Q&A November 20, 2020

SHP COVID Modeling Project Probes Secondary COVID Transmissions

Hannah Fung — a PhD candidate in biology at the School of Humanities and Sciences and a member of the SHP COVID modeling team — talks to us about a new study that shows 17% of COVID-19 patients pass the virus onto others in their households.
Family sits on couch with masks
Nelly Kovalchuk/Adobe Stock Photos

A lot of research has come out about COVID-19 quarantines and what families should do to protect older relatives and school-aged children. But what about people living with someone who has the virus?

Now a team of Stanford researchers with the Stanford-CIDE Coronavirus Simulation Modeling Consortium has taken a deep dive into household transmission of SARS-CoV-2.

Their study, published in Clinical Infectious Diseases, finds that the secondary attack rate within households is 17%. This means that on average, once the first person in a previously healthy household becomes infected with SARS-CoV-2, the others have a roughly 17% chance of being infected by that person. Note: The study doesn't account for household members who catch the virus from the second person in the home to be infected.

Their findings highlight the infectiousness of SARS-CoV-2 and the challenges for people caring for infected household members while protecting others from infection.

The consortium — developed by associate professor of medicine Jeremy Goldhaber-Fiebert, PhD, and a half-dozen other Stanford Health Policy faculty members, along with CIDE colleagues in Mexico — comprises 25 researchers across campus and around the world who work together on a range of COVID-19-related data analyses and modeling projects.

I talked to the lead author of the study, Hannah Fung, a PhD candidate in biology at the School of Humanities and Sciences and a member of the modeling team, about their recent study.

What was the ultimate goal of the study?

Our goal was to muster the best available evidence on infection risk among people living with someone with SARS-CoV-2, both to inform disease control policies and to improve the accuracy of epidemic forecasts. We also wanted to identify key gaps in our understanding of SARS-CoV-2 transmission within households.

It surprised me that the risk of someone with COVID-19 infecting a household member isn't actually higher than 17%.

Yes, many people have told me that they expected the household secondary attack rate to be higher, given the prolonged, close contact among household members.

There are two things I'd like to note here. First, the household secondary attack rate estimates are designed to capture the fraction of people infected by the primary case (the first person to be infected in a household); these people can go on to transmit the virus to other people in the household.

Second, the estimates reflect any reductions in transmission as a result of mask use and/or other protective strategies that households employ. As a result, the estimates we report might be substantially lower than what they would be in the absence of those measures.

You found that studies that tested contacts more frequently tended to detect a higher number of coronavirus cases in the household. Doesn't that stand to reason?

Yes, it makes a lot of sense that studies that tested contacts more frequently tended to report higher numbers of secondary cases. This has important implications for how we interpret existing estimates of household transmission.

Specifically, studies that only tested contacts once may have underestimated the true extent of SARS-CoV-2 transmission within households. We should keep this in mind when determining when it is safe to end quarantine and isolation.

You looked at 22 studies from around the world that analyzed 20,291 household contacts. Household transmission estimates ranged from a low of 3.9% in the Northern Territory, Australia, to 36.4% in Shandong, China. Did you find any cultural variances that surprised you?

With only 22 studies meeting our inclusion criteria, it's difficult to say anything definitive about cultural variances, given differences in health systems, epidemic phase, and testing and quarantine practices.

To address the question of how household transmission varies with culture, we need more studies from a wider range of locales -- particularly South Asia, Latin America and Africa, which account for a substantial proportion of the global caseload.

What are some practical takeaways from your results for the average American family and how they respond to the disease in their homes and surrounding communities?

Consistent with other studies, we find that household transmission may be an important driver of epidemic growth. While preventing spread in public settings is a critical first step, a full-fledged strategy for reducing transmission must also involve interventions within households.

Practically speaking, people who may have been exposed to SARS-CoV-2 should, whenever possible, quarantine from household members until they are free from infection. This includes using a separate bathroom and sleeping in a separate room. During this time, all household members should wear masks when using common areas.

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