The Shoestring Covid tracker uses the CDC community transmission levels released as part of their Feb. 12 2021 guidance for school reopening. They are based on a combination of a per capita weekly new case rate and the test positivity rate, as shown in this table:
The Shoestring Covid tracker uses just the new case rate, since the test positivity rate is often not available, and because it usually wouldn’t matter (e.g. it would be very unusual to have a “low” new case rate and the greater than 5% positivity that would change the classification to “moderate”).
These transmission rates can be used for decision making, for communities, businesses and institutions, or individuals. For example, from July 27th 2021 until Feb. 25th 2022, the CDC recommended that masks be worn indoors in communities with substantial transmission or greater, that is, with 50 or more new cases per 100K in a week. On Feb. 19th 2022, Bob Wachter, Chair of the UCSF Department of medicine, published a Twitter thread explaining his reasoning for maintaining a similar level (10 per 100K per day = 70 per week) as a threshold for indoor mask wearing, even given the changed circumstances from mid-2021. (Update 3/24: this piece from Inside Medicine supports the 50 per 100K threshold for universal indoor masking).
On Feb. 25th 2022, the CDC released new community levels and masking guidance. The new metric uses a single new case rate threshold of 200 per 100K per week, combined with the per capita rate of new Covid-19 hospital admissions, and the percentage of staffed hospital beds occupied by Covid-19 patients, to classify communities as having Low, Medium or High levels. The indoor mask recommendation applies for communities with a High level, which is reached when new cases exceed the 200 per 100K per week rate, and there are either 10 or more new admissions per 100K, or 10% or more hospital beds occupied by Covid-19 patients. This is four times the previous new case rate threshold, plus an added hospitalization rate requirement.
The CDC 2022 guidance is controversial. On the day it was released, the president of the AMA issued a statement that included the following:
But even as some jurisdictions lift masking requirements, we must grapple with the fact that millions of people in the U.S. are immunocompromised, more susceptible to severe COVID outcomes, or still too young to be eligible for the vaccine. In light of those facts, I personally will continue to wear a mask in most indoor public settings, and I urge all Americans to consider doing the same, especially in places like pharmacies, grocery stores, on public transportation…Gerald E. Harmon MD, President American Medical Association, Feb. 25 2022, https://www.ama-assn.org/press-center/press-releases/ama-statement-cdc-covid-19-updates
The new levels are of limited value for individual decision making. For example, at the Medium level, the guidance states that “[I]f you are immunocompromised or high risk for severe disease [t]alk to your healthcare provider about whether you need to wear a mask and take other precautions (e.g., testing)”. The Medium level could occur with any new case rate. Presumably, a healthcare provider’s advice on masking should take the community transmission level into account, but the new CDC guidance provides no basis on which that could be done. In addition, individuals may want to take precautions as new case rates rise, before hospitalizations begin to increase and trigger a change in the CDC 2022 community level. Furthermore, the new CDC guidance gives no help in determining the circumstances under which individuals might want to wear masks to protect community health, as the president of the AMA urges us to do in the above statement.