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Vaccines and Coincidences

People who oppose vaccinations, including the vaccines against the virus that causes COVID-19, like to cite adverse effects reported to something called the Vaccine Adverse Event Reporting System (VAERS). One that caught our eye recently came from an organization called Children’s Health Defense, which recently stated in its newsletter that as of February 12, 2021 there had been 111 reports of adverse events involving pregnant women who had received COVID-19 vaccines, 31% of which were miscarriages or preterm births.

Such a claim, taken without any other information, would seem worrisome. The article in the Children’s Health Defense publication goes on to describe several heartrending stories of women who allegedly had been experiencing normal pregnancies, received one or two doses of a COVID-19 vaccine, and lost their pregnancies. It takes issue with any recommendation that pregnant women get the vaccines and highlights the fact that at present we have no safety data from formal clinical trials that included pregnant women.

It does take some unpacking to understand a story like that, something that people at Children’s Health Defense must be fully capable of doing but avoid in order to make their point. So, we will undertake the task for you in order to illustrate just how easy it is to turn an isolated piece of information about vaccines into a dramatic and frightening tale.

Limitations of VAERS Reports

First, what is the VAERS? It was created in 1990 by federal law and is run jointly by the U.S. Centers for Disease Control and Prevention (CDC) and Food and Drug Administration (FDA). VAERS allows—actually encourages—anyone with any knowledge of an adverse event that occurs after any FDA-licensed vaccine is received to make a report. That includes healthcare professionals, drug companies, and members of the general public. CDC and FDA use VAERS reports to identify vaccine adverse side effects that are too rare to be picked up during the clinical trials of a vaccine that typically involve tens of thousands of people. They are looking for “signals” that might indicate a very rare but clinically important problem with a vaccine.


The Vaccine Adverse Events Reporting System (VAERS) allows anyone aware of an adverse event occurring after a vaccination has been received to report it to federal regulatory authorities. These reports do not necessarily mean that the vaccine caused the adverse event (image: Shutterstock).

It is never certain, however, that a reported adverse event is actually related to the vaccine. We know of a case in which a previously healthy elderly woman died hours before she was scheduled to receive a COVID-19 vaccine. Had she died a few hours after the vaccine, a report could have been made to VAERS identifying the death as vaccine related. In fact, it would have been a coincidence.

Most VAERS reports turn out to be just that, coincidence. When millions of people receive vaccines, many things that occur naturally will occur after so many people have received a vaccine. If an illness occurs in one in a million unvaccinated people, then one in a million vaccinated people is probably going to get it as well. Again, that’s called coincidence, two events occurring in close proximity to each other without one causing the other.

Miscarriages Are Unfortunately Common Events

Now, what about the reports of miscarriages after receiving the vaccine? The article by Children’s Health Defense says that 31% of 111 adverse event reports following a COVID-19 vaccine in pregnant women involved miscarriage or preterm birth, so we will assume that is 34 cases. It turns out that between 10 and 20 percent of all pregnancies end in miscarriage. That means that if just 340 pregnant women received vaccine, we would expect that by chance between 34 and 68 of those pregnancies would result in a miscarriage.

In fact, as of February 3, about 10,000 pregnant women received a COVID-19 vaccine. So, if the vaccine were associated with a higher than expected risk for miscarriage, we would expect to see more than 1000 to 2000 miscarriages to that point. In fact, as Children’s Health Defense notes, only 34 were reported to VAERS.


Miscarriages occur during 10 to 20 percent of pregnancies, so it is inevitable that some will occur by chance after large numbers of pregnant women receive COVID-19 vaccines (image: Shutterstock).

Like the elderly woman who died hours before getting a shot that we mentioned earlier, there are undoubtedly women who unfortunately suffered miscarriages shortly before a scheduled vaccine and others shortly afterwards. We are dealing with very large numbers here and all kinds of events will happen by chance after someone is vaccinated. The fact that only 34 cases of miscarriage were reported following COVID-19 vaccines actually highlights how random the reporting system really is.

Our calculations do not prove that COVID-19 vaccines are safe for pregnant women, nor do they prove that vaccines are not involved in miscarriages. They only show that picking out VAERS reports to frighten the public about vaccines is a very misleading business. FDA and CDC will have to explore every case of a miscarriage following vaccination that is reported to VAERS to see if there is any evidence that COVID-19 vaccines elevate risk. It is clear that the numbers alone—and the dramatic stories that anti-vaccination activists concoct about them—do not constitute any danger signal in themselves. Miscarriages are upsetting enough without making women blame themselves for having what is most likely entirely unrelated COVID-19 vaccinations.

There is a case to be made that pregnant women should be given priority to get COVID-19 vaccines because evidence shows they are at increased risk for severe disease and death if infected. Animal studies did not indicate any increased risks for the Moderna/NIH and Pfizer/BioNTech mRNA vaccines on pregnancy outcomes. That is part of the reason the American College of Obstetrics and Gynecology (ACOG) recommends that COVID-19 vaccines “should not be withheld” from pregnant people. The decision about whether to be vaccinated is essentially left up to the pregnant woman according to the ACOG recommendations, to be made weighing on the one hand the fact that safety data from formal studies in pregnant women is not yet available and on the other the increased risk of severe illness and death from COVID-19 among pregnant women. That’s obviously not an easy choice, but misleading women into fearing the vaccine will cause miscarriage by encouraging misinterpretation of VAERS data is clearly not helpful in making the decision.

Mining VAERS data is a scare tactic that those opposed to vaccinations will undoubtedly continue to exploit. Doing so enables them to concoct dramatic anecdotes of individuals allegedly harmed by vaccines. How many stories of vaccinated pregnant women who go on to have healthy babies would it take to counteract them? Or do we need to offer dramatic stories of unvaccinated pregnant women who become seriously ill or even die from COVID-19? Scientists have a natural aversion to arguing their case that way, preferring numbers like those we’ve given to anecdotes. Consequently, we have no such stories to offer in defense of COVID-19 vaccines, but we will continue to address every attempt to use VAERS data to cast doubt on them.

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