We visualize how we spread COVID-19 in our homes

Although COVID-19 can be transmitted through contact with contaminated objects, most research has focused on airborne droplet transmission. Against this background, Japanese researchers ran detailed computational simulations based on real-life behavioral data to visualize how viruses spread to common household objects soon after people returned home. Their results show the importance of immediate hand disinfection upon return to avoid contact infection, reinforcing the effectiveness of good hygiene.

More than three years have passed since the outbreak of the COVID-19 pandemic worldwide, and we have yet to find an effective cure for the disease. In addition, new variants of the SARS-CoV-2 virus, some of which are more infectious, are discovered every year. Fortunately, compared to the situation in 2020, we know a lot more about SARS-CoV-2 and the ways in which it spreads from one host to another.

The virus has two main routes: droplet infection and contact infection. The first condition – droplet transmission – involves the inhalation of droplets containing the virus from the carrier during normal breathing or conversation. Such infection can be effectively prevented by using face masks and increasing ventilation. In contrast, contact infection occurs when viruses are transferred from contaminated objects such as railings, doorknobs, and common 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 attracted few researchers. “Studies on the transmission of COVID-19 have mostly focused on droplet infection without considering the risk of contact infection from viruses in the body or things brought in from outside,” explains Professor Setsuya Kurahashi from the University of Tsukuba in Japan, “Studies” based on actual research data, the risk of contact infection at home is low, and this is a recognized problem of COVID in the -19 study.”

To address this lack of knowledge, a research team led by Professor Kurahashi recently conducted a study to assess the risk of contact transmission of COVID-19 in 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 objects in the home and can be mitigated. Their article was published in the journal The limits of physics January 12, 2023.

First, the team sent out more than 1,100 surveys to gather information about what people do in the first 30 minutes after returning home. They analyzed these survey responses and extracted information about people’s behavior, location, and objects touched during this period. Next, they used this information to create probability tables that describe the probability of moving from room to room and coming into contact with various common objects. In addition, the team conducted laboratory experiments to determine how well the viral load can be transferred from different materials to the skin and vice versa.

Based on this information, the team ran a variety of simulations to analyze the spread patterns of the virus in a typical two-bedroom household. In one of them, they focused on how viruses are transferred between rooms and on objects shortly after a person carrying the virus on their hands returns home. In another, they analyzed what happens when another person returns home shortly after the first person has already spread the virus indoors. Finally, they study how the virus spreads through the joint actions of an infected person recovering at home and an infected person returning home from abroad carrying the virus.

The results of these simulations revealed that what we do after returning home largely determines how efficiently the virus spreads around the household. It turns out that simply washing your hands after arrival is not completely effective, since you have probably already come into contact with various indoor areas, spreading the virus. Rather, hand disinfection at the entrance proved to be better for preventing contact spread.

These findings provide much-needed information on how SARS-CoV-2 and other viruses can spread indoors through contact with household items. “Although the use of masks and safe distance can prevent the spread of droplets, the risk of contact infections had not been properly measured before our study, and general hand disinfection has been the only recommendation,” says Professor Kurahashi. “By accurately visualizing the number of viruses infecting household objects, we have shown that disinfecting hands at the right time and in the right place is a very important turning point,” he concludes.