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Delaying the second dose of COVID-19 vaccine could benefit more people by averting more cases of COVID-19 than the authorized two-dose vaccine regimen, a modeling study found.

A "flexible" dosing strategy with a highly effective vaccine could avert an additional 23%-29% of COVID-19 cases versus a "fixed" strategy where doses are given 3 weeks apart, reported Joshua Salomon, PhD, of Stanford University in California, and colleagues in a research letter in

The FDA already came out strongly against this idea, calling any changes to dosing schedules "premature" and "not rooted solidly in the available evidence."

Nevertheless, Salomon and colleagues derived their estimates from existing Pfizer/BioNTech mRNA vaccine trial data. In the model, the "fixed" strategy held 50% of vaccine supply in reserve for the second dose. Estimating 6 million vaccine doses per week, the "flexible" strategy consisted of 10% of supply in reserve for the first 3 weeks, 90% for the next 3 weeks, and 50% thereafter. The group also incorporated the companies' finding of 52.4% vaccine efficacy for a single dose.

Under the flexible strategy, an additional 2.4 million people would receive two doses of vaccine, Salomon and colleagues estimated.

They examined additional scenarios to "disadvantage" the flexible strategy, and found it "continued to produce greater overall benefits than the fixed strategy even when we assumed that protection would drop to zero if the second dose was not received within 6 weeks after the first dose." The only analysis favoring the fixed strategy assumed a low first-dose efficacy and a collapse in supply.

"Under most plausible scenarios, a more balanced approach that withholds fewer doses during early distribution in order to vaccinate more people as soon as possible could substantially increase the benefits of vaccines, while enabling most recipients to receive second doses on schedule," Salomon and colleagues wrote.

In a related , Ruanne Barnabas, DPhil, and Anna Wald, MD, both of the University of Washington in Seattle, went a step further. They endorsed the idea of giving one dose of the Pfizer/BioNTech or Moderna vaccines, since efficacy of both vaccines after the first dose was "above the 50th percentile threshold put forth by the [FDA] as the minimum threshold for efficacy." Furthermore, they added that once the immune response was more mature, "the efficacy of a single dose may be higher."

They argued that with this single-dose vaccination strategy, a higher proportion of the population would be protected; it promotes equity and reduces sequelae of reactogenicity; and has the potential to accelerate pandemic control.

Barnabas and Wald acknowledged that while detailed data for a single vaccine dose are not available, "public health bodies have flexibility in their authority to recommend and implement a vaccination program that does not stringently reflect the product label."

An by Thomas Bollyky, JD, of the Council on Foreign Relations in Washington, D.C., attempted to examine both sides of the argument. He added that while a "powerful argument exists for doing something with less-than-perfect results" if it can help more people, it is not clear whether giving one dose of COVID-19 vaccine would do that.

Specifically, Bollyky noted several obstacles, including supply constraints, administration constraints, and even demand constraints if high-risk people are hesitant to be vaccinated, and said all solutions must take all these obstacles into account. For example, a single dose of the current vaccines could alleviate supply constraints, but aggravate demand and administration constraints, he wrote.

For now, Bollyky said these single dose or delayed second dose vaccine regimens need more evidence, through clinical testing and observational studies, to see if they will generate similar immunity to the two-dose regimens authorized by the FDA.

"The United States is unfortunately demonstrating the public health axiom that what saves lives is the practice of vaccination, not just the existence of vaccines," Bollyky wrote. "Models of alternative strategies -- particularly those that make large assumptions about vaccine efficacy in the absence of reliable data -- should not drive policy without a full consideration of the challenges of implementation and potential unintended effects."

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