Infection Rate Calculator
Calculate infection attack rate, case fatality rate (CFR), and infection fatality rate (IFR) from epidemiological data. Compare outbreak severity and transmission across populations.
🦠 Epidemiology📐 Attack Rate = cases/population x 100🧬 Biology
Number of cases
Population at risk
Number of deaths
Number tested positive
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Infection Rate Calculator | — | Attack Rate = cases/population x 100 | percent |
Step-by-Step Examples
Example 1
Food Poisoning Outbreak
School event: 450 sick out of 500 attendees, 0 deaths.
- Attack rate = 450/500 x 100 = 90%
- High attack rate typical of common-source outbreaks
- Probable vehicle: shared contaminated food
✓ Attack rate = 90%
Example 2
COVID-19 Early Data
City: 5000 cases, 250 deaths, 300000 pop.
- Attack rate = 5000/300000 x 100 = 1.67%
- CFR = 250/5000 = 5%
- True IFR lower if many asymptomatic cases missed
- Early COVID CFR appeared high due to limited testing
✓ Attack rate 1.67%, CFR 5%
Example 3
Influenza Season
500 flu cases in 10,000 person community, 3 deaths.
- Attack rate = 5%
- CFR = 3/500 = 0.6%
- Seasonal flu IFR typically 0.01-0.05% (most deaths in elderly)
✓ 5% attack rate, 0.6% CFR
Real-World Applications
Outbreak Investigation
Attack rate identifies high-risk subgroups and transmission vehicles.
Surveillance
Population-level incidence rates track epidemic trajectory.
Vaccine Effectiveness
Vaccine efficacy = 1 - (attack rate vaccinated / attack rate unvaccinated).
Comparative Epidemiology
CFR and IFR comparisons inform public health prioritization across pathogens.
Common Mistakes to Avoid
⚠️
Confusing CFR and IFR
CFR = deaths / confirmed cases (testing-dependent). IFR = deaths / all infections including undetected. IFR is always <= CFR.
⚠️
Ignoring lag time
Deaths lag infection by 2-4 weeks. Calculating CFR from simultaneous cases and deaths underestimates true mortality.
⚠️
Population denominator errors
At-risk population must be defined carefully. For foodborne illness: only event attendees count, not entire city population.
Frequently Asked Questions
What is case fatality rate? ▾
CFR = deaths / confirmed cases x 100%. Depends heavily on testing intensity. High testing = more mild cases detected = lower CFR.
What is infection fatality rate? ▾
IFR = deaths / all infections (confirmed + unconfirmed) x 100%. Requires seroprevalence studies to estimate total infections. More accurate disease severity measure.
What is attack rate? ▾
Proportion of exposed population that develops disease. High attack rate indicates high transmissibility or high dose exposure. Can vary by subgroup (age, vaccination status).
What is secondary attack rate? ▾
Probability that a susceptible contact of an infected person becomes infected. Measured in households or close contacts. Related to but distinct from R0.
What is herd immunity threshold? ▾
1 - 1/R0 (see R0 calculator). When population immunity exceeds this threshold, each infected person infects on average less than 1 other person.