Today, I’m going to talk through a few topics. First, the emerging data on the possible risks of serious COVID during pregnancy. Second, to what extent vaccination protects against those risks (a lot) and how much your risk is elevated even post-vaccine. Finally, new evidence on the extent to which antibodies are passed to infants during pregnancy (it’s good news).
(One thing I am not going to talk about is evidence on the safety of vaccines during pregnancy. There is a variety of evidence, well summarized here, indicating a good safety profile. At this point, millions and millions of COVID-19 vaccines and boosters have been given in pregnancy, with no red flags.)
COVID risks in pregnancy
There is increasing evidence that COVID illness, especially serious illness, presents significant risks during pregnancy. The most recent data on this comes from an article in the Journal of the American Medical Association that compared about 12,000 pregnant patients without a positive COVID test to 2,300 with a positive result. This study focused entirely on data from 2020, before any of the patients were vaccinated.
The authors found that 13.4% of the patients with a positive COVID-19 test had a bad outcome (death, serious morbidity associated with hypertension, postpartum hemorrhage, or other infection). This occurred in only 9.2% of the patients without a positive COVID test. Perhaps most notable for discussion later, the increased risk was apparent only in patients with moderate or more severe COVID-19 (those with shortness of breath and clinical evidence of lower respiratory disease). For patients with milder symptoms (fever, cough, loss of taste, nausea, diarrhea, etc.), the risk of serious outcomes was the same as in the non-COVID group.
A second paper, using data on 3,500 patients in the U.K., found elevated risks for serious outcomes for both mother and baby; this data was, again, prior to vaccination. Of note: the U.K. data (and the public health authorities there in general) suggests that much of the COVID risk to infants is a result of preterm birth, which itself is a result of maternal illness. That is, serious COVID-19 illness results in circumstances for the mother that require delivery of the baby, sometimes prematurely. This preterm delivery then puts the infant at greater risk.
CDC data also shows an elevated risk of stillbirth with COVID-19 infection, which may relate to issues with the placenta.
Our information is still somewhat incomplete. The CDC reports out some data on COVID-19 outcomes for pregnant people in its data tracker. Based on the tracker, it is difficult to learn anything about the relative risk for the pregnant group; the agency provides only counts, not rates, and no way to compare with the same age group for the non-pregnant. One thing that does jump out is that the death rate for pregnant people was much higher in the Delta wave than in either the initial wave or the Omicron wave.
Pulling this all together, it seems clear that the risks associated with COVID-19 in the unvaccinated are much higher during pregnancy. And we know that, in general, vaccination lowers the risk of serious COVID illness. The question is, then, to what extent are fully vaccinated people still at greater risk in pregnancy?
How much does vaccination change this?
What I think many people — myself included — would really like to see is the hospitalization rate for pregnant women and non-pregnant women in the same age group, broken down by vaccinated versus unvaccinated. The evidence above suggests that we’d clearly see higher hospitalization rates among the pregnant group if unvaccinated. The question is, do we also see that among the vaccinated and, if so, how much higher?
In principle, such data could exist. It even seems possible that the CDC has such evidence at its disposal. If it does, though, the agency has not published it, and the data it posts is insufficient to calculate it. Data from the U.K. is better, but it doesn’t get all the way there.
Still, there is significant evidence that getting vaccinated dramatically lowers the risks associated with pregnancy. Cohort data from hospitalizations in the U.K. covering the period from January 2020 through November 2021 showed serious risks associated with COVID in pregnancy. Notably, however, it found that no fully vaccinated pregnant women were admitted to hospitals during the sample period.
A comprehensive study of pregnant patients in Scotland found that the risk of critical-care hospital admission was almost 14 times as high in unvaccinated people as in fully vaccinated people. There, researchers saw only one critical-care hospital admission among 550 fully vaccinated pregnant people.
These papers line up with other statements from the U.K. data sources — for example, that 98% of pregnant people admitted to the hospital for COVID are unvaccinated. A recent summary paper looked at parallel numbers in other countries and found that critical-care admissions were effectively completely limited to unvaccinated people. Last week, a new paper using closer to comprehensive data from the UK again showed the vast majority of hospital admissions were among the unvaccinated.
From this we get, first, that vaccination before or during pregnancy is an extremely good idea. It dramatically lowers the risk of serious illness or hospital admission (just as for non-pregnant people). Moreover, looking back at the evidence on the risks of COVID to babies, the discussion above suggests that it is the serious illness in mothers that produces the risk in babies. If vaccines lower this risk to moms, they would also lower the risks to babies.
What we do not have is the direct comparison of vaccinated pregnant to vaccinated non-pregnant people. However, I think we can use logic to put some of this together. When we look at data on vaccine efficacy against serious illness, it has been very consistently in the range of 90% to 95% across various groups and variants. If we think (as is likely) that vaccines perform similarly well in pregnant as in non-pregnant people, then both would see this reduction. If COVID risks are higher for pregnant people in general, then they are likely somewhat elevated for vaccinated pregnant people too.
In practice, the actual increase may be quite small. Most of the evidence on risks in the unvaccinated suggests a risk elevated by a factor of 50%, or even doubling. This seems like a lot, but the risk of serious illness from COVID-19 in a vaccinated and boosted woman in the (say) age range of 25 to 45 is extremely, extremely low. So even doubling that risk is still a very small number.
Pregnancy puts you at elevated risk for many illnesses, and it makes sense to be a bit more cautious. But this isn’t limited to COVID. COVID is a special risk for the unvaccinated, but it likely falls into more of the “normal” elevated risk for the vaccinated.
Baby immunity
Perhaps the most exciting new developments in the pregnancy-baby-COVID space relate to research on the immunity that infants seem to get as a result of maternal vaccination.
First: a very recent paper tested about 1,350 umbilical-cord blood samples from vaccinated pregnant women for evidence of COVID-19 antibodies. Cord blood will indicate antibody levels in the infant. The authors found that regardless of the timing of vaccination, the cord blood showed antibodies. The highest antibodies were among women who were vaccinated in the early third trimester (or got a booster at that time). This timing makes sense, given what we know about antibodies, and (as I pointed out in October) it lines up with the timing we use for pertussis vaccines in pregnancy.
Second, even more directly, the CDC put out a study in mid-February showing a 60% reduction in COVID-19 hospitalization for infants under six months as a result of vaccination during pregnancy. The sample size for this study is small, so we cannot really be precise about the size of the risk reduction. But it is significantly different from zero. Moreover, lining up with the other evidence, vaccination later in pregnancy was more protective than vaccination early in pregnancy.
My best guess is that, based on these studies, there will be a push for a COVID booster in later pregnancy, similar to what we do with the Tdap vaccine for pertussis, to provide some early infant protection. But until then, if you are pregnant and you’ve had a recent booster — or you’ve recently had Omicron — you’re protecting your baby already.