Fast-acting parents are thought to have curbed the spread of whooping cough from a Christchurch primary school.

About 120 students were off sick at Paparoa Street School last week following a whooping cough outbreak that began with an unvaccinated child.

Canterbury Medical Officer of Health Alistair Humphrey said the outbreak started with an unvaccinated family.

"They've been very good, they've now gone and got themselves vaccinated. They've realised how bad this infection is, so it's very, very important for as many of us as possible to get vaccinated."

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The number of potential cases was starting to drop off, "because of the fantastic way in which the school have jumped on it so quickly, and the parents have kept their sick kids at home, we haven't seen it spread beyond the school," Mr Humphrey said.

"We're not out of the woods yet. I'm hoping by next week we'll see the numbers come down considerably."