February 29, 2024

“We do not recommend mixing Covid-19 vaccines,” Public Health England chief says

A nurse prepares to administer the Pfizer-BioNTech Covid-19 vaccine at Guy's Hospital in London, on December 8, 2020.
A nurse prepares to administer the Pfizer-BioNTech Covid-19 vaccine at Guy’s Hospital in London, on December 8, 2020. Frank Augstein/Pool/AFP/Getty Images

Mixing Covid-19 vaccines is not recommended, Public Health England’s Head of Immunisations Dr. Mary Ramsay said Saturday, after government guidance was updated this week to say the interchangeability of Covid-19 vaccines was a “reasonable” option.

“We do not recommend mixing the Covid-19 vaccines – if your first dose is the Pfizer vaccine you should not be given the AstraZeneca vaccine for your second dose and vice versa,” Ramsay said in a statement.

“There may be extremely rare occasions where the same vaccine is not available, or where it is not known what vaccine the patient received. Every effort should be made to give them the same vaccine, but where this is not possible it is better to give a second dose of another vaccine than not at all,” she added.

Ramsay clarified the UK’s position on vaccine mixing after an update to the government’s vaccine playbook on Dec. 31. 

What did the updated guidance say? Thursday’s guidance said if the same vaccine is not available, or if the first product received is unknown, “it is reasonable to offer one dose of the locally available product to complete the schedule.”

“This option is preferred if the individual is likely to be at immediate high risk or is considered unlikely to attend again,” it added.

Which vaccines are the UK using? The UK authorized emergency use of the Pfizer/BioNTech vaccine on December 2 and the Oxford University/AstraZeneca vaccine on Dec. 30.

The guidance recommends both vaccines to be administered in two doses, a minimum of 21 days apart for Pfizer/BioNTech and 28 days apart for AstraZeneca, with longer term protection provided by the second inoculation.

UK guidelines contradict US approach: The updated UK guidance contradicts guidelines in the United States for the two vaccines it has authorized, Pfizer/BioNTech and Moderna. The Centers for Disease Control and Prevention has noted that the authorized Covid-19 vaccines “are not interchangeable with each other or with other Covid-19 vaccine products,” and that “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.”

The CDC adds, however, that “if two doses of different mRNA COVID-19 vaccine products are inadvertently administered, no additional doses of either product are recommended at this time.”