Household Transmission of Severe Acute Respiratory Syndrome Coronavirus 2 in the United States: Living Density, Viral Load, and Disproportionate Impact on Communities of Color

Abstract

Background: Households are hot spots for severe acute respiratory syndrome coronavirus 2 transmission.

Methods: This prospective study enrolled 100 coronavirus disease 2019 (COVID-19) cases and 208 of their household members in North Carolina though October 2020, including 44% who identified as Hispanic or non-White. Households were enrolled a median of 6 days from symptom onset in the index case. Incident secondary cases within the household were detected using quantitative polymerase chain reaction of weekly nasal swabs (days 7, 14, 21) or by seroconversion at day 28.

Results: Excluding 73 household contacts who were PCR-positive at baseline, the secondary attack rate (SAR) among household contacts was 32% (33 of 103; 95% confidence interval [CI], 22%-44%). The majority of cases occurred by day 7, with later cases confirmed as household-acquired by viral sequencing. Infected persons in the same household had similar nasopharyngeal viral loads (intraclass correlation coefficient = 0.45; 95% CI, .23-.62). Households with secondary transmission had index cases with a median viral load that was 1.4 log10 higher than those without transmission (P = .03), as well as higher living density (more than 3 persons occupying fewer than 6 rooms; odds ratio, 3.3; 95% CI, 1.02-10.9). Minority households were more likely to experience high living density and had a higher risk of incident infection than did White households (SAR, 51% vs 19%; P = .01).

Conclusions: Household crowding in the context of high-inoculum infections may amplify the spread of COVID-19, potentially contributing to disproportionate impact on communities of color.

Keywords: SARS-CoV-2; household transmission; living density; secondary attack rate; viral load.

Figure 1.

COVID-19 Household Transmission Study enrollment and SAR. Among 100 households that completed the 28-day follow-up, household contacts were excluded if they had evidence of prior infection (negative PCR and positive antibody test at enrollment), were possibly infected at the same time as the index case based on a common exposure event, or negative PCR testing could not be confirmed with a negative antibody test at day 28. Of the remaining 176 household contacts of 91 index cases, 41% (73) were already PCR-positive at baseline and thus excluded from the primary SAR analysis. During study follow-up, 33 incident severe acute respiratory syndrome coronavirus 2 cases were identified, yielding a SAR of 32% (33 of 103). Among the 33 secondary cases, 22 were identified by both PCR and seroconversion from day 1 to day 28, 4 were identified by PCR only, and 7 were identified based on seroconversion. Abbreviations: Ab+, antibody positive; D, day; PCR, polymerase chain reaction; SAR, secondary household attack rate.

Figure 2.

Severe acute respiratory syndrome coronavirus 2 viral burden is correlated within families. The viral load obtained at enrollment from nasopharyngeal (NP) swabs in households with multiple COVID-19–positive household members is shown (n = 42 households). Each vertical row in red depicts an individual household, with circles delineating the log viral load of each member within the household. Gray-shaded circles represent values derived from a nasal midturbinate swab if NP sampling was not performed. This was based on a linear regression equation generated from more than 100 study participants with positive viral load from both NP and nasal midturbinate swabs [11]. Households are depicted across the x-axis in order of decreasing viral load. Data drawn from 148 participants. The intraclass correlation coefficient  = 0.45; 95% confidence interval, .23–.62; P value < .001. Abbreviation: COVID-19, coronavirus disease 2019.

Figure 3.

Bayesian phylogeny showing high relatedness within household infections, indicating household transmission. High-density amplicon sequencing was performed on all available viral isolates from 10 households with secondary infections to assess relatedness between infections. Whole-genome sequences were assembled according to the Wuhan reference genome, assigned to major clades, and then used for Bayesian phylogeny reconstruction. Index cases within each numbered household are in bold. Household contacts are numbered sequentially starting with the index case number, eg, X-1, X-2. Minors are indicated with the letter “c” prior to the case number. Each asterisk indicates 1 study week preceding a positive quantitative polymerase chain reaction (PCR) test, ie, * indicating a D7 positive test, ** indicating a D14 positive test, and *** indicating a D21 positive test. Household contacts without asterisks were PCR-positive at baseline. Abbreviation: SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 4.

Association of index NP viral load and transmission in the household. Households with secondary cases were more likely to have index cases with high NP viral load compared with households without secondary transmission (median NP viral load log 8.8 vs 7.4 copies/mL, respectively; P = .03). Index cases who were not yet antibody-positive at enrollment, as a marker of more recent infection, are depicted to the right in gray. Abbreviations: Ab+, antibody positive; NP, nasopharyngeal.