Households are a high-risk setting for SARS-CoV-2 transmission

By Sarah MooreReviewed by Emily Henderson, B.Sc.Dec 14 2020

The results of a new study have confirmed that households, and other crowded indoor environments, are high-risk settings for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission. The findings will likely be important to guiding future COVID-19 prevention strategies and guidance, given that currently, those with confirmed or suspected infections are requisition to isolate at home, putting other members of their household at an increased risk.

Family playing inside their home. Image Credit: fizkes/Shutterstock.com

Calculating the household secondary attack rate

A team of researchers from the University of Florida, the University of Washington, and the Fred Hutchinson Cancer Research Center, Seattle, conducted a meta-analysis of PubMed articles published from October 2019 that met the search terms SARS-CoV-2 or COVID-19 with secondary attack rate, household, close contacts, contact transmission, contact attack rate, or family transmission. All articles produced by this search were included in the analysis if they held original data relating to household second attack rate. Those that did not report secondary attack rates were excluded.

Virus transmissibility can be characterized by calculating the household secondary attack rate. The secondary attack rate refers to the number of cases occurring in a household during the incubation period following exposure to a primary case divided by the total number of people in the household.

Many studies have been conducted that have collected data on factors that may impact the household secondary attack rate, such as timing, duration, and type (symptomatic or asymptomatic).

The current study aimed to estimate the household secondary attack rate of SARS-CoV-2, as well as investigate how these different factors impact secondary transmission. To do this, they analyzed the data collected from numerous published studies.

Factors that influence COVID-19 transmission at home

In total, data from 54 studies involving 77,758 participants were included in the meta-analysis. A restricted maximum-likelihood estimator model was used in the meta-analysis to yield a point estimate and 95% confidence interval for estimations of secondary attack rate for each of the analyzed subgroups.

The results showed that the estimated household secondary attack rate was 16.6%, which was significantly higher than the secondary attack rates for SARS-CoV (7.5%) and MERS-CoV (4.7%). Symptomatic cases have higher household secondary attack rates than asymptomatic index cases, 18.0% vs 0.7%, respectively. Adult contacts had increased rates compared to child contacts, 28.3% vs 16.8%, respectively.

The data showed that the estimated mean household secondary attack rate for households with 1 contact was significantly higher than households with at least 3 contacts (41.5% vs 22.8%). Finally, infection rates between spouses were found to be higher than non-spouses (37.8% vs 17.8%).

Future strategies to protect household COVID-19 transmission

Overall, the data reveals that the household secondary attack rate is higher than the overall secondary attack rate, and is influenced by certain factors. Living with someone who has tested positive for COVID-19 who is symptomatic and an adult increases the risk of secondary infection. Living with an infected spouse also increases the likeliness of infection compared with living with a non-spouse who is infected.

The team’s research highlight that households are key environments for SARS-CoV-2 transmission, and that without changes in behavior, this will likely continue. Therefore, the development of new prevention strategies aimed at protecting those in the home is essential to preventing the continued spread of the COVID-19 virus.

These strategies, such as wearing a mask at home, improving ventilation, and targeted antiviral prophylaxis, for example, may play a key role in reducing the spread of COVID-19 and ending the pandemic. It is suggested that the efficacy of such prevention strategies should be further explored.