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

Dr. Paul Offit talks about why getting the second dose of COVID-19 mRNA vaccine is so important.


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

Paul Offit, MD: Hi, my name is Paul Offit from the Vaccine Education Center. Today is Tuesday, January 12th, 2021, and as of this time, there are two different vaccines that are recommended for people in the United States to prevent SARS-CoV-2 infections. They're both messenger RNA vaccines. Now, now they are given as two dose vaccines. In the case of Pfizer, those doses are separated by 21 days. In the case of Moderna, those two doses are separated by 28 days. But, why two doses? Why not give one dose? Wouldn't it be much easier just to give a single dose?

So when these studies were done, there are a variety of sort of trials that are done by the companies when they try and figure out how much of this mRNA to give and how frequently to give it. And what they found when they did these studies, so-called phase 1 studies or dose-ranging trials, is they found what the best dose was — in the case of the Pfizer vaccine that was 30 micrograms, a microgram is a millionth of a gram; in the case of Moderna vaccine, it was a 100 micrograms given in two doses — what they found when they gave one dose and then looked for the level of virus-neutralizing antibodies in the bloodstream, it was relatively low as compared to the level of virus-specific antibodies in the bloodstream of people who have survived the infection, so-called convalescent serum. And so what they did then, because that level of antibodies was relatively low, is they gave a second dose. They found that when they gave that second dose, there was a dramatic increase in the level of neutralizing antibodies, so they’re compared very favorably to that found after a natural infection. In fact, it was in many ways higher than that found after natural infection.

The other thing they found after that second dose is it induced not only B cells to make antibodies, but they found that you could find so-called T helper cells. T helper cells are the helper cells that actually make B cells make antibodies. And when they saw those T cells, that made them feel especially good, because that meant now that you probably had fairly long-lived memory responses. So even if your antibody responses fade, because you had these memory cells, that really suggested that you would have longer-lived immunity, immunity that could last years.

So, that's why the second dose. The second dose gives you much better and likely longer-lasting immunity than a first dose alone would give you. That's why it's so important to get that second dose and to get that second dose in a timely manner.

Thank you.

Related Centers and Programs: Vaccine Education Center

Last Reviewed on Feb 10, 2021