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Delaying second vaccine doses might end the pandemic sooner

Spreading COVID-19 doses out in time would create space for more people to get their crucial first shot.

Army Sgt. John Peterson vaccinated a patient at a clinic in East Boston.David L. Ryan/Globe Staff

A few weeks back I learned that an acquaintance received “the call” informing him that an extra dose of COVID-19 vaccine had suddenly — and unexpectedly — become available and he should rush to claim it. I was happy for him. He’d had a difficult year, and to my mind, he deserved this stroke of luck. Yes, he was a man of privilege, but few would argue that the unexpected windfall should go to waste rather than into his arm. Still, it stung to hear that after getting the first dose, he was scheduled to get a second one long before more-vulnerable Americans were even eligible to get their first.

Roughly 15 million Americans are fully protected against COVID-19, meaning that they’ve had two doses of the vaccine. That leaves the vast majority of us wholly unprotected from a disease that kills thousands of Americans every day and harms many thousands more. January was the pandemic’s deadliest month so far, with 95,254 deaths attributed to the virus in the United States alone. And while deaths and hospital admissions are currently trending downward, no one knows whether more potent or contagious variant strains of the virus will reverse this trend.


So why are some of us getting two shots while most of us continue to wait for our first?

The response to this question is only partially based in science.

Studies conducted by vaccine makers Pfizer and Moderna appear to suggest that a single dose of their vaccines does not confer sufficient immunity to delay a second shot beyond four weeks. But neither Pfizer nor Moderna tested alternative spacing options before releasing their vaccines, and for good reason: They couldn’t afford to take the time. And clearly, a second dose is necessary for long-term protection.

Now a growing number of scientists believe the three- or four-week gap between doses is just a minimum. It reflects not an immunological optimum but the need for haste in proving the vaccines’ safety and efficacy.


Dr. Chris Gill, an infectious disease specialist at Boston University’s School of Public Health, notes that studies of both the Pfizer and Moderna vaccines show quite clearly that one dose offers significant protection two weeks after the injection — “roughly 90 percent effectiveness,” he said. As for when the next dose should come, “it’s not up to the companies to make the decision on spacing. Federal agencies — the FDA and the CDC — don’t have to follow the manufacturer’s advice.”

He added: “The question we need to answer is not whether two shots are better than one; both are necessary. The question is whether giving more people one shot is better than rushing to give half as many people two shots. I’m a scientist and have the luxury of looking at the data, but any third-grader can do the math. Doubling the number of people who can get at least one dose of the vaccine would save a lot of lives.”

Indeed, experience with other vaccines suggests that waiting a few more weeks between doses could even improve their efficacy. In a recent study of pneumococcal vaccines in infants, Gill and colleagues found that spacing out the second dose by an extra four weeks resulted in increased protection from the disease. And while it was not conclusive, a small clinical trial of the Oxford AstraZeneca COVID-19 vaccine (recently approved for emergency use by the World Health Organization) surfaced evidence that a longer gap between doses—two to three months rather than one — resulted in better protection.


In the United Kingdom, the Joint Committee on Vaccination and Immunisation recently announced that it would extend the gap between the first and second doses of the COVID-19 vaccines to up to 12 weeks. The reason: “Models suggest that initially vaccinating a greater number of people with a single dose will prevent more deaths and hospitalizations than vaccinating a smaller number of people with two doses.” This decision came after a multinational study concluded that the first dose of the Moderna vaccine had 80 percent efficacy and that a delay of nine to 12 weeks “could enhance the program effectiveness and prevent additional infections, hospitalizations, and deaths, compared to a four-week interval between the doses.”

Of course, it’s important that the vaccine supply not run out before people with one dose are able to get their second. But that needn’t happen. Over the next nine to 12 weeks, vaccine supply should increase as manufacturing and distribution ramps up and additional vaccines are approved. And while it’s true that scientists cannot be sure of the risks of holding back second doses for a few weeks, what we do know is that in almost all cases, one dose is preferable to no doses.

The political climate in the United States makes it difficult for scientific and public health agencies to honestly weigh the evidence on this, as changing policy on vaccine spacing may well result in confusion and will certainly prompt objections. But science itself is rarely stagnant or settled. Facing down this deadly pandemic requires both courage and the intellectual flexibility to follow the science wherever it leads, and to do everything in our power to avoid making the perfect the enemy of the good.


Ellen Ruppel Shell, faculty associate at the Pardee Center for The Study of the Longer-Range Future at Boston University, is a regular contributor to Scientific American.