In a departure from other strategies globally, the UK government said in guidelines published on New Year’s Eve that people could be given a mix-and-match of two Covid-19 shots, if the same vaccine dose was out of stock. The UK became the first country to give emergency authorisation to the Pfizer/BioNTech and the AstraZeneca/University of Oxford vaccines last month.
“This option is preferred if the individual is likely to be at immediate high risk or is considered unlikely to attend again,” the guidelines said.
This contradicts the norms published by the US Centers for Disease Control and Prevention (CDC) for two Covid-19 vaccines authorised by the Food and Drug Administration (FDA). The US health agency said that the vaccines developed by Pfizer-BioNTech and Moderna “are not interchangeable with each other or with other Covid-19 vaccine products,” adding that the safety and efficacy after mixing the doses have not been evaluated.
“Both doses of the series should be completed with the same product,” it stressed. The CDC further stated that no additional doses of Pfizer or Moderna vaccines are recommended if two doses of different products are “inadvertently” administered. Notably, both Covid-19 vaccines approved by the US FDA use the same messenger RNA technology, while AstraZeneca’s vaccine uses a replication-deficient chimpanzee viral vector based on a weakened version of a common cold virus (adenovirus).
The contradiction raised concerns over the UK government’s updated guidance, prompting a clarification from Public Health England (PHE) over the mixing the vaccines. Dr Mary Ramsay, Head of Immunisations at PHE, said in a statement that mixing of vaccines is not recommended, adding that every effort should be made to administer the same vaccine. However, Dr Ramsay stressed that it is better to give a “second dose of another vaccine than not at all.”
“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,” Dr Ramsay said.
“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.