Why Are Two Doses of the COVID-19 mRNA Vaccine Required?

Why are two doses of the COVID-19 mRNA vaccine required?

Paul Offit, MD: Hi, my name is Paul Offit. I'm talking to you today from the Vaccine Education Center at the Children's Hospital of Philadelphia. When the vaccine was first available in December of 2020, there was a limited supply. Not everybody could get it with those first two vaccines, the mRNA vaccines made by Pfizer and Moderna. And so, there was a priority list as to who could get it and who couldn't. And there was some pressure by a number of people in the media to let's just vaccinate as many people with one dose as we can, and then we'll get them that second dose, eventually.

Now, some people took that to mean that one dose was good enough. If they're saying I can just get one dose and then wait, obviously one dose is protective, so I probably don't even need that second dose. That's not right. I think that for the Pfizer and Moderna vaccines, that's a two-dose vaccine, and the Johnson & Johnson vaccine, frankly, is also a two-dose vaccine. And the reason why is what you're trying to do with these vaccines. What you're trying to do is you're trying to protect against moderate to severe disease for years. The immunological component that is responsible for that is something called memory cells. These are long-lived cells in your body, immune cells in your body, that have names like B cells, which are the kind of cells that make antibodies; T helper cells, which are the kinds of cells that help B cells make antibodies. And then there's another group called cytotoxic T cells, which are the kind of cells that kill virus-infected cells. And they're generally long lived.

So, the point is that if you're going to have fairly high frequencies in your body of these memory B and T cells, you need that second dose in order to provide longer-term, more complete protection.

So, these are two dose-vaccines, not one dose-vaccines anymore. And I think that you should feel comfortable that with that second dose, you will have fairly long-term protection against moderate to severe illness.