Karen B. Jacobson, MD, MPH, discusses a new study in which COVID-19 vaccination timing during pregnancy was associated with reduced infant infection and hospitalization outcomes. She describes how the findings should be interpreted and what clinicians can do to strengthen vaccine counseling with their patients.
ContributorKaren B. Jacobson, MD, MPHResearch Scientist & Infectious Diseases PhysicianVaccine Study Center Kaiser Permanente Northern California Division of Research
This transcript has been edited for clarity.
Welcome back to PeerPOV: The Pulse on Medicine, a podcast series by Physician’s Weekly showcasing the latest insights from your peers across the medical community.
On this week’s episode, Dr. Karen Jacobson discusses a new study that investigated whether COVID-19 vaccination timing during pregnancy impacted infant infection outcomes.
My name is Karen Jacobson. I’m an infectious diseases physician with the Permanente Medical Group and a research scientist at the Kaiser Permanente Vaccine Study Center in Oakland, California.
Infants younger than six months are too young to be vaccinated against COVID-19, but they can still experience severe disease. In fact, infants are the age group with the highest rate of hospitalization from COVID-19 aside from the very elderly, with people over 75 years of age.
Previous studies, including those done at our site by Ousseny Zerbo (the senior author on this study) and others, have showed that COVID-19 vaccination during pregnancy protected infants from COVID-19 infection in the first months of life. However, at this stage in the pandemic, many pregnant people have been vaccinated before the pregnancy began. We wondered if those vaccinations received before pregnancy still resulted in any protection for their infants. We also wondered if the protection of during-pregnancy vaccination we had seen previously would still hold up for later variants of COVID-19, including the later Omicron variants.
This was a retrospective cohort study of infants born at Kaiser Permanente Northern California (KPNC), which is an integrated healthcare organization that serves over 4.5 million people. About 40,000 infants are born each year at KPNC facilities. This cohort that we studied were born from July 2021 to June 2023.
We looked at when the birth mothers had received their most recent COVID-19 vaccine before the birth, and we categorized their exposure by when they received their most recent vaccine up to 14 days before delivery. This was categorized as:
We were interested in two outcomes. The first was any PCR-confirmed COVID-19 infection, or COVID-19 infection determined by diagnostic code. The second was COVID-19 hospitalizations, or hospitalizations within seven days before or 30 days after a COVID-19 infection.
We chart reviewed those hospitalizations to confirm they were truly COVID-19–related. We performed a Cox regression with a calendar timeline, adjusting for maternal and infant characteristics, and calculated the vaccine effectiveness, comparing a vaccine in any of these durations versus the unvaccinated mothers.
First of all, we found that vaccination in any interval before pregnancy, even very recently before pregnancy begins, was not protective in any of the analyses. However, we did find that vaccination during pregnancy, particularly in the third trimester, was protective against infant COVID-19 outcomes and was very protective against hospitalization for COVID-19. Vaccination anytime during pregnancy was 52% protective against hospitalization and 64% protective when given in the third trimester.
As expected based on past studies, we also found that protection was highest in the infant’s first two months of life. We also found that protection was high in the Delta period, became less strong during the early Omicron period, but then in the later Omicron periods (with BA4, BA5, and BQXB variants), the protection of third-trimester vaccination was actually higher than it was back in the Delta period.
That’s reassuring because prior studies that looked at Delta and the early Omicron periods had suggested that effectiveness was declining for the newer variants, but that’s not what we found. We actually found quite strong protection in the later variants. This is important because there was a continued benefit during pregnancy, even in a population with substantial prior immunity in the moms.
I wouldn’t say that our study is making any specific recommendations for the COVID-19 vaccine and how it should be given around pregnancy. We believe that pregnant patients should continue to follow the current American College of Obstetricians and Gynecologists (ACOG) guidelines, which recommend getting an updated COVID-19 vaccine as soon as it is available, regardless of whether the person is not yet pregnant or pregnant in any trimester.
Delaying vaccination risks severe illness for the pregnant person and their baby, and there’s plenty of evidence that vaccination before and during pregnancy is beneficial for the moms and for pregnancy outcomes. In no way do our findings suggest that vaccines should be delayed till third trimester in order to protect the infant. Our study suggests that for mothers whose routine, updated COVID-19 vaccine happens to fall in the third trimester, their babies are likely very protected from COVID-19.
What we can’t yet answer and would be a topic of future research is whether mothers who received an updated vaccine before pregnancy should receive an additional third-trimester booster for the purpose of protecting those infants. That is a topic for future research.
For clinicians who are interacting with pregnant people, they have two patients: the mother and the baby. They’re looking to balance care of both of those patients. I think we and others have shown the vaccines are a great way to protect both the pregnant patient and the infant. We have a lot of evidence of the benefits to both the pregnant person and the infant that can be explained to your patients. I think most parents really do want to protect their infants from any bad outcomes, and being armed with the most information about different protection measures helps parents make those decisions.
I’d like to reiterate that COVID-19 vaccination during pregnancy is an important tool for protecting infants who are too young to be vaccinated. Our findings reinforce that the timing of the vaccination matters and that pregnancy is a key window for prevention. I encourage clinicians and patients to follow current ACOG guidelines regarding COVID-19 vaccination and pregnancy, and I look forward to future studies that might elucidate whether a booster is additionally protective or helpful.
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