The Diamond Princess cruise ship is a unique COVID-19 transmission case because of the high testing capacity and the confined environment. This exploratory study aims to raise the hypothesis regarding the role of poor ventilation systems in the spread of COVID-19 by analysing count data collected by the onboard clinic during the outbreak, and considering the deck plan and design of the air conditioning system of the ship. Observed symptomatic infection rate after day 5 (incubation period median day) of the quarantine, in cabins without previous confirmed cases are compared to that in cabins with previous confirmed cases. Accordingly, the observed symptomatic infection rate in cabins without a previously confirmed case (1.2%) was higher than for cabins with a previously confirmed case (0.8%); however, the difference was not statistically significant. In addition, age did not appear to be a confounding variable. Airborne transmission of COVID-19 through the ventilation system onboard could explain the higher than expected virus spread into cabins without previously confirmed cases during the quarantine period; thus, this study provides further potential evidence of coronavirus transmission by aerosols. Conflicting results from other studies involving the Diamond Princess outbreak are also discussed in light of our results.