Jan. 22, 2021 -- The CDC has updated its guidance on how long people can wait between the first and second doses of the Pfizer/BioNTech and Moderna COVID-19 vaccines.
If the second dose can’t be scheduled in the recommended time frame -- 21 days after the first dose for Pfizer and 28 days for Moderna -- people can wait up to 6 weeks, the CDC said in an update published Thursday.
“The second dose should be administered as close to the recommended interval as possible,” the CDC said. “However, 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.”
There’s limited information about how well the vaccines work outside that time frame, the CDC said. If the second dose is given after 6 weeks, there’s no need to restart the series.
CNN said the new CDC guidance “appears to clarify earlier language that said ‘there is no maximum interval between the first and second doses for either vaccine.’ ”
The guidance is significant because the CDC had advised states not to hold back vaccines for second shots so that a maximum number of people can get the first dose. States are adding vaccination sites as they complain about vaccine shortages.
Anthony Fauci, MD, the director of the National Institute of Allergy and Infectious Diseases and the leading voice on the coronavirus in the United States, told CNN that people would be “taking a chance” if they follow the new CDC guidance.
"You're taking a chance. The data from the clinical trials showed that in the Moderna trial, you should get the boost 28 days after the prime. That's what I got; I got it exactly 28 days later. When you're dealing with Pfizer, it’s 21; that's where the data show is the optimal effect," he said.
He added that delaying the second dose might not be “a big deal.”
The Pfizer and Moderna vaccines are the only ones being given in the United States at this time. Both use two shots of messenger RNA (mRNA) to create an immune response against the coronavirus. People should get both doses of the same vaccine, the CDC says.
Other COVID vaccines in clinical trials, such as the one being developed by Johnson & Johnson, require only one shot, which would provide a logistical advantage over the two-shot vaccines.