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Flawed CDC Study Wrongly Concludes COVID Vaccines Safe in Pregnancy

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By shisu_ka/Shutterstock


The Centers for Disease Control and Prevention (CDC) earlier this spring recommended women who are pregnant, recently pregnant, who are trying to become pregnant now or who might become pregnant in the future get the COVID-19 vaccine.

The CDC made the recommendation after concluding, in a Jan. 7 Morbidity and Mortality Weekly Report, that data support the safety of COVID vaccination during pregnancy.

By comparing COVID vaccination during pregnancy to those unvaccinated during pregnancy, the agency determined COVID vaccines were not associated with preterm birth or with delivering a child who was born smaller or less developed than expected, also known as small-for-gestational-age (SGA).

In this article, we examine flaws in the CDC study that led to the agency’s wrongful conclusion that COVID vaccines are “safe” for pregnant women.

First, some background.

Including Pregnant Women in Clinical Trials

Pregnancy is a precarious time not just for the expectant mother but most importantly the developing fetus. Expectant mothers are advised not to drink alcohol or caffeinated beverages and not to eat raw foods such as sushi and deli meats.

A lot of medications are contraindicated during pregnancy including simple pain meds like non-steroidal anti-inflammatory drugs (Ibuprofen), antidiarrheals, decongestants, antihistamines, nasal sprays and expectorants.

Women are advised not to take these medications during pregnancy because they pose potential risks to the developing fetus.

For decades, expectant mothers have been considered a vulnerable group to be shielded from potential harms of research for the sake of their fetuses’ health.

In 1977, the U.S. Food and Drug Administration issued guidelines excluding pregnant women and women “with childbearing potential” from phase I and phase II clinical trials, where new drugs are tested for safety and efficacy.

This view stemmed, in part, from tragedies caused by two now-infamous drugs that were widely prescribed to pregnant women in the mid-20th century: thalidomide, which caused thousands of children around the world to be born with flipper-like limbs and other birth defects, and diethylstilbestrol, which was linked to higher rates of cancer in both mothers and the daughters born to them.

This view changed however in 1993, with the passage of the National Institutes of Health Revitalization Act, which sought to increase gender and racial diversity in clinical trials.

Federal regulations currently require any study involving pregnant women to meet 10 criteria, including that, “where scientifically appropriate,” data first be collected on pregnant animals and non-pregnant human subjects to assess risk, and that any risk to mother or fetus be “the least possible for achieving the objectives of the research.”

Reproduction Toxicity Studies in Animal Models Hinted at Dangers Early On

While the companies developing the COVID-19 vaccines have done preliminary studies in animals, their studies were limited to rodents. The vaccine makers did not conduct studies on non-human primates, recognized as the closest animal models to humans regarding genetics, physiology and behavior.

Nevertheless, Moderna’s own Assessment Report to the European Medicines Agency Committee for Medicinal Products for Human Use on March 11, 2021, included a study for reproductive and developmental toxicology on female rats during gestation.

The report noted (page 50: Reproduction Toxicity) an increase in the number of fetuses with common skeletal variations of one or more rib nodules and one or more wavy ribs. Additionally, the number of pups born to vaccinated rats was lower than the number in the unvaccinated rats.

Most importantly, the authors explicitly stated, “In this study, no vaccine dose was administered during the early organogenesis [the period during embryonic development of an animal when the main body organs are formed], to address the direct embryotoxic effect of the components of the vaccine formulation.”

One month earlier, Pfizer reported in its Feb. 19, 2021, Assessment Report to the same committee that pregnant rats demonstrated a greater-than-2x increase in pre-implantation loss in exposed animals compared to controls.

The authors of the Pfizer report further stated (Page 50: Reproduction Toxicity) that “a very low incidence of gastroschisis, mouth/jaw malformations, right-sided aortic arch, and cervical vertebrae abnormalities” occurred in litters of exposed rats, and that these findings were within historical control data.

This finding brings up an important question: Why compare the incidence of these major congenital abnormalities with “historical” controls and not with the controls themselves?

As late as April 2021, the CDC still maintained there was limited data surrounding the safety of COVID vaccines for women who were pregnant or breastfeeding. The agency advised women who were pregnant or breastfeeding to consult with their physician before getting vaccinated.

But were obstetricians made aware of the potential safety signals appearing in animal models?

And how were physicians able to decide whether or not a COVID vaccine was appropriate for their pregnant patients if the CDC wasn’t offering any guidance at that time?

CDC’s Latest Study: A Closer Look at the Details

Using data from the Vaccine Safety Datalink — a CDC vaccine safety monitoring system the public cannot access — the CDC study identified 46,079 pregnant women with live births and gestational age.

