Interpretation
The safety of SARS-CoV-2 vaccination during pregnancy has been
established in current short-term reports.(7, 8) Despite this, more than
50% of pregnant women have reported hesitancy towards COVID-19
vaccination,(19, 20) and the coverage rate among this population is only
approximately 30–50%.(7, 21)
Our study focused on a highly vaccinated pregnant population, where only
four out of 89 participants were unvaccinated and excluded from the
analysis. Seventy-two participants received one or more vaccinations
during pregnancy, and half of them completed the primary series with a
booster dose. All the participants received first-generation vaccines
designed for the Wuhan strain.
Participants with and without SARS-CoV-2 infection showed similar
characteristics in terms of vaccination history and type. Our high
vaccination coverage rate allowed for a focused analysis of the effects
of vaccination and infection in the pregnant population. Other
characteristics, including gestational age, maternal general condition,
comorbidities, patient age, and BMI, were similar between the two
groups. The cesarean rates differed because our hospital serves as a
tertiary center where patients without SARS-CoV-2 infection were mostly
admitted because of high-risk pregnancy.
The timing of vaccination is a critical factor that affects the level of
cord serum antibodies, particularly in pregnant individuals without
prior SARS-CoV-2 infection. Previous reports have demonstrated the
safety of SARS-CoV-2 vaccination during the third trimester and its
ability to reduce the risk of neonatal adverse outcomes.(7) However,
optimal trimester for maternal vaccination remains a topic of discussion
due to limited cord samples and follow-up intervals in previous
studies.(22) Our study, on the other hand, revealed that neonates born
to mothers who received a booster dose of SARS-CoV-2 vaccine during the
third trimester, especially those without prior SARS-CoV-2 infection,
had the highest level of cord serum anti-S concentration (Fig. 2) and
the most effective protection against both Wuhan and Omicron strains.
(Fig. 3)
In the immunocompetent general population, anti-S and anti-N antibodies
appear 8–14 days after the onset of symptoms.(23) In our study, delay
in the elevation of cord serum anti-S antibody levels was also observed
in comparison to maternal serum levels, both following vaccination and
infection. Our study found that the transmission ratios of both anti-S
and anti-N in maternal-fetal pairs had a comparable association with the
time variable, up to 150 days from the last vaccination or infection.
Previous observations of a negative correlation between transmission
ratios and viral load (12, 24) may be attributed to the decline in viral
load over time. Based on these findings, we can conclude that recent
SARS-CoV-2 infection may induce a booster vaccine-like effect on
antibody titers in pregnant women and their fetuses. Furthermore, our
study suggests that the interval between antigen exposure and blood
sampling may be the primary determinant of the transplacental antibody
transmission ratio.
Our results are consistent with previous experience with the combined
tetanus-diphtheria-pertussis vaccine. Tetanus-diphtheria-pertussis
immunization 8–12 weeks prior to delivery provides newborns with the
highest antibody titer,(25) similar to the suggestion for optimal timing
of SARS-CoV-2 vaccination in the third trimester. We believe that our
results can be replicated for future variants of the
SARS-CoV-2.