In September, a man boarded a flight from Dubai to New Zealand. Not 48 hours before he had tested negative for Covid-19. Days after disembarking, while quarantining in compliance with national containment policy, he and six other passengers of the 86 on board were confirmed to be infected with the virus SARS-CoV-2.

According to a preprint case study released by New Zealand health authorities, at least four of the infections occurred in-flight—all tracing back to one man who, at least by the time he stowed his belongings and took his seat, was presymptomatic but shedding active virus. The chain of transmission was confirmed afterwards via genomic analysis.

Incoming travelers to New Zealand are required by law to quarantine for two weeks in designated hotels upon arrival, where they will be monitored and tested at least twice before being released. The samples of those who test positive are isolated, genomically sequenced, and compared to determine their exact origins. This was how health authorities were able to identify the four passengers who contracted the virus in-flight, as well as the man who gave it to them.

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Originally published on Forbes (November 23, 2020)