Lacks context : The CDC study also looked at the impact of policies allowing indoor dining, finding that these were associated with an increase in COVID-19 cases and deaths. Not providing this information doesn’t give the full context about the CDC study.
RECLAMACION: “The CDC has admitted face masks do little to prevent the spread of COVID-19.”; “In a new study, the CDC found face masks had a negligible impact on coronavirus numbers that didn’t exceed statistical margins of error.”; “The CDC added it still recommends wearing face masks, although it admitted such mandates do not make any statistical difference.”
REVIEW
This article published by the One America News Network (OAN) on 7 March 2021 claimed that a recent study from the U.S. Centers for Disease Control and Prevention (CDC) found that masks had a “negligible impact on coronavirus numbers”. The 5 March 2021 study referenced by OAN was published by Guy et al. [1] in Morbidity and Mortality Weekly Report (MMWR), the CDC’s weekly journal. Contrary to the article’s claims, the CDC study found the opposite effect: that wearing face masks reduced the spread of COVID-19.
The CDC study analyzed the association between COVID-19 cases and deaths that occurred between 1 March and 31 December 2020 and two policies: state-issued mask mandates and allowing on-premises restaurant dining. Thirty-nine U.S. states and Washington D.C. issued mask mandates starting in April 2020. While 49 U.S. states and D.C. prohibited on-premises dining in March and April 2020, all lifted these restrictions by mid-June.
The authors of the CDC study evaluated county-level data on mask mandates and restaurant closures as well as data on county-level COVID-19 cases and deaths from state and local government websites. There are over 3,000 counties in the U.S. and, depending on state laws, a county can decide to opt out or enforce a state-issued mask mandate. Such was the case in Kansas where, when a state-wide mask mandate was issued on 3 July 2020, 81 counties (77 percent) opted out.
The study used regression analysis, which is a statistical test that researchers use to determine relationships between variables in a data set, to examine the relationship between county-level policies and COVID-19 cases and deaths.
The association between COVID-19 case and death data and policies was examined over five 20-day periods following implementation (1-20, 21-40, 41-60, 61-80 and 81-100 days following implementation) and compared to two pre-implementation periods (60-41, 40-21 days before implementation).
In terms of mask mandates, the CDC study found that they were associated with a slower growth rate in COVID-19 cases during each 20-day time period. Mask mandates were associated with a 0.5 percentage point decrease in the number of COVID-19 cases during the first period, followed by 1.1, 1.5, 1.7 and 1.8 percentage point decreases for the subsequent four 20-day periods. The same trend was observed for COVID-19 deaths growth rates, where mask mandates were associated with a reduction in the growth rate of COVID-19 deaths. Note that these reductions are “percentage point” reductions on the growth rate; many people misunderstood these as meaning that masks only reduce the number of cases by about 1%, but that is not what it means.
The OAN article erroneously stated that the CDC study found that “face masks had a negligible impact on coronavirus numbers” and that this impact “didn’t exceed statistical margins of error”. In fact, the CDC study found that “mask mandates were associated with statistically significant decreases in county-level daily COVID-19 case and death growth rates within 20 days of implementation”.
The CDC study also found that policies allowing on-premise restaurant dining were associated with increases in the growth rate of COVID-19 cases and deaths. Specifically, allowing on-premises dining was associated with a significant percentage point increase in COVID-19 cases for three 20-day periods after the policies were implemented (0.9% at 41-60 days, 1.2% at 61-80 days, 1.1% at 81-100 days) and COVID-19 deaths for the last two 20-day periods (2.2% at 61-80 days, 3.0% at 81-100 days).
Though the OAN article does not mention the association between policies that allow in-person dining and the increase in both COVID-19 cases and death growth rates, some researchers pointed out the importance of this finding. In a conversation with Mike Stobbe of The Associated Press, William Hanage, a professor of infectious disease evolution and epidemiology at Harvard University, said the study “makes the case quite strongly that in-person dining is one of the more important things that needs to be handled if you’re going to control the pandemic”.
Joseph Allen, the director of the Healthy Buildings Program at Harvard T.H. Chan School of Public Health, told the New York Times’ Roni Rabin that the results of the CDC study were not surprising. “What’s surprising is that we see some states ignoring all of the evidence and opening up quickly, and removing mask mandates and opening full dining”. In the U.S. states like Texas and Mississippi recently lifted mask mandates and many cities, such as Portland in Oregon and New York City, are easing restrictions on indoor dining.
The CDC study doesn’t establish cause-and-effect relationships between the policies and COVID-19 numbers, but the regression analysis showed an association between mask mandates and decreases in the growth rates of COVID-19 deaths and cases, while also showing that policies permitting on-premises dining were associated with an increase in the growth of COVID-19 cases and deaths. Hanage told The Associated Press that “it’s always very, very hard to thoroughly nail down the causal relationships”. But he added that the study, when taken together “with all the other stuff we know about the virus, supports the message” for continued mask usage and the risks of indoor dining.
The CDC study builds on previous research showing that wearing face masks are effective at reducing the spread of COVID-19, including one study that estimated that the implementation of face masks in the U.S. prevented more than 200,000 COVID-19 cases by May 22, 2020 [2], and the issues with indoor dining, which is a known risk factor associated with SARS-CoV-2 infections [3].
In summary, the OAN article misinterprets the results and conclusions from the CDC study, leaving readers with the inaccurate notion that mask mandates have no impact on the spread of COVID-19 and that the study’s results were not statistically significant. Furthermore, it distorts the CDC’s position on face masks by writing that the CDC “admitted face masks do little to prevent the spread of COVID-19”. On the contrary, at a press briefing at the White House on 5 March 2021, CDC Director Rochelle Walensky said the study demonstrated that “you have decreases in cases and deaths when you wear masks, and you have increases in cases and deaths when you have in-person restaurant dining.”
REFERENCES
- 1 – Guy Jr et al. (2021) Association of State-Issued Mask Mandates and Allowing On-Premises Restaurant Dining with County-Level COVID-19 Case and Death Growth Rates — United States, March 1–December 31, 2020. Morbidity and Mortality Weekly.
- 2 – Lyu et al. (2020) Community Use of Face Masks and COVID-19: Evidence From a Natural Experiment of State Mandates in the US. Health Affairs.
- 3 – Fisher et al. (2020) Community and Close Contact Exposures Associated with COVID-19 Among Symptomatic Adults 18 Years in 11 Outpatient Health Care facilities – United States, July 2020. Morbidity and Mortality Weekly.
