On Friday, Governor Newsom outlined a new blueprint for reopening California – county by county. Replacing the watch list is now a color scheme based on two metrics: case rate, and positivity rate (percent of positive tests). The color determines how businesses can operate within that county.
The governor described the new framework as “simple, stringent and slow.” But let’s delve into it a little further
State director of Health and Human Services Dr. Mark Ghaly said case rates and positivity rates are the best, earliest numbers on which to base reopening decisions. The first metric is new daily cases per 100,000 population – those who have been following posts here will recognize this rate. The threshold rate to move from purple to red is to drop below 7. Los Angeles County is currently at 10.49 (Figure 1 below), although there is likely at least a week of lag to this value.
So where do these color levels actually come from? They appear to be loosely based upon the Global Health Institute at Harvard’s key metrics for COVID suppression. Working backwards, they note that at a level greater than 25, a municipality has passed into uncontrolled spread and “will require the use of stay-at-home orders and/or advisories to mitigate the disease.” Los Angeles County levels exceeded 25 for the entire month of July (Figure 1).
At case rate levels between 10 and 25, community spread is accelerating at dangerous levels, most typically on its way upwards. Of interest, most counties will plateau at a rate between 1 and 10 but the Harvard group really only considers a region to be ‘on track’ at a rate under 1 – calling this the ‘green’ level.. “At the green level, jurisdictions are on track for containment so long as they maintain maintenance levels of viral testing and contact tracing, sufficient to control spikes and outbreaks. It is not enough to get to green; one also has to plan to stay green.” Los Angeles’ rate was last below 1 the week of March 13th (Figure 1).
Test positivity rate seems to have been taken from World Health Organization guidelines which recommends a positive test rate of less than 10% for viral containment. However, the countries most successful in containing COVID have rates of 3% or less. There is an important caveat to this recommendation made by the WHO – which is that “percentage positive samples can be interpreted only with comprehensive surveillance and testing of suspected cases.” The percent positive is, in fact, a critical measure because it gives us an indication how widespread infection is in the area where the testing is occurring—and whether levels of testing are keeping up with levels of disease transmission. To Governor Newsom’s credit, he has expanded testing to double its current capacity. There are two ways to decrease test positivity: 1. Fewer infections or 2. More testing. Either serve the same end.
Lastly, I do very much concur with the “slow” aspect of reopening. In this revamped plan, instead of 14 days with measurements below the mandated levels to move from one tier to another, it will now take 21 days. Further, while the state will require 21 days in order to lift restrictions, it will only take 14 days of too-high numbers to move to a more restrictive state. This shows that Dr. Ghaly and the State Health Department have learned something from the debacle that was the July surge. Counties like Los Angeles went blindly ahead with successive reopening steps without realizing or accounting for the mounting delays in test result returns. Therefore, once it became clear that COVID-19 was surging, it was too late to put on the brakes. 21 days should provide enough time to thoughtfully examine case data – given that case reporting lags during July in our clinic were 11-12 days at one point.
I do have two critiques of what is otherwise a very reasonable and thoughtful plan. Traditionally, infectious diseases are tracked through an SEIR model where individuals are categorized as: susceptible [S], exposed [E], infected [I], and recovered [R]. Thus far, the State of California has made no attempt to quantify those that remain susceptible. Such would require an extensive, population-based serosurvey – something that has been done in New York State. If I were pressed for an answer, I would estimate that 85-90% of the California population remains susceptible with significant geographic variance.
The second critique relates to contact tracing – which is the manner in which we identify those exposed [E] in the SEIR model. In Los Angeles County, contact tracing has been abysmal. Only 63.6% of overall contacts have completed their interview since the start of the pandemic and that number slips 60.8% of those exposures occurring within the last week. The current recommendation is that at least 90% of contacts for each new case must be traced within 48 hours in order to contain COVID. With such a low percentage of contacts successfully traced, it is unlikely that the state will be able to successfully identify and isolate sources of disease spread fast enough to prevent new outbreaks.