Can different COVID vaccines be used for first and second doses?

The following advice is extracted from the 20 May 2021 NHS England publication:

The recommendation is that the same vaccine is given in the second dose as the first. If a different second vaccine is considered, a detailed informed consent process is to be followed. It may be appropriate to give a different second vaccine in the following situations, following PHE guidance:

Exceptional Circumstances

• The Green Book states that the same vaccine used for the first dose must be used for the second, except in very exceptional circumstances. These exceptional circumstances are:
  • If the first product received is unknown or if they received a brand that is not available in the UK. In these circumstances every effort should be made to determine which vaccine the individual received for their first dose and to complete the two dose course with the same vaccine.
  • If the patient initially had the Pfizer vaccine in an LVC or HH clinic and has since become housebound. In these circumstances as the COVID-19 vaccine AstraZeneca can be transported, a second dose with this vaccine can be given.
  • Those who experienced anaphylaxis reactions with the first dose of one brand of vaccine may be offered another vaccine if advised by an allergy specialist.
  • Following MHRA guidance, patients who have experienced major venous and/or arterial thrombosis occurring with thrombocytopenia following vaccination with any COVID-19 vaccine should not receive a second dose of COVID-19 Vaccine AstraZeneca.

What is the second dose guidance for pregnant women?

PHE’s Green Book advises that ‘Pfizer and Moderna vaccines are the preferred vaccines for pregnant women of any age, because of more extensive experience of their use in pregnancy. Pregnant women who commenced vaccination with AstraZeneca, however, are advised to complete with the same vaccine’. ‘If a woman finds out she is pregnant after she has started a course of vaccine, she may complete vaccination during pregnancy using the same vaccine product (unless contra-indicated). Alternatively, vaccination should be offered as soon as possible after pregnancy.’