Michael Baker. Photo: Supplied / Department of Public Health
This story has been updated since publication.
An infectious disease expert whose own child is among those potentially exposed to measles this week says he has confidence in the public health advice to schools.
Wellington Girls' College has told students in years 9 to 11 to stay home after a third measles case there, against official advice.
Hundreds of students were considered close contacts after a girl with measles was in class on Tuesday and attended prizegiving the next day.
The school said it had confirmed cases on Monday, Tuesday and another on Friday.
Otago University epidemiologist Professor Michael Baker said public health officials were constantly making "tough calls".
"They have to balance all of these competing factors and give the best advice they can. It may not always look optimal from the point of view of people who are peripherally involved, but I think it's important that they are just left to get on with it."
Officials would be basing their advice on a range of information from contact tracing and genome sequencing, which would help them work out chains of transmission and "where effort has to be applied to contain the outbreak", he said.
Generally, people who had received two doses of the measles, mumps and rubella (MMR) vaccine were "98 percent or 99 percent" protected from measles.
"It also gives what's called 'sterilising immunity', so you can't pass the virus on to other people, so there is no reason to exclude people from events or from going to school if they're vaccinated."
At the same time, schools were also having to make their own decisions based on competing factors, he said.
"It's not an enviable task trying to manage a large school when you're trying to protect your students, your staff and their families from this virus.
"And hopefully they're getting good advice from health officials and then they're balancing these various considerations and doing the best they can for the people they're looking after. So it's not easy for anyone involved."
Wellington Girls' College principal Julia Davidson said despite already having one confirmed measles case earlier in the week, the school went ahead with the prizegiving after being cleared to do so by public health.
This week they were opting for a more cautious approach, despite being told by the Ministry of Education they should keep the school open, she told Morning Report.
"We are ignoring their advice at the moment by keeping people at home for three days. We don't feel comfortable with that at the moment."
The school had staff members with babies too young to be vaccinated and immune-compromised relatives, and it also wanted to protect senior students doing exams.
"That was part of the reason the board decided to keep the other kids at home. Exams start on Tuesday [Tuesday] and we just wanted them to have a clean run."
Wellington Girls' College principal Julia Davidson. Photo: Supplied /
A letter from Director-General of Health Audrey Sonerson sent to health and education officials on Friday and seen by RNZ, said keeping students in the classroom remained "a key priority" while balancing the public health risk.
Audrey Sonerson. Photo: Public Service Commission Te Kawa Mataaho
"Where there has not been a case of measles discovered at a school, students should continue to attend school regardless of their vaccination status," she wrote.
When a case was found, the National Public Health Service (NPHS) would match school rolls with immunisation records.
Students who had not received two doses of MMR may be asked to stay home if they were close contacts of a confirmed case, "while fully immunised students should continue to attend school once their vaccination status has been verified by the NPHS".
"As director-general of health, I have communicated an expectation to NPHS that it will streamline this matching process to be able to inform schools, students and their families as quickly as possible to ensure children are not missing time at school."
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