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Workers prepare boxes of the Pfizer-BioNTech COVID-19 vaccine for shipment at a Michigan factory, Dec. 13, 2020. (AP) Workers prepare boxes of the Pfizer-BioNTech COVID-19 vaccine for shipment at a Michigan factory, Dec. 13, 2020. (AP)

Workers prepare boxes of the Pfizer-BioNTech COVID-19 vaccine for shipment at a Michigan factory, Dec. 13, 2020. (AP)

Tom Kertscher
By Tom Kertscher August 17, 2021

Claim that Pfizer COVID-19 vaccine is 42% effective leaves out key details

If Your Time is short

  • The study, which has not yet been peer reviewed, looked at people who had received at least one dose of the two-shot vaccines. It said the Pfizer vaccine’s effectiveness in preventing COVID-19 infection dropped to 42% in July. 

  • Overall, the study said that both the Pfizer and Moderna vaccines are highly effective in preventing hospitalization.

  • As the virus’ delta variant has taken hold this summer, the vast majority of COVID-19 hospitalizations and deaths have occurred among people who were not vaccinated.

With so much emphasis on vaccination to fight the coronavirus pandemic, it was startling to see one of the three vaccines used in the United States pegged as only 42% effective. 

An Instagram post using that figure has spread widely on social media.

The post is an image of a tweet by conservative activist Jack Posobiec. The tweet says:

"BREAKING: Biden Admin Concerned as Mayo Clinic Study Shows Pfizer Vaccine Dropped to 42% Effective."

A similar headline appears on an article in Human Events, where Posobiec is a senior editor.  

The Instagram post was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.)

We found that it leaves out important context about the study and its findings.

How the study was done

The Mayo Clinic study was posted Aug. 9 on a website for preprints. Preprint studies are preliminary, in that they have not yet been peer reviewed by other experts, and "should not be used to guide clinical practice," says a notice posted with the study.

The researchers compared the effectiveness of the Pfizer and Moderna vaccines in the Mayo Clinic Health System (Arizona, Florida, Iowa, Minnesota and Wisconsin) from January to July. (The Johnson & Johnson vaccine was not part of the study.)

The researchers analyzed the records of adults who had received at least one dose of either vaccine and had not tested positive for COVID-19 prior to their first dose. Both the Pfizer and Moderna vaccines call for two doses.

Key findings

The study found the following:

What gained notice were the figures for July, when the more contagious delta variant took hold across the U.S. and hit Florida especially hard. For that month alone, the vaccines’ effectiveness in preventing hospitalization remained high — 81% for Moderna’s and 75% for Pfizer’s — but was lower for preventing infection — 76% for Moderna’s and 42% for Pfizer’s.

Calling the study’s data "very preliminary" and "limited in geographic scope," Mayo told PolitiFact in a statement:

"We caution against drawing conclusions about vaccine effectiveness from a preprint study, which is intended only to be helpful to the scientific community and has not yet undergone the rigor of peer review."

The study authors noted: "It is important to realize that most vaccines are not 100% effective, particularly against asymptomatic infections. For example, the estimated effectiveness of seasonal influenza vaccines has ranged from 19-60% over the past decade."

It’s also worth noting that since the delta variant became the dominant strain of the virus, COVID-19 hospitalizations and death have occurred predominantly among people who have not been vaccinated.

In total, as of Aug. 9, 2021, more than 166 million people in the United States had been fully vaccinated and, based on reports from 49 U.S. states and territories, there were 8,054 breakthrough cases in which the person was hospitalized or died, according to the latest figures from the federal Centers for Disease Control and Prevention. That comes out to fewer than one-hundredth of 1%.

Seven states, including some with the lowest vaccination rates, accounted for about half of new cases and hospitalizations in the previous week, the White House said at a briefing Aug. 5. They include Florida (49.7% fully vaccinated), Texas (44.6%), Missouri (42.3%), Arkansas (37.7%), Louisiana (37.7%), Alabama (35%) and Mississippi (35.2%). As of Aug. 12, according to Mayo Clinic, 50.6% of the U.S. population is fully vaccinated.

Our ruling

An Instagram post claims: "Mayo Clinic study shows Pfizer vaccine dropped to 42% effective."

The Mayo study, which is not yet peer-reviewed, looked at people who had received at least one dose of the two-shot Pfizer and Moderna vaccines. It found that from January to July 2021, the Pfizer vaccine was 85% effective in preventing hospitalization and 76% effective in preventing infection.

In July alone, after the virus’ more contagious delta variant had taken hold, Pfizer’s vaccine was 75% effective in preventing hospitalization but only 42% in preventing infection.

The statement is partially accurate, but leaves out important details. We rate it Half True.

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Claim that Pfizer COVID-19 vaccine is 42% effective leaves out key details

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