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Defining vaccine efficacy: How Pfizer and Moderna COVID-19 vaccines compare

INDIANAPOLIS (WISH) – On Thursday the U.S. Food and Drug Administration will consider Moderna’s COVID-19 vaccine candidate for emergency use after granting Pfizer emergency use authorization for its vaccine last week.

Moderna has said their vaccine candidate is 94.5% effective while Pfizer’s vaccine shows 95% efficacy, but what do those numbers mean for people taking the vaccine?

“That half a percent doesn’t make any real difference. The two vaccines can almost be treated as equal,” said Dr. Paul Calkins, Associate Chief Medical Executive at IU Health.

To put those numbers into perspective Dr. Calkins says the measles vaccine, which is the gold standard in vaccines, is 97% effective and in a good year the flu vaccine is between 50% to 60% effective.

“Any vaccine that’s 75% or even 80% effective is better than no vaccine at all,” said Dr. Calkins.

He also says efficacy depends on several factors like a person not getting sick before the second dose, getting the second dose on time and how many people get the vaccine. Factors that will become more clear in the coming months as vaccine distribution continues to expand.

That said, Dr. Calkins also suggests that vaccines are based on population and that half percent between the two companies could be more varied in the real world.

“In a clinical trial you have someone calling all the participants and making sure that things happen exactly on time and that everything is perfect. And real world people don’t have that, they have lives, they they have schedules, maybe they missed their second dose or they get it at the wrong time. So we should assume we might not see 95% in the real world but it should be really high,” Dr. Calkins adds.

Aside from efficacy, the biggest difference between the two vaccines are Moderna’s two doses are 28 days apart while Pfizer’s is 21.

Despite these differences, Dr. Calkins says he would be very surprised if Moderna’s candidate was not granted emergency authorization this week.

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