The clinically extremely vulnerable should all be vaccinated by early next month, Northern Ireland's chief medical officer has said.
Appointments for people with conditions like some cancers have to be carefully planned because their needs can be complex.
Some people receiving chemotherapy may need to have their treatment adjusted, Dr Michael McBride added.
"We are making massive progress, we are rolling out the vaccine," he said, with the over-80s group almost completed.
Dr McBride told BBC Radio Ulster's Stephen Nolan Show: "I am confident that we will have completed over-70s and clinically extremely vulnerable through February and perhaps into early March.
"There is a huge number of clinically extremely vulnerable."
Those at particularly high risk from coronavirus include recipients of organ transplants, people undergoing radiotherapy, or those with a severe lung condition like cystic fibrosis.
Dr McBride said the health service is making significant progress in inoculating the over-65.
He added: "We are ahead of schedule, it is a tribute to everyone involved."
There have been instances of people who are ineligible trying to book appointments by telephone and then abusing staff when turned down.
Dr McBride added: "Please be patient, we will get to you.
"We will get this vaccine to you as quickly as we possibly can, subject to the supply of vaccine."
Northern Ireland's Department of Health said on Friday that people who received a shielding letter because they are clinically extremely vulnerable to Covid-19 can now book a vaccination appointment.
The appointments will be available at seven regional vaccination centres.
Online and telephone booking for those aged 65-69 were introduced late last month, with a surge of initial appointments made.
Demand has slowed since then.
As of today, 401,966 vaccines have been administered in Northern Ireland, of which 373,414 were first doses and 28,552 were second doses.
It comes as a further nine people have died in after testing positive for Covid-19.
Another 407 people have tested positive for the virus, the Department of Health confirmed on Friday.
There are 488 Covid-positive patients in hospital, including 58 in ICU.