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The Most Revolutionary Act

Flawed CDC Study Wrongly Concludes COVID Vaccines Safe in Pregnancy

By  Madhava Setty, M.D. Jennifer Smith, Ph.D.

The Centers for Disease Control and Prevention’s recommendation that pregnant women get the COVID-19 vaccine is based on limited data, poorly matched cohorts and inexcusably low representation of pregnant women in their first trimester.

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.


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.”



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