Vaccine Efficacy: What to Know

It has been roughly one year since the United States declared a national emergency in response to the COVID-19 pandemic. Our efforts to social distance, wear masks, and stay home whenever possible have contained the virus as best we can. However, these efforts only go so far to stop it. The advancement of therapeutics and the arrival of vaccines are key in our fight to end COVID-19. While there is much deserved excitement around the COVID-19 vaccine, there is misinformation about the effectiveness of vaccines. One key concept to understand is vaccine efficacy.

Vaccine efficacy refers to a vaccine’s outcomes during clinical trials. These tests are constructed in order to ensure the vaccine is safe and effective by passing efficacy tests. Scientists develop testing of vaccines between a vaccinated group of healthy people and an unvaccinated or control group. Efficacy is measured based on how well the vaccination group fares with COVID-19 infection and transmission compared to the unvaccinated group. If a vaccine has an efficacy of 95% for example, that means there was a 95% decrease of COVID-19 infection and transmission in the vaccinated group compared to the unvaccinated group.

Vaccine efficacy does not refer to: 

  • the number of people that the vaccine protects out of 100 (for example, a 95% efficacy rate does not mean the vaccine protects 95 people out of 100); 
  • the percentage chance you will contract COVID-19 (for example, a 95% vaccine efficacy rate does not mean there’s a 5% chance you will get sick with COVID-19); 
  • or a grading scale, where the vaccine is graded out of 100 compared to other vaccines.

While vaccinations are not absolutely perfect at completely preventing COVID-19 infections and transmissions, they are overwhelmingly effective at preventing COVID-19 hospitalizations and deaths.

The same safety standards apply to all available vaccines. All vaccines made available to the public have passed their efficacy tests. The safety of a vaccine does not depend on when the vaccine was made available, how many shots it requires, etc.

Specific to the COVID-19 vaccine, the efficacy ratings differed when each was approved by the Federal Drug Administration for emergency authorization use, but the ratings of the three vaccines available today cannot be compared as they are specific to the clinical trials that produced them, and those trials were not conducted in the same ways or during the same time period. 

Timing is a critical component, too. The Moderna and Pfizer/BioNTech studies finished enrolling participants in their phase 3 trials in October and reported their results in late November. The Johnson & Johnson phase 3 trial only finished enrolling participants in December 2020 and reported their results in January.

That means the Johnson & Johnson vaccine was tested during one of the most severe stages of the pandemic, when transmission, cases, and hospitalizations were at their worst in many places around the world, including the US. The trial also captured efficacy against the new variants of SARS-CoV-2 (the virus that causes Covid-19) which began circulating at this point in some parts of the world. Several of these variants have shown themselves to be more contagious, deadlier, and more likely to evade protection from vaccines and prior immunity.

The vaccines do have some important differences. The Johnson & Johnson vaccine is one dose, while the others require two. It also can be stored at refrigerator temperatures, while the others require freezer temperatures. 

The three vaccines available all have one thing in common: In clinical trials, all were shown to reduce the fatality rate of COVID-19 by 100% compared to their placebo groups. 

Together, we can defeat COVID-19 by supporting the science behind vaccinations, getting our vaccines as soon as they’re available, and continuing to social distance from those after us in line.