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Pfizer and Moderna did not skip animal trials

November 25, 2020 GMT

CLAIM: The Pfizer and Moderna vaccines are mRNAs vaccines that skipped animal trials because using mRNA vaccines on animals triggers dangerous inflammation.

AP’S ASSESSMENT: False. Pfizer and Moderna did not skip animal trials when testing their COVID-19 vaccines. 

THE FACTS: As the race to authorize the use of COVID-19 vaccines continues, posts online are spreading misinformation about vaccine trials. 


Recent posts are criticizing Pfizer and Moderna, who both announced that their vaccines were more than 90 percent effective at preventing COVID-19. One tweet with more than 4,000 likes suggests that the two companies did not conduct animal trials when testing their vaccines because testing mRNA vaccines on animals could cause dangerous side effects. 

The vaccines from Pfizer-BioNTech and Moderna both rely on messenger RNA. MRNA vaccines contain a genetic code that trains the immune system to recognize the spike protein on the surface of the virus to generate an immune response. 

Due to the urgent need for a vaccine in a surging pandemic, Pfizer and Moderna were given approval to simultaneously test their vaccines on animals while they were conducting Phase 1 trials on humans. The vaccines were tested on mice and macaques. 

“They overlapped preclinical studies with the early phases of the trials,” said Dr. William Moss, executive director for the International Vaccine Access Center at Johns Hopkins University. “In fact one of the reasons we are even talking about vaccines now just 10 months later is that some of the phases in which vaccine development normally occurs were overlapped rather than done sequentially.”

Posts online appeared to suggest that the animal trial phase was skipped completely when testing the two vaccines. 

University of Pennsylvania professor of medicine Dr. Drew Weissman, who has been studying mRNA and mRNA vaccines for decades, said they do not cause dangerous inflammation to animals. Along with the vaccines for Pfizer and Moderna both passing animal trials, they also passed clinical trials on humans where they were tested on more than 70,000 people. 


“Clinical trials for 75,000 people show it’s safe and it’s 95 percent effective,” Dr. Weissman said. “That’s pretty good data to convince people that it is OK.”

A serious safety issue would have surfaced in the trials if there was one, said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston and co-director of the Texas Children’s Hospital Center for Vaccine Development.

While mRNA is a new technology, experts have been working on it for years. 

“It’s not just as though these technologies were just starting to be developed during the pandemic,” Dr. Moss said. “There has been a lot of preparatory work.” 

After announcing its successful results, Pfizer is now seeking approval for emergency use of its COVID-19 vaccine from the Food and Drug Administration. But as news about the vaccines continues to make headlines, medical professionals warn that misinformation from people who oppose vaccines will continue to spike online. 

“Now they are going to go after the COVID-19 vaccine,” Dr. Hotez said. 


This is part of The Associated Press’ ongoing effort to fact-check misinformation that is shared widely online, including work with Facebook to identify and reduce the circulation of false stories on the platform.

Here’s more information on Facebook’s fact-checking program: