Related topics

Claim about Pfizer vaccine effectiveness lacks key context

August 25, 2021 GMT

CLAIM: The Pfizer vaccine is only 42% effective against COVID-19.

AP’S ASSESSMENT: Missing context. The claim stems from a single preprint study that found in July the Pfizer vaccine’s efficacy at preventing COVID-19 infections, including asymptomatic and mild ones, dropped to 42%. A coauthor of the study, as well as other experts, told the AP the figure should not be used to discredit the Pfizer vaccine since the shot continues to maintain strong protection against severe disease and death, which are key metrics.

THE FACTS: As the Pfizer vaccine gained official FDA approval Monday, some social media users pointed to a recent finding that the shot was only 42% effective at preventing COVID-19 infection in July. The posts used that figure to suggest the vaccine is ineffective and to question why it earned the regulatory agency’s highest stamp of approval.

The 42% figure comes from an Aug. 8 study that has not yet been peer reviewed or published in a scientific journal. It was conducted by the health care technology company nference and the Mayo Clinic in Minnesota. The study tracked vaccinated and unvaccinated people from January to July to compare the Pfizer and Moderna vaccines’ effectiveness at preventing any type of infection — asymptomatic and symptomatic.

The report found the Pfizer vaccine was 76% effective at preventing infection throughout most of the study period, but dropped to 42% during the month of July at the same time the Delta variant surged in the U.S.

But the figure is lacking in proper context, especially when it comes to the vaccine’s continued effectiveness at preventing severe illness and death, according to Dr. Andrew Badley, a coauthor of the study and Mayo Clinic infectious disease specialist.

The study researchers determined the Pfizer vaccine remains 85% effective against hospitalization from COVID-19 and 100% effective in preventing death among study participants. Badley said using just the 42% figure to undermine the FDA’s approval of the vaccine or to suggest it does not work amounts to “an incorrect analysis of the data.”

He added that there is no data to suggest the vaccine’s effectiveness at preventing severe disease and death has declined over time.

“What our report has shown is that both mRNA vaccines, Pfizer and Moderna together, maintain very very strong protections against hospitalization and death and as a vaccine recipient those are the primary outcomes we’re interested in,” Badley said.

However, he said the research does point to the need to develop booster shots to bolster waning immunity over time, an issue experts predicted early on.

“Effectiveness is expected to wane over time,” Dr. Anna Durbin told The Associated Press in an email. Durbin is a professor at the Johns Hopkins Bloomberg School of Public Health studying vaccines for COVID-19.

“The really excellent news is the continued high efficacy against severe/hospitalized illness and death,” she wrote, emphasizing that the study proves the vaccines are still working. “These data only further support the high effectiveness of these vaccines.”

Pfizer reported in late July that six months into its original study, the vaccine remained 97% protective against severe COVID-19. Protection against milder infection decreased slightly, from a peak of 96% two months after the second dose to 84% by six months.

Dr. Hana Mohammed El Sahly, a molecular virology and microbiology professor who works with the Vaccine and Treatment Evaluation Unit at the Baylor College of Medicine, cautioned against pulling any one number from single studies without full context.

“What happened in these clinical trials is that both Moderna and Pfizer vaccines had efficacy against symptomatic disease in excess of 90%, so I think that number kind of stuck in the minds of all of us and we started calling any drop from this particular number problematic, when it is not,” El Sahly said. “The good news, the flip side of this, is that the effectiveness against severe disease is maintained.”


This is part of AP’s effort to address widely shared misinformation, including work with outside companies and organizations to add factual context to misleading content that is circulating online. Learn more about fact-checking at AP.