Health guidance issued Tuesday recommends that fully vaccinated people wear masks indoors if they’re in areas with “substantial” or “high” transmission of Covid-19. But what do “substantial” and “high” actually mean?
The two terms are part of the US Centers for Disease Control and Prevention’s four-tiered system that measures the level of community transmission in each county: low, moderate, substantial and high. On the CDC’s website, you can run a search by state or county and click around the color-coded map to see how active the virus is where you live.
A county’s level of transmission is based on just two metrics: new Covid-19 cases per 100,000 people and the positivity rate, both measured over the last seven days. The basic idea is that these show how much virus is spreading around us, CNN Medical Analyst Dr. Leana Wen said.
“The level of coronavirus transmission is an important determinant of how much risk you’re in, if you’re vaccinated or unvaccinated,” she said.
Specifically, a “Low” transmission is considered no more than 10 cases per 100,000 people, or a test positivity rate of less than 5%. “Moderate” transmission is 10 to 50 cases per 100,000 people, or a positivity rate between 5% and 8%. “Substantial” transmission is 50 to 100 cases per 100,000, or a positivity rate between 8% and 10%, and “high” transmission is 100 or more cases per 100,000 people or a positivity rate of 10% or higher.
If a county has values in two different transmission levels, then the CDC uses the metric that is higher.
On the CDC’s map, low counties are represented in blue, moderate counties are in yellow, substantial counties are in orange and high counties are in red. Counties vary in size so it’s also a good idea to check a city, state or municipal health department website.
As of Wednesday, about 50% of counties have high transmission and 17% have substantial transmission, which covers wide swaths of the South and West, according to CDC data. About 27% of US counties are considered to have moderate transmission and only about 9% have low transmission.
Health officials say this new guidance, an update from May, reflects the latest science on the more transmissible Delta variant and evidence that suggests vaccinated people can still spread the virus. The vast majority of spread still appears to come from unvaccinated people, who are at much higher risk of severe illness that can send people to the hospital or kill them.
The guidance on mask wearing is meant to remind people who are fully vaccinated that they might be able to infect others, CDC Director Dr. Rochelle Walensky said Tuesday.
The strengths and weaknesses of the CDC’s system
Several medical experts took issue with parts of the CDC’s decision to update the mask guidance and its specific metrics.
For one, the CDC’s system solely is based on new cases and positivity rate – but not vaccinations, hospitalizations, deaths or any of the other relevant metrics experts have come to know since the pandemic began.
“If you were to ask me how I define a community that has high transmission, I say, ‘I don’t look at any one number,’” said Dr. Peter Hotez, dean of the National School of Tropical Medicine at the Baylor College of Medicine. “I don’t look at positivity rate, the per-100,000 rate, I don’t look at hospitalizations or deaths. I look at it all in aggregate to really get a sense of what’s happening on a really high level.”