An outbreak of the measles virus has hit the Waikato with eight confirmed cases and a further three possible cases reported.

Waikato District Health Board (DHB) has been notified of 18 possible cases of measles. Eight cases were confirmed, three are under investigation and seven have been determined not to be measles.

The DHB has warned the outbreak could spread further due to several known incidents of infected people attending large events and schools while ill.

One person who tested positive for measles attended the regional Tainui Waka Kapa Haka festival, held on April 16 in Hamilton, where around 5000 gathered.

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The DHB said any secondary cases from the festival would be expected to be developing symptoms this week.

There have been no confirmed cases yet, but attendees should look out for symptoms until April 30.

Another person who tested positive for measles attended Nga Taiatea Wharekura School while infectious.

The first symptoms of measles are fever, runny nose, sore runny eyes, a cough and, later on, a rash.

Of the eight confirmed cases, all patients were treated in the Waikato Hospital Emergency Department.

Three required hospitalisation, but have been since discharged.

The measles virus is highly infectious and can be very serious. It is spread by tiny droplets in the air and is one of the few diseases that can spread easily to those nearby.