Fact Check-Pfizer documents do not show that COVID-19 vaccine has 12% efficacy

Social media users have incorrectly claimed that documents released by Pfizer prove that its COVID-19 vaccine has a 12% efficacy.

The false claim has spread widely across Twitter (here, here, here, here, here and here) and on Facebook (here, here, here and here).

One tweet, which received more than 43,000 likes, said: “Pfizer docs that were released shows a 12% efficacy rate, not 95% like we were told” (here).

A spokesperson for Pfizer told Reuters that this was false.

The figure appears to stem from a Substack article (here), which makes the claim that Pfizer documents reveal the efficacy rate of its COVID-19 vaccines is 12% - not the 95% reported in clinical trials.

In the clinical trial, scientists calculated vaccine efficacy using the number of “confirmed” cases reported in the trial.

But in the article, vaccine efficacy is calculated using the number of “suspected but unconfirmed” cases.

Because suspected cases were much higher than confirmed cases, it results in the lower efficacy rate of 12%.

Experts at Meedan Health Desk explained to Reuters how using suspected cases can be misleading.

Pfizer’s study protocol (here) shows that a “suspected” case of COVID-19 is recorded when a participant has symptoms that include a fever, cough, shortness of breath, chills, muscle pain, sore throat, loss of taste/smell, diarrhea, and vomiting.

When a person presents with these symptoms, the protocol explains, they are tested for COVID-19 with a PCR test. If the test is positive, the participant is then recorded as a confirmed case.

Since COVID-19 symptoms overlap greatly with other ailments, such as seasonal flu or the common cold, it is therefore more accurate to use confirmed cases when calculating efficacy.

In the trial, the vaccine demonstrated 95% efficacy in preventing COVID-19 in those without prior infection, at least seven days after the second dose (here).

This means the percentage of people who received the vaccine were at a 95% lower risk of developing COVID-19 than those who did not.

Meedan scientists explain that while 95% is the “true efficacy” of the vaccine, effectiveness can be impacted by factors such as time and different virus variants (here , here).


Misleading. The 12% figure appears to stem from an article that calculates vaccine efficacy using the number of “suspected but unconfirmed” COVID-19 cases recorded in the clinical trial, rather than the number of “confirmed cases”.

This article was produced by the Reuters Fact Check team. Read more about our fact-checking work here .