Researchers from Public Health Scotland and the University of Edinburgh said there had been concerns that a weakened immune system may reduce the effectiveness of the coronavirus vaccines.
They used data from more than 100,000 people to determine its impact on reducing severe forms of the disease among the clinically extremely vulnerable.
The results showed the efficacy of a single vaccination dose in protecting against severe Covid-19 was as high or higher in those eligible for shielding.
But there was not enough data to judge the vaccine’s effect on those who have received a solid organ transplant.
The clinically extremely vulnerable are about six times more likely to experience severe Covid-19 leading to hospital admission or death, than someone the same age without their condition, the researchers said.
Dr Nicholas Phin, director of public health science at Public Health Scotland, said the results were “hugely encouraging”.
He said: “However, caution is still needed. Restrictions continue to ease and case rates remain high, meaning that all people, especially those who are at most risk, should continue to take extra precautions until fully vaccinated with two doses.”
Co-author of the study, professor Helen Colhoun of the Institute of Genetics and Cancer at University of Edinburgh, said: “Through using a pre-existing register that had been used to determine the eligibility for the national shielding support programme, we were able to identify those with certain conditions or on certain drugs that were likely to lead to extreme vulnerability to Covid-19.
“We found that in most categories of vulnerability, the vaccine was highly effective in reducing the risk of severe Covid-19.
“However, for those who have received a solid organ transplant, there was not enough data to demonstrate the vaccine’s effect due a low number of people in this group, meaning further studies are needed.
“In the meantime, we would still encourage recipients of organ transplants to get fully vaccinated if they have not done so already.”
Scotland’s deputy chief medical officer Nicola Steedman said the study provided “very welcome initial evidence” the vaccine works as well for vulnerable people.
She said: “While full details of the Covid booster vaccination programme are still to be finalised, we expect this will begin with those at highest risk this autumn, with our preparations already firmly underway for that.
“Around 96 per cent of those on the highest risk list in Scotland have received at least one dose of vaccination and 93 per cent have received both doses.
“We urge any of those who haven’t yet come forward to be vaccinated to do so now if you are able.”
Scotland’s national clinical director Professor Jason Leitch had warned scientists on Tuesday to expect Scotland to be hit by a ‘winter wave’ of Covid-19 and other respiratory viruses despite high levels of vaccination.
Prof Leitch said there would likely be a fourth wave, but it was unlikely to lead to another lockdown.