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How long is too long to wait for second vaccine dose?

Posted at 5:02 PM, Feb 19, 2021
and last updated 2021-02-19 19:04:20-05

TUCSON, Ariz. (KGUN) — Bad weather across the country has delayed shipments of the COVID-19 vaccines. That has caused several counties in Southern Arizona to push back vaccination appointments.

For those waiting on a second dose of either Moderna or Pfizer vaccine, how long of a delay is too long?

"I would be concerned about that because you don't get full efficacy until you get that second dose," said Dr. Anthony Fauci, NIAID Director.

Dr. Fauci has been a strong advocate of sticking to the prescribed timeline for the two-shot Pfizer and Moderna COVID-19 vaccines.

It is what I experienced during the Moderna vaccine trial, as one of nearly a thousand volunteers here in Tucson.

The clinical trials only looked at efficacy of the two-shot vaccination.

Pat Parris receives the second COVID vaccine injection in Moderna phase 3 trial.

The doses were 28 days apart for Moderna, 21 days for the Pfizer clinical trial.

But delays do happen, like the winter weather currently delaying shipments to Arizona.

The CDC has issued guidelines for the second dose in case of delays.

The CDC recommends the second dose be administered "as close to the recommended interval as possible."

But if that's not feasible because a delay is unavoidable, the CDC website says "the second dose of Pfizer-BioNTech and Moderna COVID-19 vaccines may be administered up to 6 weeks (42 days) after the first dose."

There is still limited data on how effective the vaccines are if the second dose is given beyond the six-week window.

"It gets risky, and that's the reason why we prefer to keep it on the time that the clinical trials said," explained Dr. Faucui.

Dr. Fauci says a separate clinical trial would have to be done to study the efficacy of the vaccines with the two doses given more than 42 days apart.

But he says that would take several months. By then, there should be plenty of vaccine doses available.