Event Risk Calculator — Coronavirus
Our COVID-19 event planning tool helps you estimate the probability that at least one infected person will attend your event, and the expected number of infected attendees, using active case data for your area.
🦠 Event Risk Assessment
How Dangerous Are Big Events?
Gatherings and events increase the risk of COVID-19 transmission. The larger the event, the higher the probability that at least one infected person will be in attendance. This risk depends on three key factors:
- Active cases in your area — the number of people currently infected in your city, county, or region
- Hidden (undetected) cases — research suggests that for every detected case, approximately 4 more go undetected due to limited testing and asymptomatic infections
- Event size — the more people attending, the higher the probability of encountering an infected individual
How We Built This Calculator
Our event risk calculation follows a three-step process:
- Count active cases: We use the number of new cases detected in the last 14 days (the typical duration of a COVID-19 infection) to estimate the current number of active, detected cases.
- Estimate hidden cases: Based on published research on the identification and estimation of undetected COVID-19 cases, approximately 80% of infected people won't get tested. This means for every detected case, there are about 4 undetected cases (the hidden case multiplier).
- Calculate event risk: Using the total estimated infected population and your event size, we compute both the expected number of infected attendees and the probability of at least one attending.
The Math Behind It
Total infected = active cases × hidden case multiplier
Infection rate = total infected / population
Expected infected at event = event size × infection rate
P(at least 1 infected) = 1 − (1 − infection rate)event size
How to Stay Safer in Crowds
If you must attend an event, here are evidence-based strategies to reduce your risk:
- Wear a well-fitting mask — N95/KN95 masks provide the best protection, followed by surgical masks
- Choose outdoor over indoor — outdoor events have significantly lower transmission risk due to air circulation
- Maintain distance — keep at least 2 meters (6 feet) from others when possible
- Practice hand hygiene — wash hands frequently or use hand sanitizer
- Get vaccinated — vaccination significantly reduces the risk of severe illness and death
- Consider event size — smaller, well-ventilated gatherings are safer
- Monitor local case counts — attend events when case counts in your area are low
Understanding the Hidden Case Multiplier
The hidden case multiplier (default: 4) accounts for the significant number of COVID-19 infections that go undetected. Reasons for undetected cases include:
- Asymptomatic infections — many infected people never develop symptoms
- Mild symptoms — some people experience symptoms so mild they don't seek testing
- Limited testing — testing may not be readily available or accessible in all areas
- Delays in reporting — there is often a lag between infection and case reporting
You can adjust the multiplier based on your area's testing capacity. Areas with extensive testing may use a lower multiplier (2–3), while areas with limited testing may use a higher one (5–10).
Frequently Asked Questions
What does the probability percentage mean?
The probability represents the chance that at least one person at your event is currently infected with COVID-19. For example, a 20% probability means there is roughly a 1-in-5 chance of having at least one infected attendee.
Where can I find active case data for my area?
Check your local health department website, national health agencies (like the CDC in the US), or global dashboards. Look for the number of new cases reported in the last 14 days.
Why is the hidden case multiplier set to 4?
Research published in scientific journals estimated that approximately 80% of COVID-19 infections are not detected. This means for every 1 detected case, there are about 4 additional undetected cases, resulting in a multiplier of approximately 5 times the detected number (or 4 additional for every 1 detected).
Is this calculator still relevant?
Yes. While the pandemic landscape has changed with vaccines and treatments, the mathematical model remains valid for any infectious disease risk assessment at gatherings. The principles apply whenever you want to estimate the likelihood of encountering an infected individual at an event.