With nearly half of all states planning to open COVID-19 vaccine eligibility to all adults by mid-April, your turn to receive the vaccine may be closer than ever before. With three currently authorized vaccines available, each with different dosing schedules and varying efficacy rates, you may be wondering whether one vaccine is better than another or which one is best for you. Health officials maintain that the best vaccine for you is the one you can get. To understand why this is the case, let’s break down what vaccine efficacy means, why you shouldn’t necessarily compare efficacy between vaccines, and what is most important when considering your decision to be vaccinated.
Understanding Vaccine Efficacy
Clinical trial data for Pfizer, Moderna and Johnson & Johnson showed that vaccine efficacy rates were 95%, 94% and 66% respectively. A common misinterpretation of these numbers is that out of 100 people, the Pfizer vaccine would protect 95 people from COVID-19 and only 5 people would get sick, whereas the Johnson & Johnson vaccine would only protect 66 people from COVID-19 and the remaining 34 would get sick. This is simply not true.
Let’s break things down to understand how vaccine efficacy is calculated and what it actually means.
People enrolled in a clinical trial are split into two groups – half receive the vaccine and half receive a placebo – and those people are monitored to see who gets sick with COVID-19 and who doesn’t. There were 170 total cases of COVID-19 among the approximately 40,000 clinical trial participants.
If the 170 cases were split evenly between the vaccine group and the placebo group, with each group reporting 85 cases, the vaccine efficacy would be 0%. You would have an equally likely chance of getting COVID-19 whether you were vaccinated or not.
If all 170 cases were in the placebo group and 0 cases in the vaccine group, the vaccine would have 100% efficacy.
In actuality, the trial showed that of the 170 cases, 162 were in the placebo group and only eight were in the vaccine group, which equates to a vaccine efficacy of 95% [calculation for vaccine efficacy: [1 – (8/170) x 100 = 95%]. This means that the people in the vaccine group were 95% less likely to get sick from COVID-19 than the people who did not receive the vaccine.
Why You Shouldn’t Compare Vaccine Efficacy Rates
In order to compare efficacy of one vaccine to another, the vaccine trials would have needed to be studied at the exact same time, in the same population and under the same conditions – but this didn’t happen. The Pfizer and Moderna trials both took place between July and November 2020, with the Pfizer data coming mostly from the United States and the Moderna data coming exclusively from the United States, whereas the Johnson & Johnson trial took place between September 2020 and January 2021, mostly outside of the United States, in countries such as South Africa and Brazil. This is notable because at the time of the Johnson & Johnson trial, South Africa and Brazil were experiencing rising case numbers and new variants were emerging, which became the dominant strains of the virus in these countries. In fact, the data from South Africa showed that most of the cases detected in the Johnson & Johnson trial were from one of the variants, not the original strain that was in the United States over the summer when the Pfizer and Moderna trials were taking place. If you look specifically at Johnson & Johnson’s US-specific data, which showed a 72% efficacy rate, there was a huge rise in the number of cases during the time their trial took place. The fact that these trials all took place in different countries, with different populations, under different conditions means that it’s difficult to compare their vaccine efficacy rates.
Why Vaccine Efficacy Isn’t the Most Important Number
While vaccine efficacy is an important metric by which to evaluate the success of a vaccine, it’s important to remember that the ultimate goal of the COVID-19 vaccine was not to protect against every single infection, or even infections that result in mild or moderate symptoms. The goal of this vaccine was to protect against COVID-19 that results in serious illness requiring hospitalization or death. The Pfizer, Moderna and Johnson & Johnson vaccines are all 100% effective at preventing against hospitalizations and death from COVID-19.
So rather than focusing on which vaccine has the highest efficacy rate in a clinical trial, we should be focusing on which vaccine will protect us from serious illness or death, and which vaccine will help end this pandemic. And the answer to that question is ... any of them.
Resources
- https://www.nejm.org/doi/full/10.1056/NEJMoa2034577
- https://www.nejm.org/doi/full/10.1056/NEJMoa2035389
- https://www.youtube.com/watch?v=K3odScka55A
- https://time.com/5945177/covid-19-vaccine-effectiveness/
Lianne Jacobs, Product Analyst, has a master’s degree in public health from Yale University. She is the only indoor cycling instructor who can’t ride a bike. She enjoys traveling the world, laughing at her own jokes, and tricking her husband into eating baked goods made with hidden vegetables.