Vaccine Efficacy Calculator - Severe Cases Prevented

A vaccine efficacy calculator is a public-health planning tool that translates per-vaccine efficacy and a population size into a people-saved estimate: the severe COVID-19 cases the same vaccination program prevents versus no vaccination. Pick a named vaccine, choose a population, and the calculator reports severe cases without vaccination, severe cases with vaccination, and the people-saved difference.

• • Compare named COVID-19 vaccines side by side: Swap between Pfizer, Moderna, AstraZeneca, J&J, Sinopharm, and Sputnik V to see how the people-saved figure changes.
• • Translate trial efficacy into real-world prevention: Override the efficacy slider to model 92% Pfizer efficacy versus a 70% real-world effectiveness estimate in older adults.
• • Plan a school, city, or national campaign: Test what 'X percent of severe cases prevented' means for a 500-person school, an 8.4 million-person city, or a 330 million-person country.
• • Run a sensitivity check on the severe rate: Stress-test by lowering the severe rate to 5% or raising it to 30% for older and high-risk populations.

The two halves of the result answer different questions. Severe cases without vaccination is what a 100% infected, unvaccinated population would experience. Severe cases with vaccination is what the same exposed population would still experience after the chosen vaccine.

When you want to combine vaccine efficacy with a specific event size and local prevalence, the COVID Event Risk Calculator pairs the people-saved figure with the chance an infectious person actually attends.

The calculator reports four numbers: severe cases without vaccination, severe cases with vaccination, people saved, and relative risk reduction. Each named vaccine loads its published trial efficacy into the efficacy field.

• Population: The number of people covered by the vaccination program, from a 500-person high school to a world population of 8 billion, or a custom number.
• Vaccine efficacy: The share of severe cases the vaccine prevents, as a percentage. Each named vaccine loads its published value; you can override the number for sensitivity testing.
• Severe complication rate (unvaccinated): The share of unvaccinated COVID-19 cases that become severe. The default 20% matches the CDC-aligned unvaccinated baseline.
• Baseline severe cases: Severe-case count with no vaccination, equal to population times the severe rate.
• After-vaccination severe cases: Severe-case count after vaccination, equal to baseline severe times (1 - efficacy).
• People saved: Severe cases prevented by vaccination, equal to baseline severe minus after-vaccination severe.
• Relative risk reduction: The vaccine's efficacy expressed as a percentage between 0 and 100.

The order of operations matters. Population is multiplied by the severe rate first to give baseline severe cases. Only after that does the efficacy assumption scale the after-vaccination number.

The named-vaccine table is a convenience: the override slider can re-enter any value from 0 to 100 percent. The override is the right place to enter a real-world effectiveness number from a cohort study.

According to CDC COVID-19 Vaccine Benefits, the 2023-2024 COVID-19 vaccines reduce the risk of critical illness by almost 70% in the first 2 months after vaccination, with protection decreasing to about 50% by 10 months.

If you also need the population-level susceptible-infectious-recovered curve behind your assumed 100% exposure, the Viral Infection SIR Calculator models the epidemic dynamics that produce the case count in the first place.

Four ideas are enough to understand and trust the numbers this calculator reports.

A 92% efficacy does not mean 92 of 100 vaccinated people are immune. It means the severe-case rate in the vaccinated group is 8% of the unvaccinated rate.

For a sharper read on the severe-vs-fatal distinction in the after-vaccination number, the COVID Mortality Risk Calculator breaks the same people-saved logic down by age and case-fatality rate.

Pick a vaccine and a population first, then let the vaccine efficacy calculator fill in the published efficacy and a 20% severe baseline. The result panel updates as you change the inputs.

1. 1 Choose a named COVID-19 vaccine: Pick Pfizer-BioNTech, Moderna, AstraZeneca, J&J, Sinopharm, or Sputnik V. The efficacy field updates to the published value.
2. 2 Select a population or choose Custom: Pick a school, city, country, continent, or the world, or Custom to enter your own number.
3. 3 Review the default efficacy and severe rate: Vaccine Efficacy shows the published value. Severe Complication Rate defaults to 20% (the CDC-aligned unvaccinated baseline).
4. 4 Read the four results in the right panel: The black People-Saved card is the headline. The rows below show baseline severe, after-vaccination severe, and relative risk reduction.
5. 5 Override efficacy or severe rate for sensitivity: Replace the auto-filled efficacy with a real-world effectiveness number, or drop the severe rate to 5% for a low-risk population.
6. 6 Compare a second vaccine for the same population: Change only the vaccine dropdown. People saved, baseline severe, and relative risk reduction all refresh, so the comparison stays apples-to-apples.

For the US at 330 million people with the Pfizer-BioNTech vaccine (92% efficacy) and the 20% severe baseline, the calculator returns 66,000,000 baseline severe cases, 5,280,000 after-vaccination severe cases, and 60,720,000 people saved. Switching the dropdown to Johnson & Johnson (66% efficacy) drops the people-saved figure to 43,560,000 for the same population.

When you want to plan a daily first-dose rate against a national coverage target, the US Vaccine Strategy Calculator pairs the people-saved number with the pacing required to reach a percent-vaccinated deadline.

Translating a percentage into a count helps different audiences act on the same efficacy number.

• • Turns trial efficacy into a planning number: The calculator converts 92% efficacy into a per-population severe-cases-prevented count that a school board, a city council, or a national health agency can plan around.
• • Compares named vaccines on the same population: Switching the dropdown keeps the population and the severe rate constant, so the people-saved difference between Pfizer and J&J is a fair head-to-head.
• • Stress-tests assumptions in seconds: Override the efficacy slider with a real-world effectiveness number, or drop the severe rate to 5% for a young healthy cohort, and the result panel shows the lower-bound figure immediately.
• • Pairs with related vaccine calculators: The same people-saved logic anchors related calculators on the site, so the assumption you settle on here can flow into event risk, vaccine strategy, and mortality-risk calculations without re-deriving the math.

The biggest benefit is shared vocabulary. Two planners looking at the same 92% efficacy number can disagree about whether a 330 million-person country is well protected, but with the calculator they both see the same 60.72 million severe cases prevented.

If you also need to model who is in line for the doses that drive these people-saved numbers, the Ireland Vaccine Queue Calculator estimates vaccination queue timing by age, dose, and supply rate.

The same vaccine can produce very different people-saved numbers depending on the inputs. The biggest factors are listed first.

• • The calculator assumes 100% of the population is exposed to the virus. In a real outbreak, exposure is far below 100% and the people-saved number is an upper bound.
• • The severe complication rate is a single round number across the whole population. Real risk is higher for older adults and people with underlying conditions, lower for healthy young adults.
• • The model does not separate one-dose and two-dose vaccines, partial series, or breakthrough infections.

The best way to use this tool is to read the people-saved number as a planning upper bound, then narrow it with your local coverage fraction, severe rate, and real-world effectiveness.

According to World Health Organization - Sinopharm feature story, the Sinopharm COVID-19 vaccine shows 79% efficacy against symptomatic SARS-CoV-2 infection 14 or more days after the second dose.

According to The Lancet - AstraZeneca vaccine trial, the AstraZeneca vaccine shows 76% efficacy at preventing symptomatic COVID-19 in the published pooled analysis.

When you want to see how the people-saved figure breaks down by cardiovascular and metabolic risk factors, the CVD Risk Calculator estimates baseline severe risk for the high-risk groups that drive the largest share of the after-vaccination number.