Melbourne has temporarily shut down one of its quarantine hotels for deep cleaning after several different people seemed to catch it from within the facility.

The 'working theory', according to Victorian chief health officer Brett Sutton, is that the virus may have spread further than expected because someone with COVID in the hotel had to use a nebuliser for medical reasons.

It's one of a couple of times in the past week or so that someone's caught the virus in a quarantine hotel, apparently without any close contact with an active case.

So on today's Coronacast, what does this mean for our understanding of how the coronavirus spreads through the air?

Also on today's show:

* Do quarantine hotels have an airflow problem?

* How did a NSW returning traveller test negative twice, then positive after their 14-day quarantine?

* What did the WHO fact-finding team discover about the origins of the virus on their trip to China? Is there any chance it escaped from a lab?

* Do vaccines for other diseases only prevent severe illness and not infection, like we keep hearing about coronavirus vaccines?

(NOTE: This episode was recorded before two additional cases were announced by the Victorian Government)