Why can you mix and match booster jabs in pregnancy?

29 October 2021

We have been asked on WhatsApp about the Covid-19 booster vaccines in pregnancy. In particular people have asked why the booster vaccine is deemed to be safe in pregnancy, and why mixing and matching vaccines for the booster is sometimes recommended (depending on which vaccine someone initially received) but not between first and second doses. 

We have written more about who is eligible for booster vaccines in pregnancy before, and this advice has been evolving. The most recent recommendation from the Joint Committee on Vaccination and Immunisation (JCVI) is that all adults over the age of 18 (and those over 16 who have certain health conditions, caring responsibilities, or live in households with people who are immunosuppressed) should receive a booster vaccine.

The JCVI has also updated its advice to say that boosters should be offered after three months has elapsed since the second dose, rather than six months. 

There are special recommendations for individuals who are immunosuppressed. 

Dr Pat O’Brien, consultant obstetrician and gynaecologist and Vice President of the Royal College of Obstetricians and Gynaecologists (RCOG), previously told Full Fact: “We encourage all pregnant women who are eligible to get the COVID-19 booster vaccine, to ensure that their protection against COVID-19 is maintained to a high level.”

The RCOG has since said that it is “encouraging all pregnant women to schedule their booster jabs as soon as it’s been three months since their second vaccine to ensure that their protection against COVID-19 and the new variant is the best it can be”.

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Why can you mix and match the booster? 

The JCVI has advised that anyone eligible for a booster should receive an mRNA vaccine, regardless of which vaccine they had for their first and second doses. At the moment, the two available mRNA vaccines in the UK are the Pfizer or Moderna vaccines. The JCVI advises a booster vaccination with AstraZeneca only where mRNA vaccines are not appropriate. 

The JCVI said that data from the COV-BOOST trial, which looked at using seven different types of vaccine as a third dose, indicated that booster doses of Covid-19 vaccines are generally well tolerated and provide a substantial increase in vaccine-induced immune responses. The JCVI said that mRNA vaccines “in particular” provide a strong booster effect, regardless of which vaccine was used for the first two doses. It said that it advises a preference for the Pfizer vaccine to be offered as the third booster dose irrespective of which vaccine was used initially, or alternatively individuals may be offered a half dose of the Moderna vaccine.

The JCVI said that there is good evidence that the Pfizer vaccine is well tolerated as a third dose and will provide a strong booster response. It recommends a half dose of Moderna rather than a full dose because it should still produce a good booster response, but be better tolerated.  

What evidence is there for the use of booster vaccines in pregnancy?

The COV-Boost study itself did not include pregnant women, and excluded women who were planning to become pregnant in the next three months.

A spokesperson for the RCOG told Full Fact: “Although there is not yet any published data on the booster vaccine in pregnancy, we are expecting to see published data soon, as there is a current study in Israel which is looking into the benefits of the booster dose in all cohorts of people”.

Full Fact also asked the Medicines and Healthcare Products Regulatory Agency (MHRA) about what is known on the safety of the booster vaccine in pregnancy. 

Dr Alison Cave, MHRA Chief Safety Officer told Full Fact: “Over 92,000 pregnant women have received at least one dose of COVID-19 vaccine in England and Scotland and no concerns of the safety of the vaccines have been raised. This conclusion is supported by extensive international data from the rollout of the vaccines in other countries.” 

She added that the side effects observed from booster doses (which are the same vaccines as used for the first and second doses of vaccine) are “in line with those seen after the first two doses”. 

She said the MHRA “want to reassure women that booster doses are therefore considered to be safe in pregnant women”.

Dr Cave also said that patient safety is a “top priority” and said that the safety of all Covid-19 vaccines in pregnant women will be kept “under close review”.

Why can’t you mix vaccines between first and second doses?

As we have written before, pregnant women who have received a first dose of AstraZeneca vaccine are recommended to have a second dose with the same vaccine, rather than “mix and match” doses. This has caused confusion because only the Pfizer or Moderna vaccines have been recommended as the preferred vaccines for pregnant women coming forward for a first dose. 

Public Health England (which has now been largely replaced by the UK Health Security Agency or UKHSA) has previously told us that mixing of first and second doses of the vaccine between different brands is not currently recommended, except in particular circumstances such as people who experienced anaphylaxis or blood clots with low platelets after the first dose. 

The UKHSA Green Book, which provides health professionals with the latest information on vaccinations, states: “Individuals who experienced severe expected reactions after a first dose of AstraZeneca or Pfizer BioNTech vaccines should be informed about the higher rate of such reactions when they receive a second dose of an alternate vaccine” and references a study comparing side effects associated with mix and matching of the two vaccines against giving two doses of the same vaccine.

The Department of Health and Social Care (DHSC) told Full Fact that AstraZeneca and Pfizer are both licensed as a two dose schedule and using them in this way is most in keeping with the current evidence in terms of their safety and effectiveness. In addition, the data that there is also suggests that side effects (like chills, tiredness, localised pain and swelling among others) are more common with mixing of doses. UKHSA and the DHSC told Full Fact that this is less likely to occur with the booster vaccines because a longer period will have passed between vaccine doses.

The information included in this article contains the latest evidence and official guidance available at the time it was written. This is not a substitute for medical advice. If you require specific medical advice please consult your GP or midwife.


Update 17 December 2021

This piece has been updated to reflect changing recommendations from the JCVI.

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