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Why doctors say not to compare efficacy of COVID-19 vaccines

Pfizer and Moderna's mRNA vaccines are 95% effective in preventing symptoms of COVID-19. U.S. clinical trials found the Johnson & Johnson's vaccine is 72% effective.

HOUSTON — Three COVID-19 vaccines have been granted emergency use authorization: Pfizer, Moderna and now Johnson & Johnson.

Each time, the companies provided comprehensive information from clinical trials. Trials conducted by Pfizer and Moderna found the two-dose mRNA vaccines are 95 percent effective in preventing symptoms of COVID-19. Analysis by the FDA found Johnson & Johnson’s single dose vaccine was 72 percent effective in the U.S.

Dr. Linda Yancey is an infectious diseases specialist with Memorial Hermann Hospital.

“Comparing the efficacy between these two is kind of like comparing apples and oranges,” she said.

Dr. Yancey said the Johnson & Johnson vaccine clinical trials were done several months after Pfizer and Moderna. By then, new variants had popped up.

While overall efficacy is important, Dr. Yancey said the Johnson & Johnson vaccine worked very well in preventing severe illness. The vaccine was also effective in protecting against variants from the UK and South Africa.

"If we had a time machine and went back in time and did the Johnson & Johnson vaccines at the same time we did the Pfizer and Moderna trials, I suspect you’d get very similar results," she said. “This is a safe, effective vaccine. Anything that will give you 85 percent protection at worst against dying is a good vaccine to take.”