Few studies have focused on serologic testing for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in pregnant women.1–3 The course of development and duration of antibody responses in patients with SARS-CoV-2 infection is still unclear. Studies have shown that the majority (more than 95%) of patients with SARS-CoV-2 infection confirmed by polymerase chain reaction (PCR) testing develop immunoglobulin G (IgG) or immunoglobulin M (IgM) antibodies or both after infection, even in mild and asymptomatic cases.4–6 In a recent study from Philadelphia, Pennsylvania, serologic testing was performed on 1,293 women admitted for delivery and a seroprevalence of 6.2% was found.3 The study did not include comprehensive data concerning obstetric and neonatal outcomes. Two case reports from China documented SARS-CoV-2 antibodies (both IgG and IgM) in newborns with mothers who tested positive for SARS-CoV-2, indicating possible vertical transmission.1,7 Our literature search (Appendix 1, available online at https://links.lww.com/AOG/C123) found no previous study that investigated the frequency of SARS-CoV-2 infection in the partners of pregnant women.
This study aimed to investigate the frequency of SARS-CoV-2 antibodies in parturient women, their partners, and their newborns and to explore associations with obstetric and neonatal outcomes.
The Department of Obstetrics and Gynecology at Copenhagen University Hospital Hvidovre serves a broad population and has 7,000 births per year. From April 4, 2020, to July 3, 2020 (3 months), during the first epidemic wave in Denmark, all women giving birth at the department and their partners were approached and invited to participate in the study, along with their newborns.
Participating women and their partners had a pharyngeal swab and a blood sample taken at admission. Immediately after delivery, an umbilical cord blood sample was drawn if the newborn participated in the study. The pharyngeal swabs from the participating women and their partners were analyzed for SARS-CoV-2 RNA by PCR testing. The serum from the blood samples from women, partners, and newborns was analyzed for SARS-CoV-2 antibodies (IgM and IgG).
Samples were analyzed using the iFlash 1800 and SARS-CoV-2 IgM and IgG kits. Following a recent study on the specific assay, a negative result was defined as IgM less than 8 arbitrary units/mL and IgG less than 10 arbitrary units/mL and a positive result was defined as IgM 8 arbitrary units/mL or higher and IgG 10 arbitrary units/mL or higher. This has been demonstrated to have a sensitivity of 42.0% and a specificity of 99.7% for IgM and 94.0% and 99.3%, respectively, for IgG.8
Baseline characteristics of the included women and obstetric outcomes were recorded by cross-referencing with the electronic health records. Additionally, a short questionnaire concerning symptoms of coronavirus disease 2019 (COVID-19) since December 2019 and lung disease was completed by all participating women. Characteristics of the included partners were obtained through a short questionnaire and by cross-referencing with the electronic health records. Neonatal outcomes were recorded by cross-referencing with the newborn's electronic health records.
Data were analyzed using the statistical software R. Normally distributed comparison between groups was performed using a t test; for nonnormally distributed data, the Wilcoxon rank sum test was used. Categorical data were analyzed using the Fisher exact test. Calculation of adjusted prevalence, odds ratio (OR), attributable risk, and CI was performed using the R package epiR.9 Observed prevalence was adjusted for the known specificity and sensitivity of the assays,8 and CIs were established using the Wilson method. Concordance between family members was calculated using phi correlations, with 95% CIs estimated through bootstrapping with 10,000 replicates. Multiple regression was performed using the R package rms.10P<.05 was considered statistically significant. One of the authors (D.W.) performed the statistical analysis.
The study was approved by the Knowledge Centre for Data Protection and Compliance, The Capital Region of Denmark (P-2020-255), and by the Scientific Ethics Committee of the Capital Region of Denmark (journal number H-20022647). All participants provided written informed consent. Informed consent for the newborn was obtained from both parents or from the mother alone in case of sole parental custody.
A total of 1,810 women gave birth at the hospital in the study period, of whom 1,313 (72.5%) participated in the study. Additionally, 1,188 partners and 1,206 newborns participated.
