High household transmission of SARS-CoV-2 in the United States: living density, viral load, and disproportionate impact on communities of color
Authors: Cerami Carla , Rapp Tyler , Lin Feng-Chang , Tompkins Kathleen , Basham Christopher , Muller Meredith S. , Whittelsey Maureen , Zhang Haoming , Chhetri Srijana B. , Smith Judy , Litel Christy , Lin Kelly , Churiwal Mehal , Khan Salman , Claman Faith , Rubinstein Rebecca , Mollan Katie , Wohl David , Premkumar Lakshmanane , Juliano Jonathan J. , Lin Jessica T. 
Published: March 2021

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.

AUTHORS SUMMARY

Why was this study done?

Understanding the secondary attack rate and the timing of transmission of SARS-CoV-2 within households is important to determine the role of household transmission in the larger pandemic and to guide public health policies about quarantine.

Prospective studies looking at the determinants of household transmission are sparse, particularly studies including substantial racial and ethnic minorities in the United States and studies with adequate follow-up to detect sequential transmission events.

Identifying individuals at high risk of transmitting and acquiring SARS-CoV-2 will inform strategies for reducing transmission in the household, or reducing disease in those exposed.

What did the researchers do and find?

Between April-November 2020, the UNC CO-HOST study enrolled 102 households across the Piedmont region of North Carolina, including 45% with an index case who identified as racial or ethnic minorities.

Overall secondary attack rate was 60% with two-thirds of cases already infected at study enrollment.

Despite the potential for sequential transmission in the household, the majority of secondary cases were detected within 10 days of symptom onset of the index case.

Viral loads were correlated within families, suggesting an inoculum effect.

High viral load in the index case was associated with a greater likelihood of household transmission.

Spouses/partners of the COVID-positive index case and household members with obesity were at higher risk of becoming infected.

High household living density contributed to an increased risk of household transmission.

Racial/ethnic minorities had an increased risk of acquiring SARS-CoV-2 in their households in comparison to members of the majority (white) racial group.

What do these findings mean?

Household transmission often occurs quickly after a household member is infected.

High viral load increases the risk of transmission.

High viral load cases cluster within households - suggesting high viral inoculum in the index case may put the whole household at risk for more severe disease.

Increased household density may promote transmission within racial and ethnic minority households.

Early at-home point-of-care testing, and ultimately vaccination, is necessary to effectively decrease household transmission.