Is a Single Dose of the COVID Vaccine Okay?

Infectious disease expert, Leonard Krilov, M.D., answers this and other burning questions about vaccine effectiveness and dosing schedules.

by Sarah Ellis Health Writer

The COVID vaccines are a hot topic in social conversations these days: “Which one did you get?” “Did you have any side effects?” “When will you be fully vaccinated?” We are all going through this vaccination experience together and learning new things in real time. And though the COVID vaccine rollout in the U.S. has been nothing short of a miracle, it has also presented some major challenges: On April 28, the New York Times reported that 8% of people who got the first dose of the Pfizer and Moderna vaccines missed their second dose, according to the latest data from the Centers for Disease Control and Prevention (CDC).

That’s around five million Americans who haven’t completed the two-dose series of the mRNA vaccines, due to personal choice, lack of access, or logistical errors within the supply chain. If you or a loved one missed out on the second shot, you likely have concerns about this. Can someone be considered fully vaccinated if they only got one dose? Are they even protected at all?

To answer this and other questions about COVID vaccines, we chatted with Leonard Krilov, M.D., chairman of pediatrics and chief of pediatric infectious disease at NYU Langone Hospital-Long Island in Mineola, NY. For starters, he assured us that it’s not the end of the world if you have to get your second shot late, but don’t give up on getting it altogether. For the mRNA vaccines, two doses are better than one.

Health Central: The New York Times recently reported that 8% of people who received their first Pfizer or Moderna shot missed their second dose. What do we know right now about the effectiveness of a single dose of a two-dose COVID vaccine regimen?

Leonard Krilov, M.D.: The CDC projects 80% protection from one dose. [This data comes from a March 29 study of health care workers after their first dose of the mRNA vaccines.] That is less than the 90%+ protection after the complete series, and even 80% versus 90% is still meaningful. We also don’t know yet if immunity will wane faster if you only get one dose. And will one dose be less effective against some of the COVID-19 variants? We don’t know that yet, either.

Those are the concerns: The protection is a bit lower after just one dose, and the duration and some of the additional protection may be suboptimal. How suboptimal, I don’t think we know yet.

HC: What if your provider runs out of supply and you have to get your second dose a few weeks late? Is your level of protection diminished?

Dr. Krilov: The vaccine schedule is set by how the studies were done, so that’s where the three-week window for Pfizer and four-week window for Moderna comes from. This is based on some degree of science and guessing about the interval that will give you a significantly boosted immune response with the second dose, but it’s not an exact science. And the three- or four-week window is plus or minus two days, because even in the clinical trials, things get in the way [with scheduling].

Based on best guesses and knowledge from other vaccines, the CDC has already said that up to six weeks between doses is OK if necessary. We can anticipate that a second dose would still be beneficial a few months late, but there isn’t hard scientific data on that. We are giving the COVID vaccines on a schedule based on what we know will give the right level of protection, but even if that schedule is not quite adhered to, that doesn’t mean a second dose won’t work.

HC: Why do these approved vaccines all have differing dosing schedules? Why is Pfizer given in a three-week window, Moderna in four, and J&J in a single dose?

Dr. Krilov: The companies all did their own development, study, and rationale, and there was no existing vaccine to compare these to. Pfizer picked three weeks, and Moderna picked four weeks, based on some early data in initial trials that showed an appropriate booster response. I’m not sure that three versus four weeks is really so different, but that’s just how it was studied.

The Johnson & Johnson vaccine is a different construct—it’s an adenoviral vector vaccine. They felt in their early preliminary data there was enough to suggest adequate immune response after one dose. Certainly, in terms of compliance, for populations who have difficulty with access, it’s much easier to complete the series when it’s only one dose. And given the high level of prevention of severe disease and hospitalization, it was approved.

HC: Do people who got J&J need to be concerned that the efficacy data is lower?

Dr. Krilov: It depends on what the goal is. If the goal is preventing hospitalization or severe disease, this vaccine is right up there [with the others]. If the goal is preventing any amount of disease, then maybe the efficacy is a bit lower. But here’s the other side of the coin: With J&J, you don’t have to deal with the 8% of people who aren’t getting their second dose. You get one, and you’re compliant.

It’s a balance, and I think each vaccine has its place. The best chance we have of controlling this virus is by getting people vaccinated quickly. So, the more bullets in our gun, the better.

HC: Why do side effects differ between the two doses of Pfizer and Moderna?

Dr. Krilov: It’s not completely clear, but in general, it is suggested that there are more severe side effects with the second dose. The argument is that because you already have some immunity, your immune response is more robust.
That immune response then generates inflammatory responses, which lead to side effects. Some people take this as a good sign that you’ve developed some immunity already. It may not be exactly that; I don’t think we really know for sure.

HC: If someone has more severe side effects after their first shot (rather than their second), does this mean they might have had COVID-19 previously and not known it, and that’s why their body is primed to respond to the first shot?

Dr. Krilov: I guess it could, and there certainly are a significant percentage of asymptomatic COVID cases, but it’s not been studied rigorously enough to be able to say that. What I generally tell people is that your immune system is like your fingerprint: Even within the broad range of normal, everyone is a little different. Why some people have more sensitive reactions may be too subtle for us to understand right now.

HC: Following reports that daily vaccination rates are declining in the U.S.—and that reaching herd immunity may be unlikely—what can we expect the future to look like?

Dr. Krilov: The goal of herd immunity is to be able to protect both the individual and the herd—that is, those who may not have an optimal response to the vaccine or can’t get the vaccine. The problem is that we don’t know the exact level of herd immunity necessary. The projections have been 70%, but if we get to 60 or 65%, will we get at least some moderation of the spread? It isn’t completely understood yet. Certainly, the more people vaccinated, the better.

Here are some things that may help us get closer to that number: Adolescents and young adults are now becoming a major reservoir for COVID cases, and we anticipate in the next several days or weeks, Pfizer will get EUA approval for ages 12-17. And Pfizer recently announced that by September, even younger children will be potentially approved under EUA.

Vaccine access is also becoming less of an issue. And hopefully, longer-term data that continues to show safety and efficacy will convince those who are still uncertain or uncomfortable that these vaccines really are safe and effective. The closer we get [to herd immunity], the better control we get over this virus. But I can’t say, if we don’t quite get there, exactly what it’s going to mean.

It’s clear that vaccines are having an impact, and they are still our biggest hope for getting control of this. [COVID-19] may still remain endemic, meaning there may still be some cases, but hopefully not the type of surges we’ve seen over the last year. This is history, and we’re all learning together.

Sarah Ellis
Meet Our Writer
Sarah Ellis

Sarah Ellis is a wellness and culture writer who covers everything from contraceptive access to chronic health conditions to fitness trends. She is originally from Nashville, Tennessee and currently resides in NYC. She has written for Elite Daily, Greatist, mindbodygreen and others. When she’s not writing, Sarah loves distance running, vegan food, and getting the most out of her library card.