SARS-CoV-2 Household Transmission Risk is Significantly High

Carlos Grijalva, MD, MPH (IDWeek)

New data from a team of US-based investigators showed SARS-CoV-2 transmission risk within individual households is notably high and early into the period when persons shed the virus.

The findings, presented at IDWeek 2020, provide a better characterization of household infections—at a time when research has largely focused on community transmission and prevention.

Investigators, led by Carlos Grijalva, MD, MPH, Associate Professor at Vanderbilt University Medical Center in Nashville, provided an update to its ongoing assessment, sharing data from 18 enrolled families with at least 1 ambulatory patient with confirmed COVID-19 who was discharged to at-home care and isolation.

Median age of index cases was 37 years old, while household members was 27 years old. Prior to symptom onset (median 4 days post-first sample collected for testing), 83% of index cases spent >4 hours in a room with at least 1 household member; after disease onset, that rate was halved (44%).

More than half (53%) of non-index household numbers tested positive for the virus during follow-up, in a median duration of 4.5 days from index symptom onset. Most secondary infections (72%) were detected within the first 3 days of follow-up.

These observations suggest that transmission of SARS-CoV-2 within households is high, with many infections detected during the initial days of study follow-up.

In an interview with Contagion® during IDWeek, Grijalva discussed his team’s findings, their implication for growing understanding of SARS-CoV-2 transmission-risk settings, and what may come next in research.

Contagion:What were the most pressing outcomes of your team’s assessment into SARS-CoV-2 household transmission?

Grijalva: Our study showed that transmission of SARS-CoV-2 within households is high, higher than previously reported, and can originate from both children and adults.

The rate of index cases spending >4 hours in the same room with another household member was essentially halved following the onset of symptoms. How does this inform our understanding of people’s ability and willingness to self-isolate following a positive case?

It is encouraging to observe that household members reported that they tried to reduce their exposure to a sick household member. We think prompt isolation of individuals once they feel ill is an important strategy that could help reduce the possibility of transmission of infections within households.

What could be made of the secondary infections being detected early into follow-up?

The rapid detection of infections among household members indicate that the transmission of SARS-CoV-2 is high and occurs quickly. The estimated secondary infection rates in our study appear to be higher than rates reported in previous studies. Also, we observed substantial transmission from index cases that were both children and adults

For the sake of achieving a mean SARS-CoV-2 R₀ below 1, what does our understanding of household transmission mean?

Prospective transmission studies, like ours, help us understand how fast the infections are occurring in households. The high transmission we observed in the study suggests that prompt isolation as soon as an individual feels ill may help reduce the probability of transmission in households.

What are your plans for follow-up?

We are currently in the process of integrating data from the other US site where household transmission is being evaluated. This is an ongoing multicenter study and additional households have been already enrolled and we are in the process of testing samples and completing follow up measures.

We will soon start examining factors associated with transmission of infections, and their evolution during follow-up. We have some more detailed reports under development/review now and we anticipate those reports will be published within the next weeks.