A Centers for Disease Control and Prevention (CDC) report published on April 2 found that the two-dose COVID-19 vaccine regimen prevented 90 percent of coronavirus infections two weeks after the second dose, which is when you are considered fully vaccinated. Of the 2,479 vaccinated people in the CDC study, just three had confirmed coronavirus infections after they were fully vaccinated.

Importantly, even if you do get infected after your vaccination, your case is likely to be asymptomatic or mild, like a common cold, says Gregory Poland, an infectious disease physician at the Mayo Clinic in Rochester, Minnesota, and director of Mayo's Vaccine Research Group.

He stresses that all three authorized COVID-19 vaccines — from Pfizer-BioNTech, Moderna and Johnson & Johnson — are extraordinarily effective at preventing severe illness: “I've been a vaccinologist for four decades, and I've never seen efficacy like this in first-generation vaccination.”

What efficacy rates really mean

You've probably heard about each vaccine's efficacy rate. In their clinical trials, Pfizer-BioNTech's and Moderna's two-shot vaccines had an efficacy rate of about 95 percent, while the Johnson & Johnson single-dose vaccine had a 72 percent efficacy rate in the U.S.

If a vaccine's efficacy rate is 95 percent, you might assume that 5 out of every 100 people vaccinated people will get sick. But that's not how the math works, says Anna Wald, an infectious disease physician and epidemiologist at the University of Washington School of Medicine.

The actual percentage of vaccinated people who got COVID-19 in both the Pfizer and Moderna clinical trials was far smaller — just around 0.4 percent.

Efficacy is actually calculated by comparing people in a trial who got the vaccine to people who got the placebo, Wald says. So if you received the Pfizer-BioNTech or Moderna vaccine, “whatever your chance was [of getting COVID-19] before, it's now 95 percent less,” Wald explains.

There are two more things to know about those efficacy rates. First, none of the trial participants who received any of the authorized vaccines died of COVID-19. In other words, when it comes to what's most important — preventing death — the vaccines were 100 percent effective in the trials.

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