Pfizer-BioNTech and Moderna Vaccines in Pregnancy

On 30th of June 2021, the Royal College of Obstetricians and Gynaecologists (RCOG) published a publication on Covid-19 vaccination in pregnancy for healthcare professionals. I have extracted some interesting information on Pfizer and Moderna Vaccines which are used for pregnant mums locally.


Vaccine safety

The side effects of the two vaccines were similar in their clinical phase 3 trials. Pregnant women were not included in the large randomized controlled trials testing the safety and adverse effect profiles of the COVID-19 vaccines.


However, as of 7 June 2021, over 120 000 pregnant women from diverse ethnic backgrounds in the USA have had both vaccines with no evidence of harm being identified.


Common minor adverse effects

A report on the first 35 000 pregnant women who received either vaccine in the USA revealed that most had a minor local reaction (pain, redness or swelling at the injection site). Mild side effects like fatigue, headache or muscle aches (myalgia) were common; these usually lasted less than a few days.


About 10–20% of participants had a fever after vaccination but this was less common in pregnant mums. The incidence of nausea and vomiting was more frequent in pregnant women after the second dose of the vaccine.


In general, adverse reactions are more common after the second dose than the first dose.


Other maternal effects

About 104 out of 827 or 12.6 % of pregnant women had miscarriages, with 92.3% of these miscarriages occurring in the first trimester. The incidence are similar to those in the general population.


There have also been very rare reports of inflammation of the heart muscle (myocarditis), and inflammation of the outer lining of the heart (pericarditis) following vaccination


Fetal effects

Pregnancy outcomes following the 2 vaccines appear similar.


According to one study (Shimabukuro T et al 2021), the most common adverse outcomes among 724 live births were preterm birth (9.4%), small-for-gestational-age (3.2%) and major congenital anomalies (2.2%). These figures are similar to those of the general population. None of the mothers whose babies were born with congenital anomalies had received the COVID-19 vaccine in the first trimester or the periconceptional period.


Antibody transfer

In the United States, there were two studies of over 100 women that showed the presence of antibodies in the infant cord blood and breast milk after vaccination. There is some suggestion that timing of vaccination in pregnancy or during lactation may have an effect on the level of passive immunity conferred to the new born. Studies have also found that production of antibodies and their subsequent transfer were improved following a second dose of either vaccine. Similar to natural infection, levels of antibodies appear to remain stable for several weeks following vaccination suggesting continual transfer of antibodies during lactation. However, how much protection these antibodies confer to the baby is not yet known.


Timing of vaccination in pregnancy

There is no robust evidence to guide the timing of vaccination in pregnancy. The vaccines should be effective at any stage of pregnancy. Some women may choose to delay their vaccine until after the first 12 weeks of gestation, which is the period during which the embryo or fetus is most vulnerable to substances that may produce physical or functional defects in the baby. Pregnant women are more likely to become seriously unwell when compared to non-pregnant women and have a higher risk of their baby being born prematurely if they develop COVID-19 in their third trimester. It is therefore reasonable to aim to have the vaccine before the third trimester, bearing in mind that it takes time for immunity to develop and protection is higher after the second dose of the vaccine. Women who had a first dose of vaccine before becoming pregnant should complete the course with the same vaccine.


Benefits of vaccination

  • Reduction in severe disease for a pregnant woman.

  • Potential reduction in the risk of preterm birth associated with COVID-19.

  • Potential reduction in transmission of COVID-19 to vulnerable household members.

  • Potential reduction in the risk of stillbirth associated with COVID-19.

  • Potential protection of the newborn from COVID-19 by passive antibody transfer.


Risks of vaccination

  • Minor local reaction (pain, redness or swelling at the injection site).

  • Mild systemic adverse effects like fatigue, headache or myalgia, typically short-lived (less than a few days).

  • There has been no evidence to suggest fetal harm following vaccination and fetal harm is considered to be extremely unlikely based on evidence from other non-live vaccines.

  • Risk of fetal harm cannot be completely excluded until large scale studies of vaccination in pregnancy have been completed.


Risks from COVID-19 in pregnancy

Maternal risks:

  • Most women with COVID-19 in pregnancy will have no symptoms. However, some women will develop critical illness from COVID-19.

  • The risk of severe illness from COVID-19 is higher for pregnant women than for non-pregnant women, particularly in the third trimester.

  • There is consistent evidence that pregnant women are more likely to be admitted to an intensive care unit than non-pregnant women with COVID-19.

Fetal risks

  • Symptomatic maternal COVID-19 is associated with a two to three times greater risk of preterm birth.

  • Although the overall risk of stillbirth is small, the risk is approximately doubled with Covid-19 infection

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