CDC changes COVID-19 vaccine guidance to allow mixing Pfizer, Moderna shots in ‘exceptional situations’

The Centers for Disease Control and Prevention (CDC) on Friday updated its guidance on the coronavirus vaccines in circulation, green-lighting doctors to mix shots from Pfizer and Moderna in “exceptional situations.”

The vaccines, which both use messenger RNA (mRNA) technology, require two doses and were authorized to be administered 21 and 28 days apart, respectively. The CDC now says patients can receive either shot as long as they are given at least 28 days after the first dose. 

The updated guidance also says it’s OK to wait as many as six weeks to get a second dose of either shot.

“[E]very effort should be made to determine which vaccine product was received as the first dose, in order to ensure completion of the vaccine series with the same product. 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 reads.

It underscores that the two shots are not interchangeable and that the CDC has not fully studied whether mixing and matching the two vaccines would impede the effectiveness of either shot.

The CDC says physicians should still follow the timeline implemented under its original guidance — 21 days between shots for the Pfizer vaccine and 28 days between shots for the Moderna vaccine — but that timeline could be extended to six weeks if absolutely necessary.

The new CDC guidance comes as state and local governments across the country grapple with a shortage in vaccine doses, leading some areas to cancel appointments for first shots to ensure that those who have received the first dose have a second one waiting for them.

“The intent is not to suggest people do anything different, but provide clinicians with flexibility for exceptional circumstances,” Jason McDonald, a spokesman for CDC, told CNBC.