Factors preventing materno-fetal transmission of SARS-CoV-2

Mechanisms of SARS-CoV-2 Placental Transmission

The widespread occurrence of SARS-CoV-2 infections and the diverse range of symptoms have placed significant strain on healthcare systems worldwide. Pregnancy has also been affected by COVID-19, with an increased risk of complications and unfavorable outcomes for expectant mothers. Multiple studies indicate that SARS-CoV-2 can infiltrate the placenta, breach its protective barrier, and infect the fetus. Although the precise mechanisms of intrauterine transmission remain unclear, factors such as perinatal infection, macrophages, sexual intercourse, and the virus' interaction with host angiotensin-converting enzyme 2 (ACE2) and neuropilin-1 (NRP-1) proteins appear to play a role in this process. The integrity of the placental barrier fluctuates throughout pregnancy and appears to influence the likelihood of fetal transmission. The expression of placental cell receptors, like ACE2, changes during pregnancy and in response to placental damage. However, due to the consistent presence of others, such as NRP-1, SARS-CoV-2 may potentially enter the fetus at different stages of pregnancy. NRP-1 is also found in macrophages, implicating maternal macrophages and Hofbauer cells as potential routes for viral transmission. Our current understanding of SARS-CoV-2's vertical transmission pathways remains limited. Some researchers question the ACE2-associated transmission model due to the relatively low expression of ACE2 in the placenta. Existing studies investigating perinatal transmission and the impact of sexual intercourse have either involved small sample sizes or lacked statistical significance. This review aims to explore the current state of knowledge regarding the potential mechanisms of COVID-19 vertical transmission, identifying areas where further research is needed to fill the gaps in our understanding.

Vertical transmission and maternal passive immunity post-SARS-CoV-2

Since 2020, the highly contagious nature and various transmission routes of SARS-CoV-2 have rendered the pandemic interminable. Vertical transmission (VT) through the placenta and breast milk, which is frequent for certain virus types, is thought to exist for SARS-CoV-2 and is hypothesized by many researchers. Conversely, antibodies are produced to counteract the effect of viruses. Since newborns' immunologic system cannot produce proper antibodies, maternal antibodies are usually transferred from mother to infant/fetus to meet the need. This theory leads to the hypothesis of transmission of antibodies through the placenta and breast milk following SARS-CoV-2 infection or vaccination. This paper further discusses these hypotheses, considering consequences of fetus/infant harm versus benefit.

SARS-CoV-2 Footprints in the Placenta: What We Know after Three Years of the Pandemic

As the COVID-19 pandemic continues into its third year, there is accumulating evidence on the consequences of maternal infection. Emerging data indicate increased obstetrics risks, including maternal complications, preterm births, impaired intrauterine fetal growth, hypertensive disorders, stillbirth, gestational diabetes, and a risk of developmental defects in neonates. Overall, controversial concerns still exist regarding the potential for vertical transmission. Histopathological examination of the placenta can represent a useful instrument for investigation and can contribute significant information regarding the possible immunohistopathological mechanisms involved in developing unfavorable perinatal outcomes. Based on current evidence, SARS-CoV-2 infection can affect placental tissue by inducing several specific changes. The level of placental involvement is considered one of the determining factors for unfavorable outcomes during pregnancy due to inflammation and vascular injuries contributing to complex cascade immunological and biological events; however, available evidence does not indicate a strong and absolute correlation between maternal infection, placental lesions, and obstetric outcomes. As existing studies are still limited, we further explore the placenta at three different levels, using histology, immunohistochemistry, and molecular genetics to understand the epidemiological and virological changes observed in the ongoing pandemic.

SARS-CoV-2 Transplacental Transmission: A Rare Occurrence? An Overview of the Protective Role of the Placenta

The outbreak of the coronavirus disease 2019 (COVID-19) pandemic, caused by novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has resulted in a global public health crisis, causing substantial concern especially to the pregnant population. Pregnant women infected with SARS-CoV-2 are at greater risk of devastating pregnancy complications such as premature delivery and stillbirth. Irrespective of the emerging reported cases of neonatal COVID-19, reassuringly, confirmatory evidence of vertical transmission is still lacking. The protective role of the placenta in limiting in utero spread of virus to the developing fetus is intriguing. The short- and long-term impact of maternal COVID-19 infection in the newborn remains an unresolved question. In this review, we explore the recent evidence of SARS-CoV-2 vertical transmission, cell-entry pathways, placental responses towards SARS-CoV-2 infection, and its potential effects on the offspring. We further discuss how the placenta serves as a defensive front against SARS-CoV-2 by exerting various cellular and molecular defense pathways. A better understanding of the placental barrier, immune defense, and modulation strategies involved in restricting transplacental transmission may provide valuable insights for future development of antiviral and immunomodulatory therapies to improve pregnancy outcomes.

