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Vaccine Efficacy Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Steven Wooding
Last updated: Jan 18, 2024

Efficacy, effectiveness… All these words seem familiar, but what do they really mean to us? Does the 67% efficacy indeed suck? 🤔

Thanks to the mass vaccination, we're finally able to feel that the world is slowly going back to normal. However, our job is not done yet – the pandemic is still here, taking the lives of over 40,000 people every day.

We may ask ourselves – what can we do to convince people to finally get vaccinated? We decided to create a tool that will show you how the vaccine's efficacy saves people in real life and explain all the doubts and math that stand behind it.

Fasten your seatbelts – we're going for a ride! 📚

What is efficacy?

When we produce the vaccine, we first find out about its efficacy.

Efficacy is the very first measurement of the work of the vaccine – the scientists establish it during a clinical trial. Efficacy has a limited usage – it tells a story of a vaccine placed in almost ideal conditions in a limited amount of time. 🙂

However, we still value it as an excellent source of knowledge about the vaccine's quality.

How do we measure efficacy?

Efficacy is measured as a percentage of people who got vaccinated and didn't get the disease.

To put things simply, if we vaccinate 100 people, and 5 of them still come down with COVID, we can simply say that our vaccine's efficacy is 100 - 5 = 95%.

What is effectiveness?

Effectiveness, in comparison to efficacy, was tested in real-life conditions in a significantly larger number of people.

As we all know, life is not perfect, and the number of surprises the vaccine may face cannot be entirely predicted & replicated in a short span of a clinical trial.

How do we measure effectiveness?

Effectiveness is measured the same way as efficacy; it's the number of patients who avoided getting sick in a group of 100 vaccinated people.

Efficacy vs. effectiveness

  • The efficacy is always available earlier than effectiveness.
  • The effectiveness is usually lower than the efficacy.
  • We usually prefer effectiveness as a source of knowledge – the effectiveness is more universal and can be applied to a broader range of patients.





Participants of the clinical trial

Real-life patients




Numeric value

Usually greater

Usually smaller

Preferred source of knowledge?



Still hungry for knowledge? Try our other medical math tools:

Vaccines' clinical trials

Let's start with the very basics – in order to have the vaccine, we first need to have a clinical trial.

A clinical trial is a set of pre-defined conditions that we need to follow to treat a group of patients with a new drug. An excellent clinical trial will always compare the results of a new drug with an old drug, placebo, or a standard treatment. 💊

Clinical trials are focused on the safety of the participants – they usually have a limited amount of time, and all the patients are closely monitored for any side effects.

💡 Placebo is a substance that has no impact on a human body and a given disease. The researchers disguise it as a real drug and give it to patients to test whether the effect of the drug is caused by a coincidence or the patient's behavior.

Why did we get vaccinated?

We need to face the music – a number of people who get vaccinated will still get the disease. Then why are we so convinced that it's the right thing to do?

Well, we know that both statistics and math are on our side! 🤓

Let's think about the most popular vaccine worldwide, Pfizer; its effectiveness is close to 92%.

  • Out of 100 people vaccinated, 92 will avoid getting sick at all.
  • Out of 8 people who got vaccinated and still came down with COVID, less than 2 people will face serious disease complications.
  • None of the patients will be affected by severe side effects of the vaccine.
  • None of the patients will die of COVID-19.

Now imagine that none of these 100 people got vaccinated, and all of them caught the coronavirus (remember - almost nobody's immune to this disease!).

  • Without the vaccine, 20 people out of 100 will face serious consequences of the infection; up to 4 patients will die.
  • All the 80 lucky, seemingly healthy individuals may experience a lowered quality of life and mild worsening of health.

We all need to remember that the benefits of taking the vaccine increase along with the coronavirus threat for a given person – the more you are at risk, the more you benefit from your jab.

Just to remind you, people who are at greater risk of the severe/critical course of the disease include:

  • Older people;
  • People with BMI> 25;
  • People with diabetes;
  • People with kidney disease or kidney failure;
  • People with lung diseases, especially lung cancer; and
  • People that suffer from any kind of immunity insufficiency, such as HIV/AIDS.

*You can learn more about these diseases by checking out our diabetes risk calculator, our kidney failure risk calculator, and our lung cancer risk calculator.

AstraZeneca and J&J Jansen clotting risk

Blood clots caused by the vaccines – should we panic right now, or not just yet? 🙊 Let's find it out with our beloved friend, math.

Imagine a city with almost 9 million inhabitants – just as big as London in the UK or New York in the USA.

Let's imagine that:

  1. Everyone in the city gets COVID-19 🤒

    • 1,485,000 people will get blood clots just because of the disease.
  2. Everyone in the city is a smoker. 🚬

    • 16,200 people will get blood clots just because they smoke cigarettes.
  3. Everyone in the city takes the birth control pill 💊

    • 7,650 people will get blood clots just because of the pill.
  4. Everyone in the city gets vaccinated. 💉

    • 17 people will probably get blood clots.

What do you think is more probable – being among the 1,485,000 people affected by COVID blood clots or the 17 connected to the vaccine?

💡 We also need to consider the fact that while the risk of smoking, birth control, and COVID-19 can sum up and lead to even worse outcomes, the vaccines do not increase the clotting threat for the entire population, and only lead to the disorder in certain individuals who are prone to such rare reactions.

How to use the efficacy calculator?

It's far easier than it seems! Try our ultrashort instruction below:

  1. Choose the vaccine you'd like to try

    • …and enjoy a few interesting facts we prepared for you!
  2. Choose the population you'd like to calculate

    • It can be either a school, a city, country, continent, or the entire world.
  3. Your results are already here 🎉

    • The first number lets you know how many people in your unvaccinated population would suffer serious complications of COVID if everyone came down with the disease.
    • The second number shows you how many people would be affected if your entire population got vaccinated and had exactly the same exposure to the coronavirus as the previous, unvaccinated group.
    • The third number is the difference between the unvaccinated and vaccinated group – the number of people saved by the vaccines!


  • Centers for Disease Control and Prevention
  • The Lencet
  • Reuters


What is vaccine efficacy?

We may estimate the vaccine's efficacy during the clinical trial – a specific, controlled clinical situation, with a limited number of people and time.

If a vaccine was tested in a real-life environment, we talk about its effectiveness instead.

What is vaccine effectiveness?

If a vaccine has had its effectiveness measured, it means that it was already tested in real life – in a broader range of populations and situations, for a longer period of time.

When a vaccine was tested only during the clinical trial, in specific conditions, we talk about its efficacy instead.

What is better: efficacy or effectiveness?

A vaccine's effectiveness is usually lower than its efficacy and that's totally ok. The effectiveness gives us a fuller picture of vaccine's work in different environments, situations, and patients. That's why, when we need to choose more reliable data, we usually choose effectiveness.

How do we find the vaccine's efficacy?

To find the vaccine efficacy, we need to conduct a clinical trial. During the clinical trial, we'll be able to compute the number of people who didn't get the disease after being vaccinated. This value is called the efficacy and is given as a percentage.

Łucja Zaborowska, MD, PhD candidate
What's your vaccine?
Pfizer-BioNTech vaccine's effectiveness is 92%.

Basketball player Michael Jordan had the efficacy of 49.5% - if he wanted to be as effective as the vaccine, he'd have to almost double all his scores! 🏀→🏀🏀
What's your population?
High-school (average)
If everyone in your population came down with COVID, number of serious complications would be equal to:
Before vaccination
After vaccination 💉
People saved by vaccines:
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