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Here s How a Virus Moves in a Crowd so You Can Avoid It

The last time you were in a large crowd at a live event like a concert, did you wander from one place to the next? Or did you stay in one area for a while and move with the crowd to another spot? Your answer may offer insights into how likely it will be for you to contract -- or pass along to others -- an illness like COVID-19 in large gatherings, according to new research.

Most people in crowds tend to stay in one general area for a time before gradually moving to another for a similar duration, often alongside others around them, report researchers at the University of Amsterdam's Informatics Institute.

When their data on crowd movement was considered in a model of disease transmission from epidemiologist Hans Heesterbeek at Utrecht University, the group discovered that this occasional movement in bursts can increase the likelihood of disease transmission.

The researchers first studied movements of large groups over several hours at Johan Cruijff stadium in Amsterdam during two events: a soccer game and a DJ-led dance event. They used smartphone Wi-Fi pings as a proxy for people's behavior and discovered that people's patterns naturally consisted of alternating periods of movement and rest, the authors reported in Nature's Scientific Reports.

They looked at how this pattern of movement interacted with an infectious disease transmitted by droplets or aerosols in close contact. To do so, they had to assume the stadium's conditions were the same throughout, even though in actuality the stadium has areas with greater or lesser amounts of space, ventilation, air filtration, and other characteristics.

A virus typically needs time to move from one person to another, which during the COVID-19 pandemic was the rationale behind requiring 15 minutes of close contact with an infected individual before someone is considered at risk for contracting the coronavirus. So the natural movement of staying in one place for a time before moving to the next is a perfect scenario for giving the virus a foothold before folks move along to the next area.

When people are moving constantly from one place to another, however, there often isn't typically enough time for a virus to transmit from one person to another. At the soccer game, people tended to stay in one place longer, while the dancers moved more regularly. Both habits would increase the risk of viral transmission.

The researchers acknowledge that people move differently at different types of events, but the pattern of remaining in one area for a while before moving to the next seemed the norm in these crowds and likely in others, the authors speculate. Their research is the first step in understanding how crowd behavior can affect infection rates at large events and when risks of transmission are highest.