Long after many states and localities dropped their mask mandates, the Centers for Disease Control and Prevention (CDC) finally revised its metrics to guide community Covid-19 prevention measures. Last week, the agency went from recommending indoor masking in all schools and roughly 95 percent of U.S. counties to recommending masks in about 37 percent of U.S. counties comprising 28 percent of the U.S. population. What took so long?
The new guidelines use three metrics of virus severity to determine what the agency calls “community levels” of disease—new Covid cases and new Covid hospitalizations (both per 100,000 population in the past seven days), and the share of hospital beds occupied by patients admitted for the disease. The guidelines classify counties as experiencing low, medium, or high levels. Everyone in a high-level county should wear a mask in public indoor settings, the CDC says, but indoor masks are not recommended, even in schools, in low and medium counties.
The agency had previously relied on just two metrics—new Covid-19 cases per 100,000 people and the test positivity rate, both measured over the last seven days—to measure the level of community transmission. Counties were ranked into four tiers—low, moderate, substantial and high—with indoor masking recommended for areas of “substantial” or “high” coronavirus transmission and for all schools, regardless of tier.
The rationale for the change is to move from measuring community transmission to measuring how much strain the virus is placing on a community’s health-care system. But the old system never made much sense. It took no account of vaccination rates, even though vaccines were initially highly effective against viral transmission. And even as effectiveness against transmission decreased, vaccines remained effective at curbing the severity of Covid-19 illness. Both old-guideline metrics—case rates and positivity rates—depended on the amount of testing in a community, which varied substantially based on access to testing and peoples’ willingness to be tested.
The previous regulations set arbitrary and overly cautious thresholds. “Substantial” and “high” transmission—50 to 100 cases per 100,000, or a positivity rate between 8 percent and 10 percent; and 100 or more cases per 100,000 people, or a positivity rate of 10 percent or higher— classified nearly the entire country as high or substantial risk and thus subject to mask mandates. Focusing on transmission also communicated very little about the risk Covid-19 posed to individuals and communities. The likelihood that infection would progress to more severe disease such as hospitalization or death was a function largely of the percentage of vulnerable elderly people and people with underlying medical conditions in an area.
From the outset of the pandemic, the CDC and other public-health authorities stressed the need to “flatten the curve,” justifying mitigation measures on the need to ensure that health-care resources were not overwhelmed. Transmission was, at most, indirectly related to that goal. The newly adopted metrics of hospital admissions and percentage occupancy rates would have been better tailored to preserving medical resources.
Moreover, even if the previous guidelines had measured community transmission reliably, monitoring transmission became less important as new, highly contagious variants such as Delta and Omicron spread widely throughout both the unvaccinated and vaccinated populations. With the Omicron surge, which began three months ago and peaked about six weeks ago, breakthrough cases—infections of people with either vaccine or natural immunity—became common. The level of community transmission was irrelevant: infection rates were high everywhere, but the overwhelming majority of cases were mild.
The CDC’s revised guidance is welcome and long overdue. Its original guidance was of limited value and became immaterial with the Delta and especially the Omicron waves. The CDC says that it follows the science, but it would be much better to lead.
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