How Do You Know Whether a Vaccine Has Been Tested Long Enough?

Paul A. Offit, MD, discusses at what point a vaccine has been studied enough in order to recommend its use. He talks specifically about the HPV vaccine.


How do you know whether a vaccine has been tested long enough?

Paul Offit, MD: Hi, my name’s Paul Offit. I’m talking to you today from the Vaccine Education Center here at The Children’s Hospital of Philadelphia. One question parents ask, and I think it’s reasonable, is when a vaccine is newly licensed, or when there’s a new recommendation for an existing vaccine, do we know enough? Has it been tested long enough to be able to reassure us that it really is safe?

And I think the fair question, frankly, isn’t whether or not vaccines have been tested long enough. I think probably the fair question is, do we know enough to start giving this vaccine? So let’s take the human papillomavirus vaccine for example. When that vaccine was first recommended for adolescents in this country was in 2006, what did we know? We knew that the vaccine had been tested in about 30,000 people for seven years. We knew that it induced an immune response that would protect against people being infected with that virus and therefore protect against the cancers caused by that virus. We knew that two of the strains in that vaccine would protect about 70% of cancers caused by HPV, and two other strains would protect about 90% against anal and genital warts. We knew that it was made using a technology that had also been used to make the hepatitis B vaccine, and that that technology was at least 25 years old.

So we knew all of that. And we knew that if we chose not to give that vaccine, if we chose, for example, to wait 30 years to make sure that it didn’t cause a problem, as the hepatitis B vaccine had not caused a problem, then we would just be exposing, every year, children to … about 25,000 children to get cancer 20 years later because they weren’t getting that vaccine.

So, I think the better question is always not when do you know everything? Because you never know everything. It’s when do you know enough? And I think certainly for HPV vaccine at that time, we knew enough, certainly should be reassured by the fact that the technology that was used was about 25 years old, which is true of much of the technology that we use to make these vaccines. These vaccine have an enormous long-term record of safety. We’ve been using them since the smallpox vaccine 200 year ago.

Hope that helps, thanks.

Related Centers and Programs: Vaccine Education Center

Last Reviewed on Aug 11, 2015