See why keeping groups small can save lives in the era of COVID-19

The risk of exposing people in a group to the coronavirus goes up exponentially with event size, modeling work suggests.

1,000 total cases in the country

100% chance of infected person attending event (log scale)

There is a 0.3 percent chance of someone with COVID-19 attending a 1,000-person event when there are 1,000 cases in the country.

10

3

1

0.1

0.03

0.01

0.003

0.002%

0.001

5

10

100

1,000

10,000

Dinner

Party

Wedding

Reception

Small

Concert

Sports

Event

Event size

10,000 cases

100%

26

10

3

1

0.3

0.1

0.03

0.01

0.015%

0.001

5

10

100

1,000

10,000

Dinner

Party

Wedding

Reception

Small

Concert

Sports

Event

100,000 cases

95

100%

26

10

3

1

0.3

0.1

0.15%

0.01

0.001

5

10

100

1,000

10,000

Dinner

Party

Wedding

Reception

Small

Concert

Sports

Event

1,000,000 cases

100

95

100%

26

10

3

1

1.5%

0.1

0.01

0.001

5

10

100

1,000

10,000

Wedding

Reception

Sports

Event

Dinner

Party

Small

Concert

1,000 total cases in the country

10,000 cases

100%

100% chance of infected person attending event (log scale)

There is a 3 percent chance of someone with COVID-19 attending a 1,000-person event when there are 10,000 cases in the country.

26

10

10

3

1

1

0.3

0.3

0.1

0.1

0.03

0.03

0.01

0.01

0.015%

0.003

0.002%

0.001

0.001

5

10

100

1,000

10,000

5

10

100

1,000

10,000

Dinner

Party

Wedding

Reception

Small

Concert

Sports

Event

Dinner

Party

Wedding

Reception

Small

Concert

Sports

Event

Event size

100,000 cases

1,000,000 cases

100

95

95

100%

100%

26

26

10

10

3

3

1

1

1.5%

0.3

0.1

0.1

0.15%

0.01

0.01

0.001

0.001

5

10

100

1,000

10,000

5

10

100

1,000

10,000

Wedding

Reception

Sports

Event

Dinner

Party

Small

Concert

Wedding

Reception

Sports

Event

Dinner

Party

Small

Concert

1,000 total cases in the country

10,000 cases

100%

100% chance of infected person attending event (log scale)

26

There is a 3 percent chance of someone with COVID-19 attending a 10,000-person event when there are 1,000 cases in the country.

10

10

3

1

1

0.3

0.3

0.1

0.1

0.03

0.03

0.015%

0.01

0.01

0.003

0.002%

0.001

0.001

5

10

100

1,000

10,000

5

10

100

1,000

10,000

Dinner

Party

Wedding

Reception

Small

Concert

Sports

Event

Dinner

Party

Wedding

Reception

Small

Concert

Sports

Event

Event size

100,000 cases

1,000,000 cases

100

95

95

100%

100%

26

26

10

10

3

3

1

1

1.5%

0.3

0.15%

0.1

0.1

0.01

0.01

0.001

0.001

5

10

100

1,000

10,000

5

10

100

1,000

10,000

Dinner

Party

Wedding

Reception

Small

Concert

Sports

Event

Dinner

Party

Wedding

Reception

Small

Concert

Sports

Event

Why are large events so dangerous in a pandemic? Just look at the math: As COVID-19 cases increase across the U.S., the risk of exposing members of a group to the virus goes up exponentially with event size, according to Georgia Institute of Technology biologist Joshua Weitz.

The charts above illustrate a risk calculation that Weitz made to show the phenomenon. The risks seen here reflect the probability of someone with an illness such as COVID-19 attending an event, if the event were filled with randomly selected U.S. residents.

Even in this idealized form, the math is stark: The risk of a sick person attending an event grows rapidly with both the number of attendees and the number of total U.S. cases. Weitz says that the increased risk strongly justifies recent efforts to limit event sizes. Testing delays in the U.S. have further compounded the issue by obscuring the disease’s true prevalence.

“Things have moved fast (as they do in epidemics),” Weitz says in an email. “We should be physically distancing whenever feasible.” (Find out more about why there’s no safe event size during a pandemic.)

Weitz adds that the risk of COVID-19 spreading also goes up with the total number of events, as each gathering represents a roll of the dice that might spark an intense local outbreak.

“Your dinner party of 10 people might be OK,” he says, “but someone else's will not be, endangering us collectively.”