Protecting the Public with Limited COVID-19 Vaccine Supply: 1 Or 2 Doses?

Dr. Paul Offit explains what is missing from discussions considering giving more people a single dose of the limited COVID-19 vaccine supply, compared with fewer people getting two doses, as originally recommended.


Protecting the public with limited COVID-19 vaccine supply: 1 or 2 doses?

Paul Offit, MD: Hi, my name is Paul Offit from the Vaccine Education Center. It's February 1st, 2021. We are in the midst of a major COVID-19 pandemic. Thousands of people are dying every day and hundreds of thousands are being hospitalized. It is a true national tragedy. The good news is that we have two vaccines, both of which are now approved for use under emergency use authorization. One is the Pfizer vaccine, the other is the Moderna vaccine. Both are messenger RNA vaccines, and both are two-dose vaccines given either three or four weeks after the first dose. Because we don't have enough vaccine to vaccinate everybody as soon as possible, there have been decisions that are being made about how to best use these vaccines.

One of those decisions is maybe we should just give as many people one dose as possible and then eventually let them get the second dose. So, let me tell you where that idea comes from, what the thinking is behind that idea, and why I think that that idea is problematic. So let's say for example that you have a million doses, do you give a million doses to a million people, or do you give two doses to 500,000 people? If the vaccine like these mRNA vaccines is 95% effective, and as is being stated, one dose of the vaccine is 80% effective, if that's true, then if you gave 500,000 people two doses, then 475,000 would be protected. And if you gave 1 million doses to 1 million people, knowing that it was 80% effective, then you would protect 800,000; and 800,000 is more than 475,000, therefore, more people would be protected, so let's do it that way. And that's the argument that people have made for this sort of initial one-dose strategy, thinking you'll get the second dose at some point later.

The problem with that strategy is that there are assumptions that have been made that are incorrect. The first is the notion that a one-dose strategy would provide 80% protection. Where did that number come from? It comes from the fact that when these so-called phase 3 trials were done to see whether or not the vaccine was safe and effective, there was a period of time between dose one and dose two where some people got sick. So the question is, did they get sick from COVID-19? Were they more likely to get sick because they … when they had received the placebo? Or when they’d gotten the vaccine? Or said another way, were you less likely to get sick if you'd received the vaccine? And the answer was, yes. In the Pfizer trial, the vaccine was about 50% effective. In the Moderna trial, it was about 80% effective. So, thus was born the notion that one dose of vaccine was 80% effective. Now, all you know from that is it was 80% effective for a few weeks. You don't know how long it would be effective. You also know from the phase 1 trials, the two doses of vaccine, that second dose induced much higher levels of antibodies and also induced something called T cells, which are the kinds of cells that help cells, like B cells, make antibodies, which suggested immunological memory. Or said another way, that second dose is much more likely to give you better, longer-lasting immunity.

So what's the downside then of waiting? The CDC has actually recently said that it is reasonable to wait six weeks after the first dose, which means that instead of waiting three weeks as you would for the Pfizer vaccine or four weeks as you would for the Moderna vaccine, you wait six weeks. So, it's an extra three or two weeks, depending on how you look at it. I don't think that's a problem. What worries me is that some people might say, one dose is 80% effective, two doses is 95% effective, close enough. I'll just go with one dose. And then they sort of fall through the cracks. They really never get that second dose, which means they won't have long-lasting and better immunity.

The second thing people might say, is they might say, you know what, I had some side effects with that first dose. I've heard that the side effects can be even more significant with a second dose. Why don't I just go with one dose? And I think that what you may inadvertently do is create a large number of people who are going to wait two or three or four or five months during which time immunity will likely fade before they get their second dose. Or worse, a large number of people who won't ever get that second dose, and therefore won't have the long-lasting, better immunity we're going to need in order to stop the spread of this virus.

So, I think that the one-dose strategy is ill-conceived. I think that the messaging with that strategy may likely be misinterpreted. I think people should understand this is a two-dose vaccine, and you need to get that second dose as soon as possible, certainly no later than six weeks after that first dose.

Thank you.

Related Centers and Programs: Vaccine Education Center

Last Reviewed on Feb 02, 2021