19 June 2021

The Four COVID-19 Numbers

  1. Worldwide infection count
  2. Local RT
  3. Population immunity
  4. Local cases under treatment
Worldwide infection count is the number we want to be zero for a year.  Then there's a party. That's a ways off and it's a goal of policy and diplomacy.  Don't worry about that one particularly right now.

Local RT is the transmission rate; on average, for everyone who has it, how many people catch it?  You want this number as low as you can get it.  Errors in measurement and wanting to minimize the number of cases -- every case is a chance for a new, worse, mutation -- means you should do everything you can to keep this number low.  Mask mandates, enforced mandatory quarantines, aggressive track and trace, and barriers to inter-regional movement are all important.

Population immunity is the number of people who won't catch it.  There's a statistical relationship between RT, R0, and this number; the consensus of the knowledgeable had it around 85% for the pre-delta variants.  (delta makes this number higher.  And remember that we can never guarantee an individual won't catch it, just make it less likely.)  That's 85% of the total population, fully vaccinated.  This is not a number we can reach with current vaccines, so just vaccination is helpful, but not sufficient.  We're not going to get the worldwide (or even local) infection count to zero solely through vaccination.

Local cases under treatment is the "will the healthcare system collapse?" number.  (If it does, you expect an order of magnitude more dead.)  This is the number you only care about if those responsible have made an intolerable hash of policy responses to 2 and 3.  Focusing policy on this number is an indication of complete failure.  (Also long term system damage; medical personnel take a long time to train.)

If you live somewhere the local RT number doesn't get more public attention than population immunity, or where the local cases under treatment has to be a policy consideration, you can be confident that the policy response is poor and will stay poor.  Plan on the assumption that the only things reducing your infection risk are the decisions you make.

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