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Outbreak on ship confirms the main route of contagion

Cruize ship in the port of Daikoku, Japan, in February 2020

Carl Court / Getty Images

The results of computer simulations of how COVID-19 spread on the Australian cruise ship Princess Diamond, one of the first outbreaks outside China reported at the beginning of the pandemic, have finally been published in a scientific journal. On February 20, 2020, a month after the cruise started, more than 700 of the 3,711 people on board tested positive for COVID-19. In July last year, a team led by Parham Azimi from the Harvard T.H. Chan School of Public Health, USA, released the preliminary results of the simulations. The researchers analyzed 21,600 different scenarios to explain how the disease spread on the ship. Each scenario tested hypotheses about the values of unknown variables that may affect the viability of virus particles outside the human body. The outbreak was best explained by scenarios that assumed more than 50% of infections occur through the inhalation of droplets emitted from the mouths and noses of others when speaking. Called aerosols, these tiny droplets remain suspended in the air for more than 20 minutes and can travel several meters in an enclosed space (PNAS, February 23).