Can You Mix the COVID-19 Vaccines? Moderna and Pfizer Second Dose Explained

Three different COVID-19 vaccines are authorized for use in the U.S., with two of these requiring the administration of two doses. But can you mix and match the vaccines?

The Pfizer-BioNTech, Moderna, and Johnson & Johnson vaccines are all being administered in the United States, but whereas the Johnson & Johnson vaccine only requires one shot for immunization, the Pfizer-BioNTech and Moderna vaccines each require two doses, administered several weeks apart.

This has raised the question of whether or not it's safe or effective to receive one dose of the Pfizer-BioNTech vaccine followed by one dose of the Moderna vaccine, or vice versa.

Scientists are investigating the idea, but in the meantime they've told healthcare providers to follow current vaccine guidelines and to not mix and match COVID-19 vaccines.

"COVID-19 vaccines are not interchangeable," the U.S. Centers for Disease Control (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," it continues, adding that "every effort should be made to determine which vaccine product was received as the first dose to ensure completion of the vaccine series with the same product."

If the same vaccine is temporarily unavailable when a second shot is due, the CDC says "it is preferable to delay the 2nd dose (up to 6 weeks) to receive the same product than to receive a mixed series using a different product."

However, in "exceptional situations in which the vaccine product given for the first dose cannot be determined or is no longer available," the CDC says that "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."

Both the Pfizer-BioNTech and Moderna COVID-19 vaccines are mRNA vaccines.

The CDC also says that a single dose of the Johnson & Johnson vaccine can only be administered to somebody who has already received a shot of either the Pfizer-BioNTech or Moderna vaccine "in limited, exceptional situations where a patient received the first dose of an mRNA COVID-19 vaccine but is unable to complete the series with either the same or different mRNA COVID-19 vaccine."

In this case, the Johnson & Johnson shot may only be administered at a minimum interval of 28 days from the initial mRNA COVID-19 vaccine dose, and patients should be considered to have received a valid, single-dose Johnson & Johnson vaccination — not a mixed vaccination series.

There was a great deal of controversy when health officials in England opened the door to mix-and-match vaccinations earlier this year, despite a lack of supporting evidence that the vaccinations would still be effective.

"There is no evidence on the interchangeability of the COVID-19 vaccines although studies are underway. Therefore, every effort should be made to determine which vaccine the individual received and to complete with the same vaccine," Public Health England's (PHE) guidance reads.

However it adds that it is "reasonable" to administer a dose of a different product to individuals who don't know which vaccine they've already received, or if the same vaccine is not available.

"In these circumstances, as the vaccines are based on the spike protein, it is likely the second dose will help to boost the response to the first dose," PHE says.

The U.S. Food and Drug Administration (FDA) supports the CDC's stance.

"We have been following the discussions and news reports about reducing the number of doses, extending the length of time between doses, changing the dose (half-dose), or mixing and matching vaccines in order to immunize more people against COVID-19," the FDA says.

It describes them as "reasonable questions to consider and evaluate in clinical trials," but says that "suggesting changes to the FDA-authorized dosing or schedules of these vaccines is premature."

It adds: "Without appropriate data supporting such changes in vaccine administration, we run a significant risk of placing public health at risk, undermining the historic vaccination efforts to protect the population from COVID-19."

The Infectious Diseases Society of America further backs this up, saying, "As always, our approach against this pandemic must be founded in science, leadership, funding, collaboration and cooperation."

Furthermore, in February, Dr. Anthony Fauci told the LA Times, "I wouldn't make any changes unless you've got good data. I don't think you mix and match without results showing it's very effective and safe."

In the U.K., the University of Oxford is leading Com-Cov, a study called that aims to evaluate the feasibility of getting vaccinated using two different vaccines.

The trial will take 13 months, and involves the Pfizer-BioNTech and Oxford-AstraZeneca vaccines.

A person in a facemask in NYC
A person walks by a sign that reads, "the vaccine is our best shot against COVID-19" on the Upper West Side amid the coronavirus pandemic on March 30, 2021 in New York City. Health experts currently advise against mixing and matching different vaccines. Noam Galai/Getty Images

The graphic below, produced by Statista, shows that the U.S.'s rate of vaccination stands at 49.35 jabs for every 100 people.

According to the CDC, more than 18% of the U.S. population has now been fully vaccinated.

Covid-19 vaccination rates around the world
A Statista infographic details the different rates of COVID-19 vaccinations in the U.S. and other countries around the world as of April 2021, based on data compiled by Our World in Data. Statista