Dr Tedros Adhanom Ghebreyesus believes there is still a lack of evidence to show booster jabs are needed.
Vaccine teams at the Edinburgh International Conference Centre will deliver their final injections on September 19.
Over 66% of adults have had at least one jab as of Friday.
The milestone was reached on Wednesday just five weeks on from when doses reached one million.
NHS chief apologizes after pensioners are left standing in cold waiting for their vaccinations.