Clusters and the meaning of Ro
The problem of dealing with isolated cases and clusters of Covid-19 may be a lack of understanding about the meaning of Ro.
The virus is thought to have an Ro of 2.2 - that is, one infected person infects 2.2 other people on average. The words 'on average' are important. One infected person can, in fact, infect a large number of people depending on the circumstances. For example, in a dormitory, a large family home, a crowded workplace, a crowded family home or a place where a group resides - a single infected person could infect large numbers. Ro is only 2.2 on average. It is almost certainly much greater than 2.2 anywhere where a greater number of people occupy a space for a longer time. So a single case is probably going to infect many others in a dormitory, ship, a workplace where workers must necessarily work close to others or a 'care home' where the space is shared by many. And the others will infect still others and so on. And some of the new infections will be asymptomatic and undetected. It is probably unwise to keep those infected cases, or their contacts, together (in the dormitory, workplace or care home) because the Ro may be considerably higher than 2.2 in those circumstances, and even a 90% mitigation of viral spread will not reduce the Ro adequately.
It may be that the best way to deal with a cluster is to nip it in the bud early by removing all the people in question from the space and isolating and monitoring them separately elsewhere. Everyone in a risk prone space should probably be considered a cluster as soon as a single infected person is discovered there.
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Dr Michael Cole FRACP LLB