Of the included parturient women, 28 had antibodies against SARS-CoV-2 (2.1%). The adjusted prevalence was 2.6% (95% CI 1.7–4.0%) when taking into account the assay specificity and sensitivity. One woman had a positive test result by pharyngeal swab at delivery (Table 1). The only statistically significant difference in prepregnancy characteristics between women with and without SARS-CoV-2 antibodies is that blood type A was more common among those with antibodies (OR 2.24, 95% CI 1.04–4.83) (Table 1). Of women with antibodies, 14 of 28 (50%) reported previous symptoms of COVID-19; 383 of 1,285 (31%) women without antibodies reported symptoms (OR 2.2, 95% CI 1.03–4.70). Three women with antibodies had previously tested positive for SARS-CoV-2 infection by pharyngeal swab. Two women without SARS-CoV-2 antibodies had previously tested positive for SARS-CoV-2 infection by swab (one of these had a positive result by swab 4 days before delivery). There was no significant difference in pregnancy complications or obstetric complications between the two groups (Table 1). This was further confirmed in a subgroup analysis of symptomatic mothers who had tested positive for SARS-CoV-2 infection compared with mothers with no antibodies and an analysis of only mothers with antibodies stratified by symptoms (Appendices 2 and 3, available online at https://links.lww.com/AOG/C123). Furthermore, an analysis including only mothers with IgG antibodies indicated that the low sensitivity of the IgM assay did not change the findings (Appendix 4, available online at https://links.lww.com/AOG/C123). Of the 21 newborns of the 28 women with antibodies who were tested, 14 (67%) had SARS-CoV-2 antibodies, which were also found in 3 of 1,131 (0.3%) newborns of women without antibodies (Table 1). There was a significant increase in the relative risk for newborns to have antibodies if the mother had antibodies (relative risk 280, 95% CI 88–889). This corresponded to an 81% increase in absolute risk (95% CI 62–100%).
Antibodies against SARS-CoV-2 were found in 32 of 1,188 tested partners (2.7%) (Table 2). The adjusted prevalence was 3.5% (95% CI 2.3–5.1%). There was no significant difference in baseline characteristics (eg, age, body mass index [BMI, calculated as weight in kilograms divided by height in meters squared], smoking, asthma, chronic disease) between partners with and without antibodies (Table 2). There was no increased history of travel outside Denmark since December 2019 in the group of partners with antibodies. More partners in the group with antibodies (2/32, 6%) had a prior positive test result for SARS-CoV-2 infection by swab compared with those in the group without antibodies (1/1,156, 0%) (OR 77, 95% CI 6.8–872.6). Sixty-five percent (95% CI 42–77%) of partners with SARS-CoV-2 antibodies reported symptoms since December 2019 (Table 2). None of the women or partners with antibodies in our cohort had been hospitalized for COVID-19.
Of the 1,206 newborns, 17 had antibodies against SARS-CoV-2 infection (1.4%). All were IgG-positive and IgM-negative. There were no significant differences between newborns with or without antibodies in rates of prematurity, meconium-stained fluid, sex, biometry, umbilical artery pH, or Apgar scores (Table 3). Birth weight was significantly different between the two groups (P=.015), but this association was rendered nonsignificant when corrected for gestational age and newborn birth length (P=.18). Neonatal outcomes were also not affected by the antibody status of the parents (Appendices 5 and 6, available online at https://links.lww.com/AOG/C123).
Of 1,051 families with complete serologic data (in trios of mother, partner, and newborn), we found six families where all members had antibodies against SARS-CoV-2 (Appendix 7, available online at https://links.lww.com/AOG/C123). The patterns of familial infection can be seen in Figure 1. We found that there was a 37% (95% CI 19–55) increase in the absolute risk of antibody positivity for mothers living with a partner who had SARS-CoV-2 antibodies. The absolute risk of infection was 39% (95% CI 22–59%) for the woman if her partner tested positive. There was a significantly positive concordance for infection in family members (Appendix 8, available online at https://links.lww.com/AOG/C123).
No association between previous SARS-CoV-2 infection and obstetric or neonatal complications was found. If the partner had SARS-CoV-2 antibodies, the woman had a 39% risk of infection.
