Experts debate fiddling with vaccine doses as virus rages out of control
January 4, 2021
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Vials of undiluted Pfizer COVID-19 vaccine are  prepared to administer to staff and residents at the Goodwin House Bailey's Crossroads, a senior living community in Falls Church, Virginia, on December 30, 2020.
Enlarge / Vials of undiluted Pfizer COVID-19 vaccine are prepared to administer to staff and residents at the Goodwin House Bailey’s Crossroads, a senior living community in Falls Church, Virginia, on December 30, 2020.

With the pandemic continuing to surge to new heights, debate has erupted among experts about how to get the most protection as quickly as possible from the limited supply of COVID-19 vaccines currently available.

Suggestions floated so far include delaying the second of a two-dose regimen for maybe three to four months rather than doling them out in the planned three-to-four-week intervals evaluated in large clinical trials. With this change, more people can get a first dose now, offering ­some protection. Another possibility is simply cutting doses by half, which would immediately double the number of people who can be vaccinated now. And regulators in the United Kingdom have introduced the idea of mixing regimens—with some people getting their first dose of a vaccine by one manufacturer and then getting a second dose of vaccine from a different manufacturer based on availability at the time.

Evidence supporting these suggested tweaks in the vaccine rollout are shaky at best. But some experts argue that the dire state of the pandemic warrants a new, perhaps riskier approach.

Viral pummeling

“[A]s Mike Tyson famously said, ‘Everybody has a plan until they’ve been punched in the mouth,’” Robert Wachter, chair of the department of medicine at the University of California at San Francisco, and Ashish Jha, dean of the Brown University School of Public Health, wrote in an op-ed published Sunday in The Washington Post. In the opinion piece, the pair lays out an argument for delaying second doses, writing, “When it comes to COVID-19, we’re being punched in the mouth over and over again. It’s time to change the plan.”

Wachter and Jha note that researchers haven’t looked at what happens if a person only takes one dose of a two-dose vaccine regimen. In the US, the Food and Drug Administration has authorized the use of Moderna’s mRNA COVID-19 vaccine and another mRNA vaccine made by Pfizer and BioNTech. In large Phase III clinical trials, both vaccines showed about 95 percent efficacy at protecting against COVID-19—but only after two doses given 28 days (Moderna) or 21 days (Pfizer/BioNTech) apart. It’s unclear how effective the vaccine is after just one dose and how long that protection might last.

The single-dose strategy “hasn’t been studied, so we don’t know if and when the immunity from the first shot begins to wane,” Jha and Wachter write. “But,” they go on, “data from Moderna, for example, is reassuring. It shows robust immune response four weeks after the first shot, and most experts believe it is extremely unlikely immunity would somehow plummet by week eight or even week 12 following a single shot.”

Risks and unknowns

That thinking appeared enough to sway health officials in the UK. Last week, they announced that they would allow the interval between first and second shots to be as long as 12 weeks. They also opened the possibility that people could receive first and second vaccine doses from different manufacturers—for example, a person could get a first dose of the Pfizer/BioNTech vaccine followed by a dose of a COVID-19 vaccine from AstraZeneca/University of Oxford. (The AstraZeneca vaccine is another two-dose vaccine that was approved in the UK last week. US officials expect the FDA will grant it an emergency authorization in the coming months.)

But other experts are still strongly opposed to the idea of delaying a second dose. Though immune responses may not “plummet” dramatically within a few months, they will decline over time. And, because researchers don’t yet know what level of immune responses are needed to ward off infection and disease, it’s not possible to say that the one dose will be enough to keep people from getting infected or sick.

What’s more, waning immune responses after one dose could conceivably make the coronavirus more dangerous. Some experts have noted that having the wildly circulating virus mingle with lots of people with weak immune responses could create the perfect selective pressure for the virus to adapt to evade those immune responses. Coronaviruses mutate over time naturally, and several variants—including those in the UK and South Africa—have already emerged that appear to spread more readily among people. So far, those variants do not appear to generate more severe or deadly disease. And they are not thought to evade immune responses from natural infections or the authorized vaccines. But, partially vaccinating a large number of people could lead to those nightmare scenarios, some experts fear.

Sticking to the science

With the uncertainties and risks, many experts are strongly opposed to fiddling with the vaccine plan. That includes the top infectious disease expert, Anthony Fauci. Last Friday, Fauci blasted the idea of the US following the UK’s lead in allowing a delayed second dose. “I would not be in favor of that,” Fauci said in an interview with CNN.

“The fact is we want to stick with what the science tells us, and the data that we have for both [vaccines] indicate you give a prime, followed by a boost in 21 days with Pfizer and 28 days with Moderna,” Fauci said. “And right now, that’s the way we’re going with it, and that’s the decision that is made.”

Moncef Slaoui, chief scientist of the US government’s Operation Warp Speed, echoed Fauci’s opposition to delaying second doses in an interview Sunday on CBS’ Face the Nation.

“We always said that these vaccines will be developed on the basis of science and all decisions will be made transparently on the basis of data,” Slaoui said. “Changing the decisions made, the choices made, which was to give two doses of vaccine—the second dose gives you ten times higher immune response than the first dose—without any data I think would not be responsible.”

However, intriguingly, Slaoui did offer yet another alternative to maximize the vaccine rollout:

“Here is the alternative. We know that for the Moderna vaccine, giving half the dose to people between the age of eighteen and fifty-five—two doses, half the dose, which means exactly achieving the objective of immunizing double the number of people with the doses we have—we know it induces identical immune response to the hundred microgram dose and, therefore, we are in discussion with Moderna and with the FDA.

“Of course, ultimately it will be an FDA decision,” Slaoui said. But, he added, “I think that’s a more responsible approach that will be based on facts and data.”

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