What Does a 95% Effective Vaccine Really Mean?

TL;DR

A 95% effective vaccine means vaccinated trial participants were 95% less likely to develop the trial’s defined illness outcome than placebo recipients, not that each person has a 5% chance of getting sick. In the BioNTech-Pfizer trial, 8 vaccinated participants and 162 placebo recipients developed COVID-19. Because vaccine trials used different criteria, times, places, and circulating strains, their headline percentages cannot be directly compared; read on to understand what the numbers actually reveal.

Transcript

Thanks to Brilliant for supporting this episode of SciShow. You can check out Brilliant.org/SciShow to learn more and get 20% off an annual premium subscription. This episode was filmed on March 19th, 2021. For up-to-date information on the COVID-19 pandemic, check out our playlist linked in the description. [♪ INTRO] If I offered you a choice betw... Read More

Key Insights

• ☠️ Efficacy rates for COVID-19 vaccines are based on clinical trials' primary endpoints and vary due to different trial criteria.
• ☠️ Comparing efficacy rates between different vaccines is not valid since trials were conducted at different times, locations, and with different virus strains.
• ❓ All authorized COVID-19 vaccines are highly effective at preventing death, which is a crucial outcome.
• ☠️ Real-world effectiveness of vaccines can be influenced by factors like vaccination rates, emergence of new variants, and social and racial inequities.
• 🦺 COVID-19 vaccines have undergone extensive testing and have been deemed safe and effective by regulatory bodies.
• 🌍 It is important to understand the difference between efficacy (in trials) and effectiveness (in the real world) when evaluating vaccine performance.
• 🔨 Vaccination remains a vital tool in reducing illness, hospitalization, and death from COVID-19.

Questions & Answers

Q: What does a 95% effective vaccine really mean?

It means vaccinated participants were 95% less likely than placebo recipients to develop the illness outcome defined by that clinical trial. It does not mean each vaccinated person has a 5% chance of getting sick.

Q: How was the BioNTech-Pfizer vaccine’s 95% efficacy calculated?

The vaccine and placebo groups each included about 21,000 participants. Researchers recorded 8 COVID-19 cases in the vaccine group and 162 in the placebo group; the reduction in cases among vaccinated participants produced the 95% efficacy figure.

Q: Why can’t the efficacy rates of COVID-19 vaccines be directly compared?

The trials used different case definitions and began counting cases at different points after vaccination. They also occurred at different times and places while different virus strains were circulating, so BioNTech-Pfizer’s 95%, Moderna’s 94%, and Janssen’s 66% were not measured under matching conditions.

Q: What outcomes did the BioNTech-Pfizer, Moderna, and Janssen trials measure?

BioNTech-Pfizer and Moderna counted any symptomatic COVID-19 after two vaccine or placebo doses. Janssen instead evaluated efficacy against moderate to severe cases.

Q: When did the Pfizer and Moderna trials begin counting COVID-19 cases?

Moderna began counting cases contracted 14 days after participants received the second shot. Pfizer began counting after 7 days, illustrating why even similar headline efficacy rates may rest on different criteria.

Q: Does Janssen’s 66% efficacy mean recipients have a 34% chance of getting sick?

No. The 66% figure compares illness among vaccinated trial participants with illness among placebo recipients under Janssen’s defined endpoint; it is not an individual’s absolute chance of getting sick. The transcript also notes that Janssen’s data was still awaiting peer review when the episode was filmed on March 19th, 2021.

Q: How well did the three vaccines prevent death in their trials?

All three vaccines were 100% efficacious at preventing death in the trials discussed. Reducing death and the burden on hospitals were among the main reasons health officials urged people to accept whichever vaccine they were offered, provided they did not have related allergies or other health concerns.

Q: What is the difference between vaccine efficacy and effectiveness?

Efficacy describes how well a vaccine works in controlled trials, while effectiveness describes how it works in the real world. Real-world results may be affected by community vaccination levels, whether more contagious variants become dominant, and racial or other inequities in vaccination rates.

Summary & Key Takeaways

• COVID-19 vaccines have different efficacy rates, with the BioNTech-Pfizer vaccine at 95%, Moderna at 94%, and Janssen (Johnson & Johnson) at 66% (awaiting peer review).

• Efficacy numbers are calculated based on the number of cases in the placebo group compared to the vaccine group.

• Comparing efficacy numbers between different trials is not valid due to differences in trial criteria, locations, and timing.