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CDC: COVID Vaccine Doses Can Be Spaced Up to 6 Weeks Apart

moderna vaccine vial and syringe

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Key Takeaways

  • The CDC says second doses of Pfizer and Moderna vaccines can be administered as late as six weeks after the first dose.
  • While the agency didn't give a reason for this update, the fact that states are running out of vaccines may necessitate the extension until supply is more steady.

The Centers for Disease Control and Prevention (CDC) updated its guidance for how long people need to wait between the first and second doses of mRNA COVID-19 vaccines, extending the dosing interval to up to six weeks for both the Pfizer and Moderna vaccines.

The change was part of a January 21 update to the agency's interim clinical considerations for the currently-authorized COVID-19 vaccines.

"If it is not feasible to adhere to the recommended interval, the second dose of Pfizer-BioNTech and Moderna COVID-19 vaccines may be scheduled for administration up to 6 weeks (42 days) after the first dose," the guidance says. "There are currently limited data on efficacy of mRNA COVID-19 vaccines administered beyond this window. If the second dose is administered beyond these intervals, there is no need to restart the series."

While the CDC still recommends following the dosing intervals recommended by the manufacturers whenever possible (21 days for Pfizer-BioNTech, 28 days for Moderna), an extension may help alleviate the pressure of vaccine distribution challenges.

As distribution moves through the initial phase of priority groups like healthcare workers and longterm care facility residents, states are beginning to run out of vaccine supply. At first, states were given vaccines based on the size of their adult populations, but in mid-January, the Trump administration announced states administering vaccines the fastest would receive more shots.

The Biden administration is taking further action to disseminate vaccines, announcing a plan to release nearly all available doses.

"In order to expand the supply available to states, the Administration will end the policy of holding back significant levels of doses, instead holding back a small reserve and monitoring supply to ensure that everyone receives the full regimen as recommended by the FDA," reads Biden's National Strategy for the COVID-19 Response and Pandemic Preparedness.

Even with careful supply monitoring, there will be challenges that may necessitate this new extended dosing interval. So far, states have had to navigate shipping, storage, and unforeseen logistical issues. This week in New York, a delay in Moderna shipments forced the cancellation of 23,000 vaccine appointments. And in California, state epidemiologists recommended a brief pause in administering a certain lot of Moderna vaccines because of a higher-than-average rate of allergic reactions.

What This Means For You

If your second COVID-19 vaccine appointment was cancelled, know that the CDC says you now have a slightly longer window to receive dose number two.

In addition to the dosing timeframe change, the CDC updated its interim guidance with a clarification about switching between brands for each dose.

"These mRNA COVID-19 vaccines are not interchangeable with each other or with other COVID-19 vaccine products," the CDC says. "The safety and efficacy of a mixed-product series have not been evaluated. Both doses of the series should be completed with the same product."

However, the CDC says exceptions can be made in extremely rare circumstances.

"In exceptional situations in which the first-dose vaccine product cannot be determined or is no longer available, any available mRNA COVID-19 vaccine may be administered at a minimum interval of 28 days between doses to complete the mRNA COVID-19 vaccination series," the guidance says.

Vaccine recipients should receive vaccination cards at the time of their first shot in order to keep track of whether they were given Pfizer or Moderna.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

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  1. U.S. Department of Defense. Operation Warp Speed COVID-19 press conference transcript by defense officials. Updated January 12, 2021.