English measles outbreak which began early last month has now affected 13 Otago people aged from 4 to 22 years.
Medical Officer of Health for Otago Southland Dr John Holmes said new cases could all be linked to the original four which turned up in an unvaccinated family which had travelled to Vietnam in January.
Dr Holmes said he was keeping an open mind on the possibility of more cases and it was important that if doctors thought an illness was measles that they ordered relevant blood tests.
The illness is considered rare in New Zealand, with 12 cases recorded last year.
Three of the new cases occurred in Logan Park High School pupils.
Parents and caregivers were notified last week by letter and provided with information about the illness.
None of those people who had contracted the disease had required hospital treatment, Dr Holmes said.
People needed to be aware they could have flu-like symptoms before the rash appeared (between the third and seventh day of the illness).
They are infectious from one day before the beginning of the first day of illness until four days after the rash appears.
The incubation period for the illness is about 10 days, but it can take longer than that for the rash to appear.
Logan Park High School principal Jane Johnson said one of the year 10 boys who had contracted the illness had been with a group on the Spirit of New Zealand yacht and it was a matter of establishing who within that group had not been immunised.
The parents concerned had been mindful about watching their children for signs of the illness.
The family involved in the original outbreak was not from the school.
Parents did not appear to be "at all over-anxious" about the matter.
The school had acted responsibly and was working closely with its public health nurse .
She was "pretty optimistic" the school was on top of the problem.
Dr Holmes said it was a good time for parents to check their children's vaccinations were up to date.
About 85% of those born since October 2005 have been vaccinated.
Babies are considered protected by their mother's antibodies and do not receive their first vaccination until 15 months, followed by another dose at four years.
