The CDC finds no safety risks for pregnant people who get the Pfizer and Moderna COVID-19 vaccines

• CDC data: Pfizer and Moderna's COVID-19 vaccines don't seem to pose serious risks during pregnancy.
• The preliminary findings add to evidence that the shots are safe and beneficial to mom and baby.
• The study had limitations, but getting COVID-19 in pregnancy remains riskier than the vaccines.

The Pfizer and Moderna COVID-19 vaccines don't seem to pose any serious risk during pregnancy, according to a preliminary study using data from the US Centers for Disease Control and Prevention.

The study, published Wednesday in the New England Journal of Medicine, used data from CDC's smartphone-based tracking system, called V-safe, that included 35,691 vaccinated pregnant people between December 14, 2020 and February 28, 2021.

They found pregnant folks more likely to report injection site pain than non-pregnant people, but less likely to report other side effects like headache, chills, and fever.

When looking more closely at the 3,958 participants who enrolled in a pregnancy registry, they found that 86.1% completed their pregnancies while 13.9% experienced a pregnancy loss. That's in line with what you'd expect to see in the general population, where about 15% to 20% of known pregnancies in the US end in miscarriage.

The study also found 9.4% of those in the registry experienced preterm births, and 3.2% had babies who were underweight. In general, about 10% of newborns in the US are born prematurely, and 11% in high-income countries are born small for their gestational age. No babies died.

Using another other monitoring system, called VAERS, which tracks possible adverse effects from vaccines, the researchers found miscarriage was the most commonly reported pregnancy-related negative event - 46 cases out of 221.

That didn't raise red flags because miscarriage is common and similar results were seen after the introduction of the 2009 H1N1 flu vaccine, which is now recommended in pregnancy.

"Preliminary findings did not show obvious safety signals among pregnant persons who received mRNA Covid-19 vaccines," the researchers wrote.

The results are preliminary

While the study found that most of the completed births occurred among people who received their vaccines in the third trimester of pregnancy, that could be because many of those in the registry who got their shots in the first or second trimester were still pregnant by the time the study wrapped.

Still, the study was subject to many limitations. People in V-safe were volunteers and often healthcare workers who were prioritized for vaccination, so they're not a perfect snapshot, demographically, of vaccinated pregnant people in the US.

"The registry data are preliminary, are from a small sample, and describe mostly neonatal outcomes from third-trimester vaccination; the findings may change as additional pregnancy outcomes are reported and the sample size increases, which may facilitate detection of rare outcomes," the study authors write.

Meanwhile, the data used from VAERS likely underestimates negative outcomes since not everyone reports their potentially vaccine-linked effects.

So, while miscarriage was the most commonly reported negative event, that may not reflect the true prevalence because those vaccinated after the first trimester are at a lower risk for miscarriage simply due to the length of time they've been pregnant.

Plus, researchers don't know exactly how many pregnant people in the US have gotten vaccines, so they can't accurate estimate rates of negative outcomes.

However, the findings are an important to provide "a crude sense of whether there are any unexpected safety signals in these early data." More research, of course, is needed.

Evidence is building that the COVID-19 vaccines are beneficial to mom and baby

The study adds to evidence that the COVID-19 vaccines are both safe and effective in pregnancy, and can benefit the unborn baby, too.

One earlier preprint paper found that pregnant women are protected by Pfizer's and Moderna's COVID-19 vaccines and pass high levels of disease-fighting antibodies on to their babies.

Based on the way they're made, it makes sense that the shots would be safe in pregnancy and pose "very little risk to a developing baby," pediatrician and neonatologist Dr. Jessica Madden previously told Insider. That's because, like the flu vaccine, the coronavirus vaccines do not contain live virus.

"The mRNA in the vaccine acts locally, in the muscle cells surrounding the injection site," she said. "It cannot enter into cells' nucleus, thus it has no effect on DNA." It also doesn't enter into the placenta or otherwise directly interact with the fetus.

"I feel more and more comfortable with the information that we're gaining in terms of being a stronger advocate and a recommender of this vaccine for both pregnant moms and for breastfeeding moms," Madden, who serves as medical director of Aeroflow Breastpumps, said.

The risk of COVID-19 in pregnancy is greater than the risk of vaccination

Not getting the vaccine, on the other hand, puts pregnant people at risk for contracting COVID-19, which can lead to serious complications, though the overall risk is still low.

A November report out of the CDC - the largest of its kind to date - including about 400,000 15- to 44-year-old women found that pregnant women with COVID-19 were nearly four times as likely to need ventilation and twice as likely to die than nonpregnant women with COVID-19 of the same age. COVID-19 in pregnancy is also linked to an increased risk of preterm delivery.

A study presented in January 2021 including 1,200 infected pregnant women across 33 US hospitals found that the 12% with critical or severe symptoms tended to be most at risk for complications and death, and were also more likely to have other risk factors like a higher body mass, asthma, diabetes, and hypertension.

Still, pregnant people should not feel pressured into getting a vaccine if it makes them overly anxious, Madden said during an Insider webinar. "I'm a proponent of the vaccine," she said. "I'm not a proponent of anybody feeling like they have to get this right now."