To the Editor—We welcome the first prospective study of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) household transmission in the United States (US) by Lewis and colleagues [1]. Household transmission of SARS-CoV-2 is an important contributor to R0 and cannot be realistically addressed by continuous masking and social distancing. Use of data from contact tracing likely leads to an underestimate of household transmission [2]. Prospective studies like the one from Lewis et al, that test all household contacts regardless of symptoms at multiple points in time, are needed to measure the true household secondary attack rate (SAR). The SAR of 29% among household contacts (55% among households) in [1] is impressive—double that of early estimates of approximately 10%–15% from studies that relied primarily on contact tracing data, and higher than the mean estimate of 19%(95% confidence interval, 15%–22%) found in a recent meta-analysis based on household data worldwide [3]. Even so, there is reason to believe the Lewis et al finding is not an outlier, and future estimates from the US may be even higher, due to 3 features of their study. First, a key strength of the study is its completion during a shelter-in-place period when exposure of multiple household members to a common source was less likely. In our experience, multiple family members frequently test positive after attending a communal event (eg, barbecue, funeral, vacation, wedding). Such perihousehold transmission could boost the SAR in areas with significant community-level transmission [3, 4]. Second, key demographic groups affected by the pandemic were underrepresented in the population studied in Wisconsin and Utah. The population was largely non-Hispanic, white, and healthy:< 10% had any cardiovascular disease, and none of the 188 household contacts were hospitalized [5]. Diabetes mellitus was seen in 5% of primary patients and 3% of household contacts (8 persons total), whereas a recent national US survey found a diabetes prevalence of 10.5%[6]. If the authors’ finding that diabetic individuals are more susceptible to secondary infection holds true, this would translate to a higher SAR in the US population. Finally, most households were tested after secondary transmission had already occurred, and testing was not performed frequently enough to reliably detect asymptomatic or mild cases with short-lived viral shedding. Households were sampled relatively late—a median of 11 days (interquartile range, 8–16) from symptom onset of the index case, and 83% of positive household contacts were already polymerase chain reaction positive at the initial household visit. Thus, actual secondary transmission was rarely observed. Secondary cases with mild illness may have tested negative by the initial visit, though they may have been captured via antibody testing [7]. Additionally, since samples were collected on study days 0 and 14, asymptomatic or mild cases that occurred and cleared in the 2 weeks between sampling may have been missed, as such cases may not seroconvert by day 14. Clearly, household transmission happens frequently despite efforts to selfisolate, and new strategies are needed to prevent coronavirus disease 2019 transmission in the home. As access to SARS-CoV-2 testing improves, and especially if antigen-based at-home rapid testing becomes available, increased opportunities to interrupt household transmission should become possible.