24 hours in Marin: 844 tests, 18 new confirmed cases (10 in San Rafael; 7 in Novato; 1 in Ross Valley towns), plus one in San Quentin. 0 new hospitalizations; 0 deaths
The infectious spread of a disease, like COVID, is measured by R, the infection rate. At its heart, the idea of R is very simple. R is the number of people that an infected person will get infected. When a disease first shows up, as COVID did a half-year ago, there’s no immunity, no vaccine, nothing. R then is called: R(0), pronounced R naught. R(0) for COVID was about 5 or 6 - each infected person would transmit the disease to five or more people. (Typically for season ‘flu it’s between 1 and 2; measles is scary - R(0) of 15 or higher!) This high R(0) is why the COVID initial statistics showed a 25% increase in infected people PER DAY!!!
As the contagion progresses, R drops and is replaced by R(t) … t stands for time. Why does it drop? Two major reasons, in the absence of vaccines, etc: First, if a significant number of the people in a crowd already have it (and if they cannot be re-infected), then the infected person can’t spread it to them. Second, because the people in the crowd, hopefully, can take precautions to prevent infection. (Widespread use of vaccines are incredibly powerful at limiting R, but who knows when one will be safe and effective for COVID?)
If R stays significantly above 1, it’s spreading. If it drops to 1 - each infected person only transmits the disease to one other person - and the contagion stays but doesn’t grow. If it drops below 1, the disease is on its way to extinction.
So, where is R now?
Marin County: Perhaps 1.2
Dr. Matt Willis, the county’s director of pubic health, used this estimate about 10 days ago in one of his presentations. I don’t have data to back this up or negate it. He also said that Marin had the 4th highest R in the state.
California: About 1.1
I’m not aware of a statement from the state of its R(t), so am relying on external sources. The two I like best are rt.live (run by one of the founders of Instagram) and covid19-projections (run by a machine learning grad student at MIT). rt.live is currently offline as its model is being rebuilt, but was showing - implausibly - R(t) of 0.98 yesterday. covid19-projections is currently showing California with R9t) of 1.1.
R is not trivial to calculate. There are a variety of entities - those I mention here, plus Imperial College, Johns Hopkins and others - that have published their tools (and at least one of them runs beautifully on my PC!) And just about everywhere is publishing the raw data on COVID in computer-readable form (when I refer to sources as .csv format, that’s comma-separated variables, a form used for transferring data for computers - the Los Angeles times and NYTimes maintain data repositories for this purpose). But I’m relying instead on entities with better tools and better capabilities than my own.