Of those, 10,064 (21.8%) received ≥1 COVID vaccine doses during pregnancy from Dec. 15, 2020, to July 22, 2021.

Nearly all (9,892, or 98.3%) of the pregnant women included in the study were vaccinated during the second or third trimester.

The authors found that among unvaccinated women, the rate of premature births was 7% compared to 4.9% in those who had received either one or both vaccine doses.

The rate of small-for-gestational-age in both vaccinated and unvaccinated mothers was equal (8.2%).

The authors thus conclude that “… receipt of COVID-19 vaccine during pregnancy was not associated with increased risk for preterm birth or SGA at birth.”

5 Flaws in the CDC Analysis

On closer examination, we identified the following five deficits in the CDC study:

  • Cohorts were not well matched. There were greater than three times more African American women in the unvaccinated group than in the vaccinated group. The CDC acknowledges the African American race is a risk factor for preterm birth and may be as high as 50% greater than in white women.

There were also greater than 50% more mothers in the unvaccinated group classified as having inadequate prenatal care. Obesity, also a risk for preterm birth, was also overrepresented in the unvaccinated group (29% vs 23.9%) compared to the vaccinated.

  • No adjustment for mothers with a history of preterm birth of SGA. The authors did not address this potential confounder.
  • COVID infection, another potentially important confounder, was present in the unvaccinated group at a 25% greater incidence than in the vaccinated cohort (3.5% vs 2.8%). There was no mention of when in the pregnancy the infection was detected.Viral infections early in pregnancy are particularly deleterious to the developing fetus. This should have been an important risk factor to quantify independently, especially when establishing a risk-versus-benefit ratio of vaccination.
  • The CDC data indicate a 7.7% risk of preterm birth in mothers having received one of two vaccines. This represents a 10% greater risk than in unvaccinated pregnancies. This increased risk is not mentioned in the discussion. Moreover, the adjusted Hazard Ratio (aHR) in this population is given as 0.78, indicating a 22% risk reduction in preterm birth in vaccinated mothers, seemingly conflicting with the raw data. (A request for clarification from the corresponding author was not answered).
  • The most glaring deficit in the CDC analysis is the scarcity of vaccinated mothers who received a vaccine in the first trimester in this study. The risk of untoward outcomes (birth defects, miscarriages) in pregnancy is greatest during the first third of pregnancy, a time when crucial embryonic structures are developing. This is the period of time where maternal health is particularly important, and exposure to toxins, infections and certain medicines must be minimized or eliminated entirely if possible.

Only 172 of more than 10,000 (1.7%) vaccinated mothers in the study received a vaccine in the first trimester. The incidence of preterm birth and SGA were not mentioned in this small cohort because of limited numbers.

Nonetheless, the authors arrive at the stunning conclusion: “CDC recommends COVID-19 vaccination for women who are pregnant, recently pregnant (including those who are lactating), who are trying to become pregnant now, or who might become pregnant in the future (4) to reduce the risk for severe COVID-19–associated outcomes.”

CDC Not Required to Provide Access to Its Data or Subject Its Analysis to Peer Review

The Vaccine Safety Datalink uses data reported from nine large healthcare organizations, serving only 3% of the U.S. population. The system collects electronic health data from each participating site.

This database is accessible only to researchers outside the CDC and only by request. Requests may be accommodated after a research proposal is submitted and approved by the Research Data Center of the National Center for Health Statistics.

CDC Morbidity and Mortality Weekly Reports can, as in the case of the agency’s analysis of COVID vaccine safety in pregnant women, be based on data that is not necessarily publicly available.

The agency’s analyses are not subject to peer review. Nevertheless, the reports are often widely cited as the official scientific position.


The CDC’s determination that COVID vaccination is safe in pregnant women is unfounded.

Cohorts were poorly matched. There was an inexcusably low representation of women who were vaccinated early in their pregnancy in their analysis. This is a period where any exposure to medical interventions will have a greater potential for risk to the fetus.

Broadly recommending vaccination for all pregnant women including those who are trying to become pregnant is particularly unwarranted.

This report places the CDC’s purported commitment to its mission of disease control and prevention on full display. The agency’s conclusions arrive more than a full year after the CDC authorized COVID vaccinations and are based on retrospective data alone.

In other words, the CDC is willing (and apparently allowed) to make safety determinations only after the experimental vaccines have been widely and indiscriminately deployed.

This is a shocking departure from the higher standards of prudence that are demanded during pregnancy, a time where two lives are potentially at risk and poor outcomes can lead to a lifetime of potential consequences.

It should be noted that several of the authors of this study reported potential conflicts of interest.

One author reported institutional research funding from Pfizer, and another from Pfizer and Johnson & Johnson. A third author has a career grant from the National Institute of Allergy and Infectious Diseases.

Originally published on Children’s Health Defense

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