The Pfizer COVID-19 Vaccine in Pregnancy

The information provided below is for readers based in the United States of America. Readers outside of the United States of America should seek the information from local sources.

Pfizer/BioNTech BNT162b2/“Comirnaty

Pfizer/BioNTech’s BNT162b2 vaccine is an mRNA vaccine, meaning that the effective ingredient consists of a strip of genetic material known as mRNA. This single-stranded RNA molecule carries a recipe used by cells to make a protein that provides the immune system with the ability to protect against SARS-CoV2 (the virus that causes COVID-19).

Commercially produced under the name Comirnaty, BNT162b2 vaccine has US Food and Drug Administration full approval for those ages 16 and up and has received CDC Emergency Use Authorization (EUA) for people ages 5-15.

Comirnaty’s Active Ingredient

The active ingredient in Comirnaty is a nucleoside-modified mRNA that encodes the viral spike (S) glycoprotein of SARS-CoV-2. In other words, a molecule of RNA that contains specific genetic instructions for cells of the vaccinated person to produce the spike protein of SARS-CoV2.

Comirnaty’s Delivery Vehicle

The Comirnaty vaccine is delivered via lipid nanoparticles. These lipid nanoparticles are tiny vessels used to surround the nucleoside-modified mRNA molecules. Each nanoparticle contains numerous mRNA molecules within it.

Each of these particles protects the mRNA molecules from enzymes that would otherwise destroy the mRNA and until the particles merge with the membrane of a cell of the vaccinated person.

This process releases the mRNA to the cell’s cytoplasm, where tiny cellular particles, called ribosomes, translate the message mRNA-encoded message into spike (S) glycoprotein molecules before being transported to, and displayed on, the surface of the cell.

Comirnaty’s Method of Administration

The Comirnaty vaccine is administered via deltoid IM injection.

Comirnaty’s Recommended Administration Schedule

It’s recommended that the Comirnaty vaccine be administered with two injections given 21-28 days apart (WHO) followed by a booster dose at least 5 months after the second dose for ages 12 and up and two injections, given 21 days apart for ages 5-11 years.

As up February 2022, the US Center for Disease Control and Prevention is evaluating a proposal to expand the interval between the two doses from 21-28 days to eight weeks. Some countries have been administering a fourth doses to individuals considered to be at risk. Qualification for a fourth dose varies between countries, but generally include advanced age and factors that may decrease the immune response, such as being an organ transplant recipient, obesity, and heart failure.

The Comirnaty vaccine can also be given as a booster (third shot) in people who were vaccinated initially with other COVID-19 vaccines. In other words, you may mix and match your initial doses and boosters.

The initial two doses of this vaccine have proven extremely effective in preventing severe COVID-19 disease. Studies also suggest that booster doses can raise levels of neutralizing antibodies, which can further reduce the chances of developing any symptomatic disease.

In the United States, this vaccine has full FDA approval in people ages 16 and up, while children ages 5-11 are authorized to receive the vaccine under emergency use authorization (EUA).

Regardless of whether administered with full FDA approval or EUA, the vaccine contains the same mRNA in the same lipid nanoparticle, regardless of who receives it. Children ages 12 and up receive the adult dose (30 micrograms per shot), while children ages 5-11 receive a pediatric version of the vaccine at a lower dose (10 micrograms per shot).

Clinical trials are currently in progress to evaluate a version of this vaccine for children between the ages 6 months and 4 years. The proposed dosage for this youngest age group will be 3 micrograms per shot. Because trial results have not shown two shots of the 3-microgram dose of vaccine to be very effective, researchers are presently evaluating the efficacy of a three-shot course.

Vaccine Dosage Per Shot

Adults and children ages 12+ years: 0.3 mL @ 30 μg/0.3mL Pediatric ages 5-11 years: 0.2 mL @ 10 μg/0.2mL Pediatric ages 6 months-4 years: 3 μg, likely a 3-dose regimen, not yet authorized in any country.

The only exceptions to this are in rare cases of people who previously have suffered allergic reactions to ingredients in the vaccine, such as polyethylene glycol.

Although data are not available specifically on pregnant women, as of November 2021, WHO reported that India had vaccinated 120,000 pregnant women with Covaxin.

Initial reports have revealed only the minor type of adverse effects on those women. While studies of the Covaxin vaccine in pregnancy are planned, experience with other inactivated vaccines suggests that the effectiveness and safety should be similar in pregnant women as in non-pregnant women of the same age range.

Comirnaty Vaccine Efficacy (Ages: 5 years and older, as of February 2022)

To date, this vaccine has demonstrated ~95 percent overall efficacy in preventing COVID-19. Protection against severe COVID-19 disease and death is above 99 percent for all variants, while efficacy against mild upper respiratory disease caused by the B.1.617.2 (Delta) variant has demonstrated to be slightly less.

Data from Israel suggest waning immunity against mild disease and asymptomatic infections occur-ring several months after the second dose, but data from the UK has not shown a similar waning.

Efficacy is reduced significantly in immunocompromised individuals, such as organ transplant recipients, but studies have involved mostly B cell activity (antibodies); less is known about the T cell response. There is mounting evidence that a third dose may improve immunity in people who don’t have a good immune response after two doses.

