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.