The drop-in clinic was scheduled to take place between 10am and midday for children aged five to 11, and from 12pm to 4pm for those aged 12 and over.
But people who arrived before 2pm found the clinic closed.
'I travelled to Campbeltown for my vaccine after phoning NHS Highland to confirm the clinic was on, only to find no sign of it,' said one person.
'Even staff I was able to find at the hospital didn't seem sure if there was a clinic or not. It was a shambles.
'Eventually, someone checked the ward and said there was no one there.
'And I wasn't the only one there trying to attend the clinic.'
Another person said: 'At least if there had been a sign up to say it had been cancelled or cut short, it would have explained things but it was frustrating to have to walk away not knowing what was what.'
An NHS Highland spokesman said: 'Due to an administrative error, the Covid vaccination clinic at Campbeltown Hospital on August 13 closed at noon rather than at 4pm which was the advertised time.
'We would like to apologise to the members of the public who turned up in the afternoon for their vaccination.
'We have arranged an additional drop-in clinic at the hospital for Thursday August 25 from 4pm to 6pm.
'This is open to all eligible individuals who have still to receive their 1st, 2nd dose or 1st booster.'
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