The Houston Chronicle Cites Iffy Data To Make the Case for Mask Mandates in Schools
“Are mask mandates helping Houston school districts slow the spread of COVID-19?” asks the headline over a Houston Chronicle story published today. The data presented in the article do not answer that question, although it is clear that the authors—Hannah Dellinger, Julian Gill, and Alejandro Serrano—really, really want the answer to be yes. In that respect, they resemble most mainstream news reporters, who take it as a given that “universal masking” in K–12 schools plays an important role in reducing virus transmission, then cast about for evidence to support that assumption.
“Houston-area school districts that require students and staff to wear face masks generally have experienced fewer COVID-19 cases,” Dellinger, Gill, and Serrano report. “The numbers, which offer a snapshot in time of active cases, appear to reinforce what doctors have been saying for months: requiring or strongly encouraging masks can help reduce the spread of the virus, more so when paired with other strategies, such as vaccines and social distancing.”
That is a pretty slippery summary. School districts with mask mandates “generally have experienced fewer COVID-19 cases,” except when they haven’t. Dellinger and her colleagues think the numbers “appear to reinforce” something, but it’s not clear what that thing is. Is “requiring” masks necessary, or is “strongly encouraging” them enough? Were these “doctors” talking specifically about masking in K–12 schools, or were they referring to the general question of whether face masks “can help reduce the spread of the virus”? And if mask mandates are usually “paired with other strategies,” how can the effect of this particular precaution be isolated?
Dellinger et al. focus on “active” COVID-19 infections reported by school districts last Thursday. What does “active” mean? That varies from one school district to another. The Chronicle notes that “cases in some districts are considered active for 10 days after a positive test, the length of the recommended isolation period, regardless of an individual’s actual condition.”
Did these cases result from in-school transmission? Not necessarily, which undermines any attempt to infer a causal relationship between a district’s mask policy and its infection rate. Dellinger and her colleagues note that “not all communities have the same rate of transmission outside of classrooms.” If school districts without mask mandates tend to be located in communities with higher general infection rates, that could help account for differences that might otherwise be attributed to a lack of face coverings in school.
Similarly, vaccination rates vary widely across the Houston area. In Fort Bend County, for example, the Texas Department of State Health Services reports that 74.5 percent of residents 12 or older are fully vaccinated. In Waller County, by contrast, the rate is 42.8 percent. The rate for Harris County, where Houston is located, is 63.9 percent.
These county-level data obscure differences within counties. Harris County, for example, has 4.7 million residents and covers 1,777 square miles, encompassing communities with different demographics and population densities, ranging from Houston to small towns and unincorporated areas. To complicate matters further, some school districts include parts of different counties with different vaccination rates.
Even based on county-level data, it is clear that failing to take vaccination rates into account could skew any attempt to measure the impact of mask mandates, especially if school districts that don’t require masks tend to have lower vaccination rates, which
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