
You can book a vaccine appointment here.