How Will We Know That a COVID-19 Vaccine Is Safe?

With COVID-19 vaccines being developed at “warp speed,” some people are concerned about vaccine safety. Dr. Paul Offit explains what is being done to ensure that COVID-19 vaccines will be safe and effective.


How will we know that a COVID-19 vaccine is safe?

Paul Offit, MD: Hi, my name is Paul Offit. I'm talking to you today from the Vaccine Education Center at Children's Hospital of Philadelphia. I think probably the question of the day is, how do we know whether or not these COVID-19 vaccines are safe? I think what people worry about is that here we have a virus that just made its debut in the human population; we've only really had it in hand since about a year ago, and now we're going to have a vaccine that will probably be out in just a little over a year. That's much faster than any vaccine has ever been made. So, how do we know that it's going to be safe? Well, the good news is, is we've really subjected these vaccines to the same sort of pre-licensure trials that we do for any vaccines.

So, the vaccines are being tested in so-called prospective, placebo-controlled phase 3 trials, where half of the participants get a vaccine, the other half don't. These trials are 30,000 people big, 44,000 people big, in one case 60,000 people big. So that's as big as any normal vaccine trial. The human papillomavirus vaccine trial was a 30,000-person trial. The conjugate pneumococcal vaccine trial was a 35,000-person trial. So these trials are roughly that size. So, although things are moving quickly, that hasn't moved quickly. That's as big as we always do those kinds of trials.

And what those trials will tell you is that you don't have a very uncommon, severe side effect. Now, if we test it in 20,000 people or 30,000 people, that's not 30 million people. So, you're only going to know whether or not there's a very, very rare serious side effect after these products are released, which is true, frankly, of any medical product. The good news: There are systems in place to pick up those rare adverse events should they occur.

Thank you.

Related Centers and Programs: Vaccine Education Center

Last Reviewed on Nov 13, 2020