Mask Effectiveness for Preventing Secondary Cases of COVID-19, Johnson County, Iowa, USA

Jacob Riley; Jamie M. Huntley; Jennifer A. Miller; Amelia L.B. Slaichert; Grant D. Brown


Emerging Infectious Diseases. 2022;28(1):69-75. 

In This Article

Abstract and Introduction


In September of 2020, the Iowa Department of Public Health released guidance stating that persons exposed to someone with coronavirus disease (COVID-19) need not quarantine if the case-patient and the contact wore face masks at the time of exposure. This guidance differed from that issued by the Centers for Disease Control and Prevention. To determine the best action, we matched exposure information from COVID-19 case investigations with reported test results and calculated the secondary attack rates (SARs) after masked and unmasked exposures. Mask use by both parties reduced the SAR by half, from 25.6% to 12.5%. Longer exposure duration significantly increased SARs. Masks significantly reduced virus transmission when worn by both the case-patient and the contact, but SARs for each group were higher than anticipated. This finding suggests that quarantine after COVID-19 exposure is beneficial even if parties wore masks.


On September 29, 2020, the Iowa Department of Public Health (IDPH) issued new guidance for persons who had been in contact with someone infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (hereafter called case-patients). This guidance recommended that when both the case-patient and the contact were correctly and consistently masked during an exposure, the contact should perform symptom monitoring for 14 days instead of quarantining at home. This guidance deviated substantially from that provided by the Centers for Disease Control and Prevention (CDC), which still recommended at-home quarantine after exposure to someone with coronavirus disease (COVID-19), regardless of mask use. Johnson County Public Health (JCPH) staff decided to follow IDPH guidance but also supported any persons or organizations who chose to continue to follow the CDC recommendation.

Although the IDPH change in guidance provided an opportunity to lessen the burden of the pandemic on Johnson County, we were concerned about a potential increase in transmission rates. Because data supporting this change in guidance were lacking, we designed a prospective cohort study to evaluate the potential risk for increased virus transmission by measuring the secondary attack rates (SARs) of COVID-19 between persons exposed when both parties were masked and those exposed when ≥1 person was unmasked.

The purpose of this study was to examine how effective masks are at reducing transmission of SARS-CoV-2 and, therefore, whether the new IDPH recommendation for symptom monitoring was appropriate. However, mask use is only 1 of many factors that affect SARS-CoV-2 transmission. While examining the available data, we identified several additional risk factors of interest, including symptom status, exposure setting, and exposure duration. This information enabled us to examine additional guidance relating to COVID-19, such as early release from quarantine and the potential for airborne transmission, to ensure that our recommendations did not increase the risk for transmission in the community.

After reviewing relevant literature,[2–4] we hypothesized that mask use consistent with CDC guidance[5] would reduce the SAR for COVID-19 in nonhousehold contacts from 10% to 5%. The study proposal was evaluated according to internal ethics review protocols, met the criteria for public health practice,[1] and was not required to undergo institutional review board review.