US CDC says virus patients can be switched to another vaccine for second dose

With possibility of vaccine shortages on horizon, changes may offer a way to vaccinate more people

The Centers for Disease Control and Prevention has quietly changed its recommendations for coronavirus immunizations to allow patients to switch the authorized vaccines between the first and second doses in “exceptional situations,” and to extend the interval between doses to six weeks, even though such changes have not been studied in large clinical trials.

The new guidelines were posted on the agency’s website Thursday with little public notice. With the possibility of vaccine shortages on the horizon and little expectation that supply can be increased before April, the changes may offer a way to vaccinate more people.

Read: Don't panic, you'll get vaccine, assures WHO

A CDC spokeswoman, Kristen Nordlund, said the agency’s “intention is not to suggest people do anything different but provide clinicians with flexibility for exceptional circumstances.”

Dr. Anthony Fauci, the president’s special adviser for COVID-19, has repeatedly advised against delaying the second dose or making any other changes in vaccination protocol without the data to support them.

Earlier this month, Britain quietly updated its vaccination playbook to allow for a mix-and-match vaccine regimen if the second dose of the vaccine a patient originally received isn’t available or if the manufacturer of the first shot isn’t known. Some scientists questioned the move at the time.

In the United States, two vaccines have emergency federal authorization — one by Pfizer and BioNTech, and the other by Moderna — and both rely on the same mRNA technology and call for two doses.

The updated CDC guidance still states that the authorized vaccines are “not interchangeable with each other or with other COVID-19 vaccine products.” The agency put the word “not” in bold on its website and noted that the safety and efficacy of mixing doses has not been studied.

But “in exceptional situations in which the first-dose vaccine product cannot be determined or is no longer available,” the guidelines added, any available mRNA vaccine can be used for the second dose.

With respect to dosing, the guidance says the second dose should be administered as close as possible to the recommended interval — three weeks for the Pfizer-BioNTech vaccine and four weeks for Moderna. But if that is “not feasible,” the agency wrote, the interval between doses may be extended to six weeks.