Guidelines for second dose changed quietly

But officials say new vaccine rules are intended for “exceptional” situations

The Centers for Disease Control and Prevention 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 — a high priority for President Joe Biden, who outlined his national COVID-19 strategy Thursday.

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, repeatedly has advised against delaying the second dose or making any other changes in vaccination protocol without the data to support them. But Friday he seemed open to delaying second doses for short periods.

“What the CDC is saying, sometimes, the situation is stressed where it’s very difficult to be exactly on time,” Fauci told CNN. “So we’re saying, you can probably do it six weeks later, namely, two additional weeks. Quite frankly, immunologically, I don’t think that’s going to make a big difference.”

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, saying Britain was gambling with its new guidance.

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. Until now, the CDC has adhered strictly to the recommendations from its Advisory Committee on Immunization Practices, which specifically stated that the vaccines were not to be mixed.

The updated CDC guidance still states that the authorized vaccines are “not interchangeable with each other or with other COVID19 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 that 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.

The pace of vaccination is critical not just to curbing disease and death but also to heading off the effect of more infectious forms of the virus. The CDC has warned that one variant, thought to be 50% more contagious, might become dominant in the U.S. by March.