COVID Event Risk Calculator — Infectious Person at Gathering
Enter the size of a gathering, the local active-case prevalence per 100,000 people, and an ascertainment factor for unreported infections to estimate the probability that at least one person present is currently infectious with COVID-19 or a comparable respiratory virus.
Probability that at least one attendee is currently infectious
- 1
Individual infection probability
100 ÷ 100,000 × 4 = 0.4%Reported prevalence scaled by ascertainment factor to estimate true infectious fraction. - 2
Probability all attendees are uninfected
(1 − p)ⁿ = 0.818402 - 3
Risk: at least one infectious person present
1 − 0.818402 = 18.2%
How does this calculator work?
P(at least one infectious person at event) = 1 − (1 − p)ⁿ, where p = local prevalence per 100,000 × ascertainment factor, and n = number of attendees. A 50-person event with 100 cases/100k and 4× ascertainment gives roughly 18% risk. Higher prevalence or larger events raise the probability rapidly.
Formula
How this is calculated
This model applies the complementary probability rule: the chance that at least one person out of n is currently infectious equals one minus the probability that all n people are uninfected. The individual infection probability p combines the reported active-case rate (cases per 100,000 population) with an ascertainment factor that corrects for unreported or asymptomatic cases — a factor of 4 means actual infections are estimated at four times the reported count, which was a commonly cited figure during 2020–2025 surveillance studies. Enter your local figure if better data is available.
The model assumes attendees are randomly drawn from the local population (no clustering by vaccination or behaviour), that all are equally likely to be infectious, and that "infectious" means contagious on the day — not merely test-positive at some earlier point. These are simplifying assumptions: real transmission risk also depends on ventilation, mask use, duration and distance, none of which are modelled here.
As of 2026, COVID-19 is broadly endemic in most regions. The calculator works for any respiratory disease with a known period prevalence — replace the COVID prevalence with influenza or RSV figures from your local health dashboard if relevant.
Frequently asked questions
Most national and regional public health agencies publish a 7-day or 14-day case rate per 100,000 on their websites or dashboards. Search for your country or region plus "COVID-19 case rate per 100,000". The number changes weekly, so use a recent figure.
The ascertainment factor accounts for cases that are real but not officially reported (asymptomatic, not tested, etc.). During the main COVID waves (2020–2023) estimates ranged from 2× to 10×, with 4× often used as a middle estimate. As testing rates and surveillance methods vary, treat the factor as an adjustable estimate rather than a fixed constant.
No. The percentage is the probability that at least one person at the event is infectious — not that any transmission actually occurs. Actual transmission depends on ventilation, proximity, duration, masks, immunity and viral load. This tool estimates exposure opportunity, not infection outcome.
Also known as
TG we-Calculate Editorial Team. (2026). COVID Event Risk Calculator — Infectious Person at Gathering [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/covid-event-risk-calculator
TG we-Calculate Editorial Team. "COVID Event Risk Calculator — Infectious Person at Gathering." TG we-Calculate. 2026. https://we-calculate.com/calculator/covid-event-risk-calculator.
TG we-Calculate Editorial Team, "COVID Event Risk Calculator — Infectious Person at Gathering," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/covid-event-risk-calculator
@misc{wecalculate_covid_event_risk_calculator, title = {COVID Event Risk Calculator — Infectious Person at Gathering}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/covid-event-risk-calculator}}, year = {2026}, note = {TG we-Calculate} }
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