Health experts are urging pregnant women to get vaccinated for whooping cough, amid fears the national outbreak of the disease could last years and affect thousands more people.

Whooping cough (pertussis) is caused by the bacterium Bordetella pertussis and is spread by coughing and sneezing. It is highly contagious and can cause pneumonia, seizures, brain damage and death in babies.

On December 1 the Ministry of Health declared a national outbreak with 1300 cases reported since January 1. Since then another 800 cases have been identified.

The last outbreak in 2011-2013 was particularly bad, infecting more than 11,000 people.

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Dr Anusha Ganeshalingham, a paediatric intensivist at Starship Children's Hospital, said 38 infants were admitted to the ICU with whooping cough during the outbreak, compared to 1-4 per year normally. Two died in the ICU, and a third before being brought to Starship.

"It is really distressing for our team when we are unable to save these babies and to see the absolute terror and then grief that the parents have to endure, and also the guilt they can feel when their baby dies from what is essentially a preventable illness," she said.

Since April 2017, Starship ICU has had three cases admitted, with one severe. Ganeshalingham said none of those babies' mothers had been told about the vaccine by their lead maternity carer.

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Pertussis symptoms in babies include high temperature, runny nose, sneezing and coughing, progressing to coughing fits followed by a characteristic "whoop" sound.

Eating or drinking may trigger a coughing fit, which can cause vomiting or the baby may stop breathing.

The Ministry of Health says pregnant women should get immunised between 28 and 38 weeks. Babies younger than six weeks can't be immunised, but the mother's antibodies pass through the placenta to protect the baby for the first few months of its life, Ganeshalingham said.

Babies should get their shots at six weeks, three months and five months. Siblings should get their booster shots at 4 and 11 years, and adults - especially parents - every 10 years.

Dr Helen Petousis-Harris, a senior lecturer in primary health care at the University of Auckland, said Kiwis were worryingly ignorant about the dangers of the disease.

A recent study found fewer than two thirds of people knew adults could pass the disease to infants.

Just one in four respondents were vaccinated and had received a booster shot. A further 29 per cent had not had a booster, while 46 per cent didn't know if they had been immunised at all.

"It's a tricky disease and one that puts a lot of stress on the public health system," Petousis-Harris said.

"It is critical that babies and pregnant mothers are immunised to provide a viable defence to the disease."