WELLINGTON, N.Z. (Deutsche Presse-Agentur) -- Airlines and health authorities need to consider how they can prevent sick people from boarding flights, according to New Zealand researchers who said Wednesday there was a "small but measurable risk" of contracting swine flu in the air.

The research was based on New Zealand's first experience of pandemic influenza A/H1N1 virus in April 2009, when a flight arrived in Auckland with a group of infected high school pupils returning from a trip to Mexico.

Ten members of the group had symptoms when they boarded the flight in the United States, and two other passengers became sick after the flight and were also confirmed with swine flu.

The researchers said that the timing of their illness was consistent with exposure during the flight and the known incubation period for influenza A.

"The pandemic gave us a unique opportunity to investigate the risk of influenza transmission on a flight. Because this was an entirely new virus to New Zealand, we know that the only place it could come from was from other passengers on this flight," said Associate Professor Michael Baker of the University of Otago.

"Better exit screening at airports might help detect infectious cases so that they can be managed to reduce the risk of infecting others."

The research, led by Baker and Dr Craig Thornley of the Auckland Regional Public Health Service has just been published online by the British Medical Journal.

A university statement said it was the first scientifically documented outbreak of influenza on a plane in recent times.

"It's reassuring that infections were few in number and were only seen in passengers sitting close to infected people on the flight," Baker said.

"That suggests transmission by small droplets produced by coughing and sneezing rather than via fine aerosols carried through the planes air-conditioning system."

Copyright 2010 dpa Deutsche Presse-Agentur GmbH