Measles is a viral infection that is serious for small children but is easily preventable by a vaccine.

More than 2400 people travelling on the Interislander over the holidays may have been exposed to measles.

A child, whose family had chosen not to vaccinate, likely contracted the measles in Auckland, then crossed Cook Strait on the Interislander's Kaitaki ferry twice during the holiday period.

The child was on the 8.45am sailing on December 28 from Wellington to Picton and the 2.15pm sailing on January 6 from Picton to Wellington.

There were 1219 passengers on the first crossing and 1220 passengers on the second.

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KiwiRail, which runs the Interislander, became aware of the issue after the Canterbury District Health Board (CDHB) publicly notified the case on Tuesday.

The CDHB urged people who had not been vaccinated and may have come into contact with the child to isolate themselves.

"We are contacting all passengers and crew who travelled on the two affected sailings to provide health advice from the Canterbury District Health Board," KiwiRail tourism and marketing executive general manager Ahleen Rayner said.

JOHN NICHOLSON/STUFF A child with measles crossed the Cook Strait twice, potentially exposing more than 2400 people to the infectious disease.

A post on the folk music festival Facebook page said the child, a boy, was unwell during the festival "and as a consequence he had very little interaction with other children.

"Apparently he is now bouncing back well with medical care."

Canterbury medical officer of health Dr Alistair Humphrey said the child had previously travelled to Auckland, and it was likely they contracted measles there. He said the child was not vaccinated, and said it was the family's choice.

Humphrey said the child was taken to Moorhouse Medical Centre in Christchurch. A GP there suspected he had measles, and testing confirmed this.

The family were from the North Island and had been travelling over land on their holidays. Humphrey said they would have been in numerous locations, so the CDHB had alerted people only where they could confirm exact locations and times.

"This highlights the issue around summer time when families are moving around the country, he said.

"Our only protection from an infectious measles case is that we are fully vaccinated, that's all we can do."

WERE YOU EXPOSED?

The CDHB is asking people who may have been exposed to measles at the following locations to isolate themselves, unless they have had two MMR vaccinations or are over 50 years old:

- December 28 – Interislander Ferry Wellington to Picton, 8.45am to 12pm – remain isolated until January 11.

- December 30 to January 3 – Whare Flat Folk Music Festival (near Dunedin) – remain isolated until January 17.

- December 30 and 31 – ED at Dunedin Hospital between 8.30pm to 1am – remain isolated until January 13.

- January 6 – Interislander Ferry Picton to Wellington, 2.30pm to 5.45pm – remain isolated until January 20.

MEASLES FACTS

- Measles is a highly infectious viral illness spread by contact with respiratory secretions through coughing and sneezing.

- Symptoms of measles include: dry cough, runny nose, headache, a temperature over 38.5 degrees Celsius, and a red blotchy rash starting on day four or five of the illness, usually on the face and moving to the chest and arms.

- People are infectious from five days before the onset of the rash to four days after the rash starts.

- Infected people should stay in isolation – staying home from school or work – during this time.

- The best protection from measles is to have two MMR vaccinations. MMR is available from your GP and is free for eligible people.

- Anyone believing they have been exposed to measles or has symptoms should not go to ED, an after-hours clinic or their GP. Instead call your GP any time, 24/7 for free health advice.