CDC study on COVID-19 vaccination and spontaneous abortions found no major flaw – Community News
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CDC study on COVID-19 vaccination and spontaneous abortions found no major flaw

For many months, vaccine skeptics have been sounding false alarms about the effect of COVID-19 vaccines on pregnancy and fertility.

Concerns about the risk of spontaneous abortion, also known as miscarriage, have resurfaced after a report in November in Science, Public Health Policy & the Law, a journal of James Lyons-Weiler.

“Report Finds Increased Risk of Spontaneous Abortion After COVID-19 Vaccination During Pregnancy Epidemiologists,” reads the headline on a statement from Lyons-Weiler, who aims to summarize the report’s findings. “Epidemiologists find CDC research contains major flaws.”

Lyons-Weiler, whose claims about COVID-19 we have previously fact-checked, argues in his piece that the report’s authors found a “major flaw” in a study conducted by the Centers for Disease Control and Prevention published in the New England Journal of Medicine in June. The miscarriage rate, the authors claimed, was seven to eight times higher than the CDC study presented.

However, the authors’ calculations misrepresent the findings of the CDC study by manipulating and picking data, using a similar methodology to previously circulating claims on social media about the same CDC study.

The blog post was flagged as part of Facebook’s efforts to fight false news and misinformation in the news feed. (Read more about our partnership with Facebook.)

The CDC study in question

The authors of the original CDC study, titled “Preliminary Findings of mRNA Covid-19 Vaccine Safety in Pregnant Persons,” used data from three federal reporting systems that monitor vaccine safety. They contacted 3,958 people enrolled in one of these systems approximately three months after receiving the Pfizer-BioNTech or Moderna COVID-19 vaccine.

Of those people, 712 had given birth to healthy babies, 104 had miscarried and one was stillbirth. Ten people had an ectopic pregnancy or had an abortion. The remaining 3,131 participants were still pregnant or had not yet completed follow-up.

They found 104 miscarriages out of 827 completed pregnancies, representing a miscarriage rate of 12.6% — comparable to the average miscarriage rate of 12.5% ​​to 18.7% in the general population.

The alleged mistake

The authors of the report in Lyon-Weiler magazine argue that this figure is incorrect and present alternative calculations that result in a much higher miscarriage rate of 82% to 91%.

They arrived at these numbers using the same flawed method used in social media posts we saw circulating in July. Those posts, which we fact-checked, excluded 700 people who were vaccinated in the third trimester, when the fetuses are more developed and the risk of miscarriage is reduced. Similarly, the authors excluded 700 people who were vaccinated after 20 weeks of pregnancy, the cut-off period that defines a miscarriage. This resulted in only 127 completed pregnancies. The 104 miscarriages of the 127 pregnancies were 82%.

However, this 82% is not the miscarriage rate. To determine the effects of vaccination early in pregnancy, we only look at people vaccinated before 20 weeks, but it is misleading to apply the narrow denominator of 127 to the preliminary data in the NEJM paper.

“[Y]You can’t do that because the dataset looks at all the pregnancy outcomes,” said Dr. Jen Gunter, an OB-GYN, in a blog post. “Of course, if you remove those 700 pregnancies, the miscarriage rate looks artificially high, because the only way a person got into the data set… was if the pregnancy had ended and they had been vaccinated.”

Likewise, the top range of their estimate, 91%, used a denominator that only counted people with pregnancy loss. This included 104 people who had spontaneous abortions and 10 people who had an ectopic pregnancy or had an abortion, essentially dividing the number of miscarriages by itself. Determining the frequency of a particular outcome (i.e., miscarriages) using a sample that grossly overrepresents that outcome necessarily produces an inflated figure that cannot be applied to the general population.

A more accurate assessment of whether vaccination before the third trimester increases miscarriage rates would require tracking these pregnancies to completion. “These pregnancies will eventually be the denominator to look at first and second trimester vaccination, but these pregnancies have to end and all the data has to be collected before they can be used,” Gunter said.

“There is no conspiracy to hide miscarriages here, just misuse of statistics,” Gunter said.

More data is now available

Since the CDC study was originally published, more data has become available.

The authors of the CDC study both acknowledged that there was an error in the denominator they used, as it included people vaccinated after the 20-week mark. They published another paper on Oct. 14 with updated data that now includes 2,456 pregnant people. They found that the cumulative risk of miscarriage for those at 6 to 20 weeks gestation was 14.1%, which is still comparable to the average miscarriage rate in the general population.

Other studies confirm these findings. A September article in the Journal of the American Medical Association analyzed 105,446 pregnancies based on medical records from eight health care systems in the US and found no increased risk of miscarriage after vaccination with the Pfizer-BioNTech or Moderna COVID-19 vaccine.

The CDC issued an urgent health advisory on Sept. 29 strongly recommending COVID-19 vaccination for pregnant and newly pregnant people, pointing out their increased risk of serious illness. An April multinational study found that people at any stage of pregnancy or delivery who contract COVID-19 are 22 times more likely to die than those without COVID-19, and their newborns are twice as likely to receive intensive care or death after birth. The American College of Obstetricians and Gynecologists also updated its guidelines on November 3, 2021 to recommend pregnant and recently pregnant people receive a booster dose of COVID-19.

our statement

A blog post claims a new report found a major flaw in a CDC study, claiming that the miscarriage rate for pregnant people who received the COVID-19 vaccine was several times higher than presented in the study.

However, the report’s authors arrived at these numbers by selecting and manipulating data from the CDC study, and not taking into account all completed pregnancies.

The CDC study has since published follow-up data that supports the original findings that there is no increased risk of miscarriage after COVID-19 vaccination.

We rate the claim as false.