Increased Rate of Spontaneous Pneumothorax in Infants of COVID-19 Positive Mothers

Objective The epidemiological and clinical characteristics of neonates born to women infected with coronavirus disease 2019 (COVID-19) during pregnancy were assessed, and the correlation between this infection and spontaneous pneumothorax in neonates born to mothers with COVID-19 was evaluated in the present study.

Methods Records of 14 neonates in the neonatal intensive care unit with pneumothorax were collected and analyzed. Pregnant women were routinely screened for COVID-19 before birth. This study only included the neonates of mothers positive for severe acute respiratory syndrome coronavirus 2 immunoglobulin Mand immunoglobulin G and developing spontaneous pneumothorax. Antenatal, natal and postnatal risk factors, data related to demographic, epidemiological and clinical characteristics, treatment strategies, and breastfeeding history were obtained from medical records.

Results The gestational age of the 14 neonates was 30 to 38 weeks. One male infant was born by normal spontaneous vaginal delivery, while all other infants were born by C-section. Though the mothers did not have a diagnosis of COVID-19 in their charts, they all reported one or more symptoms when interviewed. No mother had received a COVID-19 vaccination before or during pregnancy. No mother had undergone a hospital visit or doctor examination due to suspicion of COVID and COVID polymerase chain reaction test. COVID antibody titers were present during admission to hospital before birth.

Conclusion The infants of pregnant cases with symptomatic or asymptomatic COVID-19 may develop respiratory distress and pneumothorax. Observational data obtained from case series similar to what is presented here may be accepted as a potential first step to producing hypotheses to test with preclinical or clinical models if it can be expanded in larger cohorts.

Coronavirus disease 2019 and the placenta: A literature review

Coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus has been implicated in the clinical pathology of multiple organs and organ systems. Due to the novelty of the disease, there is a need to review emerging literature to understand the profile of SARS-CoV-2 in the placenta. This review sought to evaluate the literature on the mediators, mechanism of entry, pathogenesis, detection, and pathology of SARS-CoV-2 in the placenta. Systematic literature searches found 96 eligible studies. Our review revealed that SARS-CoV-2 canonical mediators, angiotensin-converting enzyme-2 (ACE2), and transmembrane serine protease-2 (TMPRSS2) are variably expressed in various placenta compartments, including the villous cytotrophoblasts, syncytiotrophoblasts (STBs), and extravillous trophoblasts (EVTs) throughout pregnancy. Placental SARS-CoV-2 and coronavirus-associated receptors and factors (SCARFs), including basigin (BSG/CD147), dipeptidyl peptidase-4 (DPP4/CD26), cathepsin B/L (CTL B/L), furin, interferon-induced transmembrane protein (IFITM1-3), and lymphocyte antigen 6E (LY6E) may increase or reduce the permissiveness of the placenta to SARS-CoV-2. EVTs express genes that code for proteins that may drive viral pathogenesis in the placenta. Viral RNA, proteins, and particles were detected primarily in the STBs by in situ hybridization, immunohistochemistry, electron microscopy, and polymerase chain reaction. Placental pathology in SARS-CoV-2-infected placentas included maternal and fetal vascular malperfusion and a generally nonspecific inflammatory-immune response. The localization of SARS-CoV-2 receptors, proteases, and genes involved in coding proteins that drive viral pathogenesis in the placenta predisposes the placenta to SARS-CoV-2 infection variably in all pregnancy trimesters, with antecedent placental pathology. There is a need for further studies to explicate the mechanism of entry and pathogenesis of SARS-CoV-2 in the placenta.

