Covid-19 vaccines are very effective

We’ve seen widely shared posts on social media claiming that the Covid-19 vaccines are less effective than people believe, based on a series of figures from an article in the Lancet.

An Instagram post shares a screenshot that quotes these figures and claims that the vaccines “reduce your chance of catching COVID-19 by: Pfizer 0.8%, Johnson & J 1.2%, Moderna 1.2%, AstraZeneca 1.3%. So basically NO prevention”.

This is false.

Another post on Twitter claims they show that the vaccines’ efficacy is “not as 95% stated by the vaccine companies”.

This is false too.

The Lancet article is a comment piece, not peer-reviewed research, as the posts also falsely claim. The figures quoted in the screenshot show the absolute reduction of people’s risk in percentage points, not percent.

In other words they describe how much of the already low risk of getting ill with Covid the vaccines took away for participants in the trials. They do not refute any of the existing evidence on how well the vaccines work.

What does the article say?

The article argues that “fully understanding the efficacy and effectiveness of vaccines is less straightforward than it might seem”.

It compares the conventional method of measuring the effectiveness of a vaccine, which is by “relative risk reduction”, with another measurement called “absolute risk reduction”.

Relative risk reduction describes how much someone’s risk of something—in this case, getting ill with Covid—is reduced by the vaccine.

So if 10 unvaccinated people in a group of 100 get ill with Covid, but only one vaccinated person gets ill in a similar group of 100, then we can say that vaccinated group have had their risk reduced from 10% to 1%, which is a reduction in their risk of 90%.

Absolute risk reduction would describe the same situation differently. If 10 out of 100 unvaccinated people get ill with Covid, then the risk of catching it in the group was 10%. In the vaccinated group, the risk was lower, at one out of 100, or 1%. This means that vaccination reduced the absolute risk of getting ill with Covid by nine percentage points. That is the absolute risk reduction.

The two measures therefore describe different things. The reduction in someone’s absolute risk depends on how much risk they were facing in the first place, whereas a relative risk reduction applies no matter how much risk you might be facing.

The Lancet article argues that absolute risk reduction figures should be included alongside relative reduction figures when vaccine studies are reported.

It reports the relative risk reductions from trials of the five Covid vaccines: “95% for the Pfizer–BioNTech, 94% for the Moderna–NIH, 90% for the Gamaleya, 67% for the J&J, and 67% for the AstraZeneca–Oxford”.

It also includes what it says are the absolute risk reductions from the trials of five Covid vaccines: “1.3% for the AstraZeneca–Oxford, 1.2% for the Moderna–NIH, 1.2% for the J&J, 0.93% for the Gamaleya, and 0.84% for the Pfizer–BioNTech vaccines.”

These are the lower numbers that were quoted in the social media posts, which use them to falsely suggest that the vaccines offer less protection than people think.

In fact, these numbers show that the people who were not vaccinated in these trials had a low risk of getting ill with Covid, but the vaccines still took away most of that risk for the people who received them.