A third baby has died from whooping cough and there are concerns that the disease is spreading undetected in New Zealand.

Te Whatu Ora said all three infants who died from the disease were under a year old. The latest death was not related to two previous two cases reported in February.

National Public Health Service clinical lead Dr William Rainger said there had been 11 cases of whooping cough reported in New Zealand so far.

“The ratio of fatalities to identified cases is much higher than in previous years, suggesting there may be undetected spread in the community,” he said.

Typically, between one and two out of every 100 babies under the age of one who are hospitalised with whooping cough will die.

Reported cases remained low, but the deaths were an “urgent reminder” that whooping cough was a serious illness, Rainger said.

“Parents should seek medical advice for their baby or young child if they have a cough that ends with a ‘whoop’ sound or vomiting.”

“The best protection is to ensure that pregnant people, babies and those who will spend time with babies are all immunised.”

Immunisation rates plummeted in New Zealand during the Covid pandemic and are yet to recover.

In December, 82.4 per cent of 24-month-olds were up to date with their vaccinations - well below the 95 per cent rate required for herd immunity. Among Māori children, the rate was 66.4 per cent.

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Whooping cough immunisations are routinely given to pregnant people at 16 weeks, and infants are immunised at six weeks, three months and five months. Boosters are given at 4 years and 11 years. The injections are free for under-18-year-olds, pregnant people, high-risk patients, and adults between 45 years and 65 years.

Pertussis, also known as whooping cough, is spread by coughing and sneezing and infectious people can pass it on a week before their symptoms start.

Initial symptoms include a runny nose, cough and fever. After a week to 10 days, the cough becomes more severe and may end with a “whoop”, dry retching or vomiting.

If it is diagnosed early, treatment with antibiotics can reduce the infection period to between two to five days. Left untreated, patients can be infectious for up to three weeks.

Te Whatu Ora said that with school holidays and the long Easter weekend approaching, people who are unwell should avoid visiting young babies. Anyone who lives with a baby and had a new or worsening cough, sneezing, runny nose or a fever should self-isolate if possible.

Immunisation is available at medical centres, Hauora and Pacific health providers and at some pharmacies.