Claim that 44% of Pfizer Covid-19 vaccine trial pregnancies ended in miscarriage is wrong

6 September 2022
What was claimed

Nearly half (44%) of pregnant women in the Pfizer Covid-19 vaccine trial miscarried.

Our verdict

This figure is based on a miscalculation. During the Pfizer trials, there were much lower rates of miscarriage among those who received the vaccine and those who received the placebo. Neither rate was unusually high.

Posts on Instagram claim that 44% of pregnant women who participated in the Pfizer Covid-19 vaccine trial had miscarriages.

They feature screenshots of an online article that makes the same claim.

But as other fact checkers have pointed out, the calculation used to get this percentage is wrong.

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Where did the figure come from?

The claim appears to have initially appeared on a website called The Daily Clout, which is run by the author Dr Naomi Wolf, who was suspended from Twitter last year for spreading misinformation about the Covid-19 vaccine. 

An archived version of the article (as the original now appears to have been deleted) claimed that there were 22 miscarriages out of 50 pregnancies that occured during the Pfizer trials.

These figures apparently came from a number of documents released in response to a Freedom of Information request in the US about the Pfizer vaccine (which we have written about before).

On pages 3,643 and 3,644 of one of these documents, which detail adverse events experienced by people in the trial after being vaccinated, there are 50 subject numbers (which identify participants) listed under the heading “subjects reporting pregnancy after dose 1”. 

The Daily Clout claimed that 22 of those participants suffered miscarriages, which would be 44% of 50. The blog lists what page numbers of the document the various miscarriage cases appear on.

Problems with the calculation

One issue with this calculation is that it appears to have double counted all individual cases of miscarriages, which are listed both as “adverse events” and separately as “serious adverse events”. For example, the blog lists two incidents of “complete spontaneous abortion” citing pages 1,806 and 3,551 of the document. But both of these refer to the same person, with subject number C4591001 1231 12313998. 

On top of doubts about the claim that there were 22 miscarriages, there also appear to be problems with the claim that there were 50 pregnancies in total. At least one subject number mentioned as having had a miscarriage (C4591001 1013 10131255 on page 219) does not appear on the list of 50 pregnancies.

So there are problems with both the numerator (22) and the denominator (50) of the calculation that gave 44%.

Before deleting the blog post, The Daily Clout later added a correction, saying, “Two analysts have reviewed this Pfizer document and reached different totals and percentages than did the author of this report”. 

So how many miscarriages were there in the Pfizer trials?

According to data from the Pfizer’s Phase 2/3 trials (these two stages were run in parallel) there were 42 pregnancies in the group that were vaccinated and three miscarriages, before the cut-off date of 13 March 2021, giving a 7% miscarriage rate. In the placebo group, there were 47 pregnancies and seven miscarriages, giving a 15% miscarriage rate. (For those in the placebo group who were later vaccinated, there were eight pregnancies and no miscarriages.)

The number of miscarriages isn’t officially recorded, but Tommy’s National Centre for Miscarriage Research estimates that one in six pregnancies (16.7%) sadly ends in miscarriage, if only those who realised and reported the miscarriage are counted.  

There’s also no evidence that the miscarriages that did occur during the Pfizer trial were due to the vaccine. We just know that they happened afterwards.

Since those trials, several studies have shown that Covid-19 vaccination is safe in pregnancy. 

And as we have written before, the Covid-19 vaccine is now recommended at all stages of pregnancy. The Royal College of Midwives, the Royal College of Obstetricians and Gynaecologists and the NHS advises pregnant women to get the Covid-19 vaccine.

Image courtesy of Mat Napo

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