COVID-19 Vaccine Efficacy: What It Means

What does the COVID-19 vaccine efficacy rate tell us about the vaccine? Learn more about the meaning of the efficacy rate and how other factors are also important to note when comparing COVID-19 vaccines.

by Raihan Rahman

What are vaccine efficacy rates?

Vaccine efficacy rates are generated from clinical trials where a large number of individuals are divided into two groups where one group receives the vaccine, and the other does not. These two groups are then free to continue their daily routine while scientists monitor whether or not they contract a particular virus. The COVID-19 vaccine efficacy rate was determined using the same method mentioned above.

For example, the Pfizer vaccine’s clinical trial for the COVID-19 virus had 43,661 volunteers. The volunteers were split into two groups: a placebo group and a vaccinated group. Each group contained 21,830 people. The results are as below:

  • Out of the 21,830 volunteers in the placebo group, 162 participants contracted COVID-19.
  • Out of the 21,830 volunteers in the vaccinated group, 8 participants contracted COVID-19.

What is a placebo group?

A placebo group is a number of individuals who received a placebo – a harmless substance that does not have any therapeutic value in order to test the efficacy of the vaccine. The use of a placebo group is regarded as standard practice when testing out the efficacy of new vaccines or medicines.

The efficacy of the vaccine was determined by how many people were infected in each of these groups. According to the CDC, the efficacy rate can be determined by this calculation:

The vaccine efficacy formula:-

For the Pfizer trial, the risk among each group can be calculated as:

  • Placebo group: 162/21830 X 100 = 0.74%
  • Vaccinated group: 8/21830 X 100 = 0.04%

Using the above formula, we can calculate the efficacy rate for the Pfizer vaccine as below:

The efficacy rate of other vaccines is determined in the same manner as the above with some different criteria such as what represents a ‘severe’ case of COVID-19 as opposed to a ‘mild’ case. An efficacy rate does not guarantee that in a group of people, a certain number of individuals WILL get infected by the virus. 

Instead, the efficacy rate tells us how well the vaccine will work to protect any particular individual from a COVID-19 infection compared to a person who is not vaccinated.

However, some factors may affect a vaccine’s efficacy rate and a lower efficacy rate may not be a true indicator of how well the vaccine protects you against COVID-19.

How do clinical trials affect efficacy rates?

For the existing COVID-19 vaccines, each of the efficacy rates generated was from various times during the pandemic. The clinical trials for the 2 American-made vaccines, Moderna and Pfizer were conducted solely in the United States from August to November of 2020.

However, Johnson & Johnson conducted their clinical trials from October to late January 2021. In comparison, the two periods had vastly different numbers of COVID-19 cases per capita with the Johnson & Johnson clinical trials having a much more severe number of cases.

Geographical Consideration

Differing from Moderna and Pfizer, the Johnson & Johnson vaccine clinical trial was largely conducted in countries other than the United States of America, mainly South Africa and Brazil. These countries not only had different COVID-19 case numbers, but the virus itself was also different.

Variant Consideration

The Johnson & Johnson clinical trial was conducted during the emergence of the B.1.351 variant in South Africa and the P.2 variant in Brazil. These variants were not only the leading number of cases in the respective country’s but they were also more contagious compared to the average strain.

Clinical Trial Criterias

In sum, a vaccine can only be compared if each clinical trial is conducted in the same region, at the same time with the same COVID-19 strain and inclusion criteria i.e. what counts as a COVID infection. Each vaccine and its corresponding efficacy rate signify the efficacy rate according to their unique clinical trial. Therefore, experts maintain that the efficacy rate may not be the only number we should be concerned with when evaluating a vaccine’s effectiveness.

Is the efficacy rate all we should look at?

Considering the whole purpose of the COVID-19 vaccine, the efficacy rate is not the only factor to consider. Since getting vaccinated still carries the possibility of getting infected, the aim of the vaccine is also to reduce the severity of the virus to any infected individual. In other words, the vaccine is designed to reduce the likelihood of severe symptoms, hospitalisation and death for COVID-19 positive individuals. According to all of the trials conducted by each of the available vaccines, each vaccine was effective in reducing the severity of the disease by at least 90%.

Why should an individual get vaccinated if it does not protect them from a COVID-19 infection? 

 An individual should get vaccinated against COVID-19 as it will help to reduce the likelihood of a serious infection of the virus. Each vaccine has been proven to be effective in eliminating the possibility of death or hospitalisation for COVID-19. Although you may get infected, the danger of the COVID-19 virus has been significantly reduced by the vaccination. 

Protect Yourself and Others

The vaccination not only helps protect you against COVID-19 but also others that may not be fit to receive the vaccine. By doing our part to lower the number of cases of COVID-19 and its extremity, we are easing the burden on healthcare facilities and professionals and moving the world closer to ending the COVID-19 pandemic.


CDC (2021) COVID-19 Vaccines Work, Available at: (Accessed: June 1, 2021).

Janssen investigational COVID-19 vaccine: Interim analysis of phase 3 clinical data released (2021) Available at: (Accessed: June 1, 2021).

Pfizer and Moderna Vaccine Trial Summary (no date) Available at: (Accessed: June 1, 2021). (2021) Why you can’t compare Covid-19 vaccines. Available at:

About the Writer
Raihan Rahman
Raihan loves psychological thriller books and horror movies but sleeps with a night light, lest the monsters get her.
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