AN 18-year-old Hobart woman with the meningococcal infection had been vaccinated against the potentially fatal bug, it was revealed last night.

Health authorities confirmed the young woman was immunised a few months ago.

However, they said no vaccine was 100 per cent effective and that people at risk should continue to be immunised.

The young woman was admitted to the Royal Hobart Hospital's intensive care unit on Sunday and it was confirmed yesterday that she had meningococcal infection.

She is now in a satisfactory condition in a hospital ward.

It is not yet known what strain of the infection she has.

Public Health acting director Avner Misrachi said all 18 cases of meningococcal this year and 24 cases last year had been either the B or C strains.

There is no vaccine yet to cover the B strain.

"If it is the C strain, no vaccine is 100 per cent effective, all vaccines offer from 90 to 98 per cent protection," he said.

"Even with measles, vaccines are 97 to 98 per cent effective and with meningococcal it [vaccination] offers about 90 per cent protection.

"If it turns out to be the C strain that she has, she could have had a much, much more serious infection if she had not been immunised."

Dr Misrachi stressed that although there was a small chance a person could still get meningococcal even if they had been immunised, the symptoms were usually less severe.

He said the young woman had spent only a short time in intensive care and had not been seriously ill.

"It is similar to when people receive a flu immunisation yet still get the flu. They usually get a milder dose," Dr Misrachi said.

"Vaccines are very safe and effective."

Dr Misrachi warned Tasmanians to continue to be vigilant especially during late winter and early spring, the peak period for meningococcal.

Early signs of the infection include headaches, neck stiffness, fever, drowsiness, nausea and skin rash.

For more information, people should contact their doctor or the Public Health Service Hotline on 1800671738.

The Mercury