More frequent border testing and a stand-down period before cleaners can board an international flight after it lands are being mooted as further layers of protection against Covid-19. The suggestions follow the case of the Auckland airport worker, who appears to have caught the virus after cleaning a plane that arrived on April 10 with a passenger who later tested positive. An investigation is underway into how the cleaner became infected, and it remains unclear whether they had any person-to-person contact with any of the passengers. "It's a huge challenge to figure out how they got infected because they were doing all the right things with PPE and being vaccinated," Otago University epidemiologist Professor Michael Baker said.