There has also been some speculation that discrepancy between Israeli and British outcomes on mild disease may relate to the timing between the two doses being extended to several weeks in the UK, and as part of an effort to get more people to receive the first dose. This discrepancy seems to suggest that increasing the interval to eight weeks could improve efficacy against the Delta variant. However, this hypothesis would require testing and may be rendered moot, when either a third dose, or a booster tweaked to the Delta variant, is approved.

Clinical testing of the low dose (10 microgram) version of the Comirnaty vaccine in children ages 5-11 has been reported to demonstrate high efficacy after two shots, approaching 100 percent against severe disease (based on data that have yet to be peer reviewed).

Implications for Pregnancy and Lactation

Vaccination against COVID-19 is recommended “when the benefits of vaccination to the pregnant woman outweigh the potential risks” (WHO).

Currently, the benefits of COVID-19 vaccination far outweigh potential risks both in pregnancy and lactation (other than in rare cases of people who previously have suffered allergic reactions to ingredients in the vaccine, such as polyethylene glycol).

However, globally, there is an abundance of policies and philosophies surrounding to what degree potential vaccine recipients should be able to assess the risk vs benefits on their own.

COVID-19 testing of pregnant women prior to being vaccinated is not necessary, nor should women delay or terminate pregnancy on account of COVID-19 vaccination (WHO).

There is no plausible mechanism supporting notions that this vaccine might present a particular danger during pregnancy or lactation. Maternal vaccination against COVID-19 may possibly protect newborns and nursing infants against endemic variants of SARS-CoV2, including the delta variant.

Pregnancy continues to be a risk factor for the development of severe COVID-19 in unvaccinated women. While healthy young people develop good, long-term immunity after the initial two shots, unvaccinated pregnant women are at elevated risk of severe disease and of undesirable pregnancy outcomes, such as preterm delivery, in the event that they do get infected.

Since a booster shot boosts levels of neutralizing antibodies, albeit temporarily (for a few months), given the short duration of pregnancy (40 weeks), the rationale is quite strong for pregnant women to receive a booster shot.

Adverse Effects Associated With Comirnaty Vaccine

Injection site: pain, erythema, swelling at injection site Systemic: fatigue, headache, myalgia, chills, fever, nausea

There have been rare, anecdotal reports of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the membrane that surrounds the heart) in adolescents and young adults, especially late teens through late 20s, especially males.

Whereas the myocarditis/pericarditis risk becomes notable in people under the age of 30, most of the small number of cases that have occurred have been mild cases, requiring just rest and minimal treatment.

In June 2021, the Advisory Committee on Immunization Practices (ACIP) of the US Centers for Disease Control and Prevention conducted a review of data and concluded that benefits of vaccination against COVID-19 outweigh the risks of myocarditis and pericarditis in all people ages 12 years and up.

The myocarditis/pericarditis risk is lower in connection with the Pfizer/BioNTech vaccine compared with the Moderna vaccine. Even younger pregnant women are not at particular risk for myocarditis/pericarditis, because of their female gender.

Along with the possibility of increasing the potency of the vaccine against the virus, the rationale for the CDC to consider expanding the recommended interval between first and second dose is that studies suggest that this would further decrease the risk of myocarditis/pericarditis as well as other more common adverse effects.

Pregnancy and Lactation Positions by Country for the Pfizer Vaccine (Comirnaty)

FDA For Recipients and Caregivers 12 years of age and older


Hause AM, Gee J, Baggs J et al. COVID-19 Vaccine Safety in Adolescents Aged 12-17 Years – United States, December 14, 2020-July 16, 2021. MMWR Morb Mortal Wkly Rep. 2021 Aug 6;70(31):1053-1058. doi: 10.15585/mmwr.mm7031e1. PMID: 34351881; PMCID: PMC8367318.

Lopez Bernal J, Andrews N, Gower C. Effectiveness of Covid-19 Vaccines against the B.1.617.2 (Delta) Variant. N Engl J Med. 2021 Jul 21. doi: 10.1056/NEJMoa2108891.

Lustig Y, Zuckerman N, Nemet I et al. Neutralising capacity against Delta (B.1.617.2) and other variants of concern following Comirnaty (BNT162b2, BioNTech/Pfizer) vaccination in health care workers, Israel. Euro Surveill. 2021 Jul;26(26). doi: 10.2807/1560-7917.ES.2021.26.26.2100557.

Montgomery J, Ryan M, Engler R et al. Myocarditis Following Immunization With mRNA COVID-19 Vaccines in Members of the US Military. JAMA Cardiol. 2021 Oct 1;6(10):1202-1206. doi: 10.1001/jamacardio.2021.2833. PMID: 34185045; PMCID: PMC8243257.

Polack FP, Thomas SJ, Kitchin N et al. Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine. N Engl J Med. 2020 Dec 31;383(27):2603-2615. doi: 10.1056/NEJMoa2034577. Epub 2020 Dec 10.

US Center for Disease Control and Prevention. Pfizer-BioNTech Vaccine Accessed October 20, 2021

US Center for Disease Control and Prevention. COVID-19 Vaccine Related Codes. Accessed Octo-ber 20, 2021

World Health Organization (WHO) The Pfizer BioNTech (BNT162b2) COVID-19 vaccine: What you need to know Accessed October 20, 2021

World Health Organization (WHO). Vaccine Tracker BNT162b2) Accessed October 20, 2021

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

Leave a Reply