A recent study found a significantly higher frequency of cesarean delivery in women with symptomatic COVID-19 (46.7%) compared with women without COVID-19 (30.9%).11 In our cohort, approximately 20% of the included women had a cesarean delivery. In the study by Prabhu et al, however, they used nasopharyngeal swab and PCR analysis,11 which reflected acute COVID-19. The authors state that obstetric management was not altered according to symptoms or COVID-19 status, and the indication for cesarean delivery was not statistically different according to COVID-19 status.11 The difference in frequency of cesarean delivery may be explained by differences in baseline characteristics of the included women (eg, fewer women in our cohort had BMIs higher than 30) or general national differences in obstetric management. The significantly higher frequency of blood type A in the women who tested positive has been documented previously.12
Our study identified 17 newborns with SARS-CoV-2 antibodies. No newborns had IgM antibodies. Starting at the second trimester, IgG passively transfers across the placenta from the mother to the fetus.13 Neonatal IgM antibodies originate in the fetus and are evidence of intrauterine infection. The presence of IgG antibodies but not IgM antibodies in the newborns is most likely passive transfer from the mother. There were three newborns with SARS-CoV-2 antibodies among 1,285 women without antibodies. Those three mothers had IgG values just below the cutoff for positive samples, so essentially these mother and newborn results are concordant.14 The mothers' test results showed either declining IgG values after a previous infection or biological high negative values. Although neonatal infection cannot be ruled out, the neonatal outcomes from our study are reassuring.
A substantial increase in the absolute risk of infection for women living with a partner who had antibodies was documented in our study. A study investigating secondary transmission of SARS-CoV-2 infection in households found that secondary transmission occurred in 41 of 124 families.15 That study, however, included only symptomatic patients. In our study, only three women and two partners with SARS-CoV-2 antibodies had previously tested positive by PCR swab, indicating that our study primarily represents asymptomatic or mild cases of SARS-CoV-2 infection. At the beginning of the epidemic, people in Denmark were tested only if they had severe symptoms with need of clinical examination.16 In April, testing of mild cases was implemented,16 and, from the end of May, all people in Denmark could be tested.17
Our cohort therefore may represent the risk of transmission for asymptomatic cases. However, this risk assumes the parturient women live with their partner and that the primary case is the partner. The pattern of disease in families is highly essential to evaluate the risk of infection and to provide guidance to health authorities.
The present study is a prospective cohort study from the largest obstetric department in Denmark. The health care system in Denmark is free of charge, which minimizes selection bias, and 95% of all births take place in a hospital. We used serologic testing and had complete data on obstetric and neonatal outcomes, and all included women were pregnant during the pandemic.
In general, our study population was young, nonsmokers, had BMIs in the normal-weight range, and had had asymptomatic or mild SARS-CoV-2 infection. Our results may therefore not be applied directly to other populations. Additionally, the women most likely contracted SARS-CoV-2 infection in the late second trimester or third trimester of pregnancy. Future studies are thus needed to assess the risk for vertical transmission in the first trimester and early second trimester.
Pharyngeal swab testing for SARS-CoV-2 infection has a sensitivity of 90–95% in symptomatic individuals.18 However, the sensitivity is likely lower in asymptomatic individuals, leading to an increased false-negative rate. The present study focused, however, on antibodies, and we used the iFlash 1800, which has shown highly accurate results.8,19 However, future studies are needed to validate the serologic assays, especially for use as a screening tool in asymptomatic populations.20 Owing to the low prevalence of SARS-CoV-2 antibodies and event rate, we cannot rule out that some of the nonsignificant findings may not generalize to other populations because of the limited statistical power.
In conclusion, we found no association between previous SARS-CoV-2 infection and obstetric or neonatal complications. We found a seroprevalence of 2.6% in the parturient women and 3.5% in their partners. Sixty-seven percent of newborns delivered by mothers with antibodies had SARS-CoV-2 IgG antibodies.
A preprint of this manuscript was submitted to medRxiv. Validation of chronic diseases in partners and validation of positive SARS-CoV-2 test results in all women and their partners has since been performed. This resulted in minor changes throughout the text but did not alter the main findings or conclusion. Additionally, we performed five new subgroup analyses (Appendices 2–6, https://links.lww.com/AOG/C123).
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