High household transmission of SARS-CoV-2 in the United States: living density, viral load, and disproportionate impact on communities of color

Abstract

Background: Few prospective studies of SARS-CoV-2 transmission within households have been reported from the United States, where COVID-19 cases are the highest in the world and the pandemic has had disproportionate impact on communities of color.

Methods and findings: This is a prospective observational study. Between April-October 2020, the UNC CO-HOST study enrolled 102 COVID-positive persons and 213 of their household members across the Piedmont region of North Carolina, including 45% who identified as Hispanic/Latinx or non-white. Households were enrolled a median of 6 days from onset of symptoms in the index case. Secondary cases within the household were detected either by PCR of a nasopharyngeal (NP) swab on study day 1 and weekly nasal swabs (days 7, 14, 21) thereafter, or based on seroconversion by day 28. After excluding household contacts exposed at the same time as the index case, the secondary attack rate (SAR) among susceptible household contacts was 60% (106/176, 95% CI 53%-67%). The majority of secondary cases were already infected at study enrollment (73/106), while 33 were observed during study follow-up. Despite the potential for continuous exposure and sequential transmission over time, 93% (84/90, 95% CI 86%-97%) of PCR-positive secondary cases were detected within 14 days of symptom onset in the index case, while 83% were detected within 10 days. Index cases with high NP viral load (>10^6 viral copies/ul) at enrollment were more likely to transmit virus to household contacts during the study (OR 4.9, 95% CI 1.3-18 p=0.02). Furthermore, NP viral load was correlated within families (ICC=0.44, 95% CI 0.26-0.60), meaning persons in the same household were more likely to have similar viral loads, suggesting an inoculum effect. High household living density was associated with a higher risk of secondary household transmission (OR 5.8, 95% CI 1.3-55) for households with >3 persons occupying <6 rooms (SAR=91%, 95% CI 71-98%). Index cases who self-identified as Hispanic/Latinx or non-white were more likely to experience a high living density and transmit virus to a household member, translating into an SAR in minority households of 70%, versus 52% in white households (p=0.05).

Conclusions: SARS-CoV-2 transmits early and often among household members. Risk for spread and subsequent disease is elevated in high-inoculum households with limited living space. Very high infection rates due to household crowding likely contribute to the increased incidence of SARS-CoV-2 infection and morbidity observed among racial and ethnic minorities in the US. Quarantine for 14 days from symptom onset of the first case in the household is appropriate to prevent onward transmission from the household. Ultimately, primary prevention through equitable distribution of effective vaccines is of paramount importance.

Figure 1.

Geography of 100 households enrolled the UNC CO-HOST study.

Figure 2.. Secondary attack rate (SAR) among susceptible household contacts.

Of 100 households included in the analysis, 99 completed one month follow-up. One household of 6 withdrew (3 infected at baseline). Among 182 susceptible household contacts, 42% (76/182) were already Infected at the time of study enrollment and 33 additional secondary cases were observed during follow-up, resulting in an overall SAR of 60% (106/176, 95% Cl 53%−67%).

Figure 3.. Secondary household attack rate.

Of 92 households included in the analysis, 53% (49/92) contained infected household contacts at enrollment, with 15 more households sustaining transmission over the next 21 days, resulting in a secondary household attacak rate of 70% (64/92).

Figure 4.. SARS-CoV-2 viral burden is correlated within families.

The viral load obtained at enrollment from nasopharyngeal swabs in households with multiple COVID-positive household members are shown. Each vertical row in red depicts an individual household, with circles delineating the log viral load of each member within the household. Circles shaded in gray represent values derived from a nasal mid-turbinate swab if NP sampling was not performed. Households are depicted across the x-axis in order of decreasing viral load. Data drawn from 98 households and 184 participants. The intraclass correlation coefficient ICC = 0.44, 95% Cl (0.26, 0.60), p-value < 0.001.

Figure 5.. Association of index nasopharyngeal viral load and transmission in the household.

Households with new secondary cases folllowing enrollment were more likely to have index cases with high nasopharyngeal viral load compared to households without secondary transmission. Index cases that were not antibody-positive at enrollment, as a marker of more recent infection, are depicted to the right in gray. Blue overlaid boxes depict 95% CIs.