Pregnancy and COVID-19 vaccines

On 16 April 2021, the Joint Committee on Vaccination and Immunisation advised that all pregnant people should be offered the COVID-19 vaccine at the same time as the rest of the population, in line with the age group roll out. 

Prior to this, they had recommended a risk-based approach and said that pregnant people with high risk medical conditions — and met the definition of being “clinically extremely vulnerable” — should consider having a COVID-19 vaccine in pregnancy. This was because they viewed that their underlying condition may put them at a higher risk of experiencing serious complications of COVID-19.

However, the guidance regarding vaccination against COVID-10 during pregnancy has changed as new research and evidence has emerged. 

It has been indicated that it is preferable for pregnant people in the UK to be offered the Pfizer-BioNTech or Moderna vaccines where available, as there is more safety data available on these vaccines in pregnancy.

COVID-19 during pregnancy

Recent news reports have suggested that there has been a marked increase in the number of pregnant people being admitted to hospital with COVID-19. In the initial stages of the COVID-19 pandemic, findings from Sweden indicated that pregnant people in Sweden with COVID-19 were five times more likely to be admitted to the intensive care unit and four times more likely to receive mechanical ventilation compared with those who were not pregnant.

More recently, the Royal College of Obstetricians and Gynaecologists has indicated that pregnant people who contract COVID-19 are at slightly increased risk of becoming severely unwell compared to non-pregnant people, particularly in the third trimester. In their guidance they reference the following emerging findings on the impact of COVID-19 on pregnancy:

    • those with symptoms of COVID-19 are twice as likely to deliver their infant prematurely;
    • pregnant people who tested positive for COVID-19 at the time of birth were more likely to develop pre-eclampsia, more likely to need an emergency caesarean, and their risk of stillbirth was twice as high (although the actual number of stillbirths remains low). This research also found COVID-19 was more common in women who were younger, of non-White ethnicity, primiparous, resided in the most deprived areas, and/or had comorbidities.

In the UK, information about all pregnant women requiring admission to hospital with COVID-19 is recorded in a registry called the UK Obstetric Surveillance System 

Research on COVID-19 vaccines in pregnancy

Research published in March 2021 found that mRNA COVID-19 vaccines are effective in pregnancy. Those currently available include: Pfizer-BioNTech, Modern; and Johnson and Johnson’s Janssen. It also found that pregnant people who have been vaccinated are passing on immunity to their newborns (in the womb via the placenta) and after birth (through breastmilk). In the UK, vaccines against illnesses, such as whooping cough, are routinely administered to pregnant people to ensure newborns acquire antibodies to protect them from the illness until their first vaccinations at 8, 12, and 16 eight weeks old.

Clinical trials and research

The clinical trials for the Pfizer-BioNTech vaccine did not include people who were pregnant or lactating. The Pfizer-BioNTech vaccine was tested on rats, prior to mating and during pregnancy. There were no vaccine-related effects recorded on fertility, pregnancy, or foetal or offspring development.

Preliminary findings from research on mRNA COVID-19 vaccine safety in pregnant people did not show obvious safety signals among pregnant people who received mRNA COVID-19 vaccines. However, the research acknowledges the importance of longitudinal follow up, to establish any additional findings amongst those vaccinated at different stages of pregnancy and if there are any other maternal, pregnancy, and infant outcomes.

So why weren’t pregnant people included in vaccine trials?

A Stanford researcher explains that women have traditionally been excluded from clinical trials, more generally. This is attributed to the fact that men were thought to be a more homogenous group, and one which did not have to to consider sex-based variables (such as hormone cycles) that might impact the medical conditions under consideration. Dr Heather Byers elaborates on this:

In addition, pregnant women are still classified as a ‘vulnerable’ population for all research studies, so investigators must take additional steps to enroll them to ensure minimum risk. Also, the lack of data about what pregnant women can safely be exposed to leads to more uncertainty. So many investigators choose to exclude them, even if they might benefit from the study intervention.

The historical legacy of thalidomide (prescribed for morning sickness during the 1950s) and diethylstilbestrol (DES) (prescribed to prevent miscarriage from the 1940s to the 1970s) has continued to inform the view that pregnant people and their children are vulnerable and should be protected from clinical research trials. Therefore, pregnant people are still almost automatically excluded from clinical trials. In the context of the COVID-19 pandemic, some have questioned the rationale of this approach.

A previous version of this article was published on 13 April 2021.


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