How are Strains of Influenza Chosen for the Annual Vaccine?

Everyone knows about influenza vaccines. For most of us, it's just a matter of making the time to get one. Another group, however, has something else to consider. An advisory group to the FDA has to decide which strains to include in the vaccine — and they have to do it months before the flu season starts. And they have to be right.

Watch this short video to find out more about the data they rely on and how they try to successfully predict the future.


Paul Offit, MD: Hi, my name is Paul Offit. I’m talking to you today from the Vaccine Education Center here at the Children’s Hospital of Philadelphia. A question that I wanted to address is, how is it that we choose different strains of influenza every year to make the vaccine? And this is sort of a section where I’m going to try to explain the science simply. I think the minute people hear the word science, they immediately turn the video off, but don’t turn it off because I’m going to make it more fun.

So influenza is different really than every other virus that we try and prevent with a vaccine. And what makes it different is that on the surface of that virus there is a protein called the hemagglutinin. Now, that’s the protein that’s responsible for binding to cells that then allow the virus to enter the cell. So, the way that your immune system works, is for every virus, is it tries to prevent the virus from binding to cells. Or, in the case of influenza, it means trying to make an antibody response against the hemagglutinin, the influenza hemagglutinin. The problem with influenza virus is that every year that hemagglutinin mutates, or changes, so much from one year to the next that natural infection or immunization the previous year doesn’t prevent disease the following year. So, every year, you have to try and make new influenza vaccines that account for those mutations that occur in the influenza hemagglutinin.

Now the way that the decision is made in terms of which strains to include is it’s made by two bodies. One is the World Health Organization that makes a recommendation for what strains should be given basically to people who are making vaccines throughout the world. The second group in the United States is called the Food and Drug Administration. There’s a particular subcommittee. I’m actually fortunate enough to be on that subcommittee that decides which strains to include.

And the way that that decision is made is which strains are circulating in the world. And for North America, us, it typically tends to be strains that come up from South America. And usually we’re right. I’d say most of the time you can predict which strains are going to be coming into the United States based on which strains are circulating in the world and which strains are circulating in South America, but you’re not always right.

So for example in the 2014-2015 year, when we picked our vaccine strains, between the time that we picked our vaccine strains and the companies then spent the roughly, you know, four to six months it takes to make and test those strains, that vaccine, or the natural circulating virus, mutated far enough away from the vaccine that the vaccine wasn’t protective, and the protective efficacy that particular season, the 2014-2015 season, was only 13 percent, which is woeful.

So, I think we’re pretty good at this generally of predicting which strains to pick, but influenza is difficult. And it’s difficult because every year it mutates and every year we need to make a new vaccine, and it’s a vaccine that's recommended for everybody that is over 6 months of age in the United States, which means, you know, a few hundred million people. And it's I think really burdensome I think to the healthcare system, and obviously difficult for parents and children to have to get this yearly vaccine. But it is a virus that causes every year several hundred thousand people to be hospitalized and tens of thousands of people to die. So even though the vaccine sometimes is only 40 or 50 percent effective, that’s better than zero percent effective, which is what the efficacy is when you choose not to get the vaccine.

Thank you.

Related Centers and Programs: Vaccine Education Center