Longitudinal seroprevalence study among pregnant people indicates waning of immunity from natural SARS-CoV-2 infections

By Dr. Chinta SidharthanReviewed by Aimee MolineuxJan 3 2023

In a recent study posted to the medRxiv* preprint server, researchers evaluated the seroprevalence and durability of antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid protein among pregnant individuals and investigated transplacental antibody transfer.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Background

Studies investigating the seroprevalence of anti-SARS-CoV-2 antibodies among pregnant people have largely been cross-sectional and during hospital deliveries. These studies miss reporting on individuals who had coronavirus disease 2019 (COVID-19) in the early stages of the pregnancy or were asymptomatic or mildly symptomatic.

While COVID-19 vaccines have been recommended in the United States for pregnant and lactating individuals and children above the age of six months, there are no vaccines yet for infants below six months of age. SARS-CoV-2 infections are known to provide immunity against reinfections and may protect pregnant individuals as well as infants during the peripartum period.

However, longitudinal studies spanning ante- and post-natal periods investigating anti-SARS-CoV-2 antibodies in pregnant people and infants are scarce. Information on the antibody responses to SARS-CoV-2 infections during and after pregnancy as well as the efficacy of transplacental antibody transfer, could help understand the protection granted by natural SARS-CoV-2 infections to the mother and the neonate.

About the study

In the present study, the researchers tested the anti-nucleocapsid immunoglobulin G (IgG) seroprevalence in pregnant individuals above the age of 18 who had sought antenatal care or were admitted to the labor and delivery medical centers or clinics affiliated with the University of Washington. Electronic health records were used to determine the severity of the SARS-CoV-2 infections and COVID-19 symptoms.

For the prospective cohort, the researchers enrolled 23 participants who tested positive for SARS-CoV-2 antibodies during the seroprevalence study and 66 other patients identified from medical records with previous SARS-CoV-2 infections. Prior SARS-CoV-2 infections were confirmed based on positive reverse transcription polymerase chain reaction (RT-PCR) or rapid antigen tests, following which blood samples were collected for immunological tests. For the longitudinal analysis, maternal and cord blood samples were also collected at one, two, and three months after enrollment in the study and delivery. Additionally, blood samples were also collected during the postpartum period, around two, four, and six months.

The chemiluminescent immunoassay was used to determine the anti-nucleocapsid immunoglobulin G (IgG) levels in the blood samples. The primary exposure was determined based on the diagnosis, symptoms, and severity of SARS-CoV-2 infections. The anti-nucleocapsid IgG levels and their durability, as well as the transplacental anti-nucleocapsid IgG antibody transfer, were the measured primary outcomes.

Results

The results reported seroprevalence of anti-nucleocapsid SARS-CoV-2 antibodies from 65 (5%) out of the 1289 selected participants of the seroprevalence study. Of these, 60% (39) did not have any documented positive RT-PCR tests during pregnancy.

The seroprevalence also differed based on ethnicity and race. White pregnant individuals constituted 57% of the group with no anti-nucleocapsid SARS-CoV-2 IgG antibodies and 37% of the seropositive group. Individuals of Black, American Indian, Native Alaskan, Native Hawaiian, or Pacific Islander groups had almost three times the seroprevalence of the White participants. Furthermore, Hispanic participants had three times higher seroprevalence compared to non-Hispanic participants.

The probability of testing positive for anti-nucleocapsid IgG antibodies was seen to decrease with increasing age, with each year decreasing the seropositive probability by 8%. Of the 65 seropositive individuals, 35% (23) exhibited COVID-19 symptoms, with 21 having mild symptoms and two showing severe symptoms. Two of the seropositive individuals were also hospitalized due to severe COVID-19.

In the prospective cohort study, 73 participants out of the 89 enrolled tested positive for anti-nucleocapsid IgG antibodies during pregnancy. Delivery samples were only available for 49 participants, and of these, 67% (33) were seronegative by the time of delivery. When the cord blood samples and blood samples of the remaining 24 seropositive individuals were analyzed at delivery, 50% (12) displayed efficient transplacental transfer of anti-nucleocapsid IgG antibodies against SARS-CoV-2.

Furthermore, the time taken for the waning of antibodies from the first positive result for anti-nucleocapsid IgG antibodies to below-positive threshold levels had a median value of 17 weeks, irrespective of the initial RT-PCR test results. Time taken for anti-nucleocapsid IgG antibodies to fall below positive threshold levels from the first positive RT-PCR test had a median value of 28 weeks.

Conclusions

Overall, the results indicated that antibodies elicited by previous SARS-CoV-2 infections in pregnant individuals might wane by the time of delivery. The authors recommended that pregnant individuals make use of the available COVID-19 vaccines to ensure continued protection against SARS-CoV-2 infections during pregnancy and offer the neonate protection from the virus during the first six months.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.