Visualize how we spread COVID-19 in our homes

While COVID-19 can be transmitted through contact with contaminated objects, most studies have focused on airborne droplet transmission. Against this backdrop, scientists from Japan performed detailed computational simulations based on real-life behavioral data to visualize how viruses spread to common household items soon after people return home. Their results illustrate the importance of immediate hand disinfection upon return to prevent contact infection, reinforcing the effectiveness of good hygiene.

It has been more than three years since the COVID-19 pandemic broke out worldwide and we have yet to find an effective cure for the disease. In addition, new variants of the SARS-CoV-2 virus are discovered every year, some with higher contagiousness. Fortunately, compared to our situation in 2020, we know a lot more about SARS-CoV-2 and the ways it spreads from host to host.

There are two main transmission routes for the virus: droplet infection and contact infection. The first mode – droplet infection – involves inhaling virus-laden droplets that are exhaled by a wearer during normal breathing or conversation. This type of transmission can be effectively prevented by using face masks and increased ventilation. Contact infection, on the other hand, occurs when viruses are transferred from contaminated objects, such as handrails, doorknobs and everyday objects, to a person’s hands and eventually to the mucous membranes of the mouth, eyes or other parts of the body.

Unfortunately, contact infection has so far received little attention from scientists. “Studies of COVID-19 transmission have mainly focused on droplet infection, without considering the risk of contact infection due to viruses on the body or belongings brought in from the outside,” explains Prof. Setsuya Kurahashi of the University of Tsukuba , Japan, out. “Studies on the risk of contact infection at home based on real survey data are few and this is a recognized problem in COVID-19 research.

To address this knowledge gap, a research team led by Professor Kurahashi recently conducted a study to estimate the risk of contact transmission of COVID-19 for indoor environments. By combining real behavioral data, agent-based modeling and computational simulations, the team shed important light on how viruses spread from residents’ hands to various household objects, and how this can be contained. Their paper was published in the journal Limits in physics on January 12, 2023.

First, the team sent out more than 1,100 surveys to collect information about what people do in the first 30 minutes after returning home. They analyzed these survey responses and pulled data on people’s behavior, location, and objects touched during this time period. They then used this data to create probability tables describing the likelihood of room-to-room transmission and that of coming into contact with various everyday items. In addition, the team performed laboratory experiments to quantify how well a viral load can be transferred from different materials to the skin and vice versa.

Based on this information, the team ran multiple types of simulations to analyze viral distribution patterns within a standard two-bedroom household. In one, they focused on how viruses are transferred between rooms and objects shortly after a person comes home with the virus on their hands. In another, they analyzed what happens when a second person returns home slightly after the first person has already spread the virus indoors. Finally, they examined how the virus spreads through the combined actions of an infected person recovering at home and a person returning from outside with the virus.

The results of these simulations showed that what we do after returning home largely determines how effectively the virus will spread through the household. It turned out that simply washing hands after entering is not entirely effective, as people have probably already come into contact with various objects indoors that spread the virus. On the contrary, hand disinfection at the entrance itself proved to be better at preventing contact transmission.

These findings provide much-needed insight into how SARS-CoV-2 and other viruses can spread indoors through contact with household items. “While the use of masks and safe distancing can prevent droplet transmission, the risk of contact transmission was not well quantified prior to our study and general hand sanitization was the only recommendation,” says Prof. Kurahashi. “By accurately mapping how many viruses attach to household objects, we have shown that hand disinfection at the right time and place is extremely important as a tipping point,” he concludes.