Pregnant people have been hit especially hard during the fourth wave of the pandemic, with some states seeing a tripling or quadrupling of hospitalizations for severe COVID-19 compared with earlier in the pandemic. Pregnancy increases the risk for hospitalization, ventilation, and death from COVID-19, as well as miscarriage, preterm labor, and stillbirth. Notably, there’s no evidence that pregnant people are more likely to get infected with the SARS-CoV-2 virus—their infection rates are similar to the general adult population—but if they do get infected, they are more likely to have worse outcomes.

The reason is twofold: There is a dampening of the immune system during some stages of pregnancy so that the mother doesn’t reject the fetus, while at other stages there is an increase in inflammation. Physical changes during the second and third trimesters also place an increased demand on the cardiovascular and respiratory systems, which can contribute to worse outcomes. 

“The increased susceptibility of pregnant women to being severely ill with COVID-19 is multifactorial,” says Andrea Edlow, a maternal-fetal medicine specialist at Massachusetts General Hospital who researches the maternal immune system. “One of the features certainly might be immune suppression, but it also might be immune dysregulation or over immune response to certain stimuli. And it also has to do with just the physiology of pregnancy as it relates to respiration.”

Despite these risks, pregnant people have one of the lowest vaccination rates in the country, with only 25% to 30% fully vaccinated. Initially, there was limited safety data on the vaccines during pregnancy because the group was excluded from the early clinical trials, so many pregnant people declined to get vaccinated. But since then, numerous studies have been published showing that the vaccines are overwhelmingly safe for pregnant people and their babies, with no increased risk for severe side effects or miscarriage. In fact, one study showed that pregnant people were less likely to have vaccine side effects than non-pregnant people.

“Early in this process we just didn’t have any data and we were saying, ‘It’s a risk benefit question and you should talk to your health care provider.’ That messaging has really shifted,” says Torri Metz, a maternal-fetal medicine expert at the University of Utah who’s researching the effects of COVID-19 on pregnancy. “I want them to understand that messaging to recommend the vaccine is because we do have data now. We have data out of the CDC demonstrating safety. We have other data demonstrating similar efficacy. And so we’re just at a different point in terms of the science where we feel a lot more comfortable telling pregnant women they should go ahead with this.”

The surge in hospitalizations of pregnant people for COVID-19 is likely a reflection of these low vaccination rates, combined with the more infectious Delta variant. Nationally, the average age of hospitalization for COVID-19 has dropped by more than 10 years because more older adults are vaccinated than younger adults, and the vaccines are extremely effective at preventing severe disease. That means that statistically, the people who are left vulnerable to infection and hospitalization are the unvaccinated, and the majority of pregnant people fall into that category.

The good news is that if they are vaccinated, pregnant people appear to respond extremely well to the vaccines. Published data from Edlow’s lab show that pregnant people produce similar levels of antibodies as nonpregnant people of the same age. Reflecting this fact, the vast majority of pregnant people who are hospitalized with COVID-19 are unvaccinated. And Edlow says that, anecdotally, if a pregnant person does experience a breakthrough infection, they’re typically asymptomatic or have mild, flu-like symptoms. 

“I think by and large, pregnant women are like everybody else, where the vaccines are mostly protecting against hospitalization and severe disease, which is what they were designed to do,” she says.

So why the recommendation for booster shots? During the meeting of the CDC Advisory Committee on Immunization Practices (ACIP) this week, data was presented showing minimal waning immunity when it comes to severe disease for several groups of people with pre-existing conditions (pregnancy was not specifically included in the data). However, overall, people with at least one underlying condition have a 10-point drop in vaccine effectiveness against hospitalization compared with healthy individuals. As a result, the advisory committee voted in favor of offering booster shots to people with pre-existing conditions that confer a greater risk for severe disease, such as diabetes, heart disease, cancer, and pulmonary diseases, just in case. Pregnancy was included on that list because of the general concerns about changes to the immune system and the overall greater risk for severe disease.

Realistically, because of low vaccination rates and timing, the booster recommendation will encompass a very small group of pregnant people. All the experts interviewed for this article said that if CDC ruled that pregnant people were eligible to receive a booster, they would support it, but the more important task is increasing vaccination uptake among pregnant people who are not yet vaccinated. 

“I think we have to retool the message to say [if you’re pregnant] you’re clearly at higher risk, and the vaccine saves lives,” says Richard Beigi, an obstetrician/gynecologist, and president of Magee-Womens Hospital at the University of Pittsburgh. “Every day, I see more reassuring data on this vaccine. Every day I see more data demonstrating that pregnancy is at high risk. Those are the two real issues, I think, and we’ve got to do better at communicating that.”