The effect of the delta SARS-CoV-2 variant on maternal infection and pregnancy

A greater proportion of pregnant women with COVID-19 have mild disease compared with their non-pregnant counterparts. Paradoxically, however, they are at higher risk of developing severe disease, requiring respiratory support and admission to intensive care. The delta SARS-Cov-2 variant is associated with increased risk of hospitalization and morbidity in unvaccinated pregnant populations. However, it is not known whether the worse pregnancy outcomes associated with the delta variant are due to a direct effect of the virus on the pregnancy, or whether this effect is mediated through more severe maternal infection. Here, we synthesize studies of COVID-19 pregnancies, focusing on the different routes of SARS-CoV-2 infection of lung and placenta, and the mechanisms of syncytial formation for each SARS-CoV-2 variant. To delineate COVID-19 complications in pregnant women, future studies should explore whether the delta variant causes greater placental infection compared to other variants and contributes to increased syncytial formation.

A review of the main placenta histopathological findings reported in coronavirus disease 2019

Several studies have reported that pregnant women are more susceptible to contracting the SARS-CoV-2 disease. However, SARS-CoV-2 infection studies have limited evidence regarding its impact on pregnancy, particularly its pathological effects on the maternal-fetal interface. This review emphasized the placental structures and immunomodulatory defense mechanism against the viral infection COVID and highlighted the spectrum of reported histopathological changes from SARS-CoV-2-infected mothers' placenta to contribute to the knowledge of the nature of this placental pathology. Further studies where collaborations that seek to maximize sample numbers analyzed can be performed to improve the generalizability and reliability of the findings. This can lead to improved knowledge on the relationship between placental dysfunction and pathology from maternal SARS-CoV-2 infection. Consequently, this can help improve maternity care delivery during the pandemic.

Calcium Signals during SARS-CoV-2 Infection: Assessing the Potential of Emerging Therapies

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a positive-sense single-stranded RNA virus that causes coronavirus disease 2019 (COVID-19). This respiratory illness was declared a pandemic by the world health organization (WHO) in March 2020, just a few weeks after being described for the first time. Since then, global research effort has considerably increased humanity's knowledge about both viruses and disease. It has also spawned several vaccines that have proven to be key tools in attenuating the spread of the pandemic and severity of COVID-19. However, with vaccine-related skepticism being on the rise, as well as breakthrough infections in the vaccinated population and the threat of a complete immune escape variant, alternative strategies in the fight against SARS-CoV-2 are urgently required. Calcium signals have long been known to play an essential role in infection with diverse viruses and thus constitute a promising avenue for further research on therapeutic strategies. In this review, we introduce the pivotal role of calcium signaling in viral infection cascades. Based on this, we discuss prospective calcium-related treatment targets and strategies for the cure of COVID-19 that exploit viral dependence on calcium signals.

Significance of the placental barrier in antenatal viral infections

The placenta provides a significant physical and physiological barrier to prevent fetal infection during pregnancy. Nevertheless, it is at times breached by pathogens and leads to vertical transmission of infection from mother to fetus. This review will focus specifically on the Zika flavivirus, the HIV retrovirus and the emerging SARS-CoV2 coronavirus, which have affected pregnant women and their offspring in recent epidemics. In particular, we will address how viral infections affect the immune response at the maternal-fetal interface and how the placental barrier is physically breached and discuss the consequences of infection on various aspects of placental function to support fetal growth and development. Improved understanding of how the placenta responds to viral infections will lay the foundation for developing therapeutics to these and emergent viruses, to minimise the harms of infection to the offspring.

COVID-19 in pregnancy: What we know from the first year of the pandemic

The COVID-19 pandemic has infected nearly 178 million people and claimed the lives of over 3.8 million in less than 15 months. This has prompted a flurry of research studies into the mechanisms and effects of SARS-CoV-2 viral infection in humans. However, studies examining the effects of COVID-19 in pregnant women, their placentae and their babies remain limited. Furthermore, reports of safety and efficacy of vaccines for SARS-CoV-2 in pregnancy are limited. This review concisely summarises the case studies and research on COVID-19 in pregnancy, to date. It also reviews the mechanism of infection with SARS-CoV-2, and its reliance and effects upon the renin-angiotensin-aldosterone system. Overall, the data suggest that infection during pregnancy can be dangerous at any time, but this risk to both the mother and fetus, as well as placental damage, increases during the third trimester. The possibility of vertical transmission, which is explored in this review, remains contentious. However, maternal infection with SARS-CoV-2 can increase risk of miscarriage, preterm birth and stillbirth, which is likely due to damage to the placenta.

Prediction of Non-canonical Routes for SARS-CoV-2 Infection in Human Placenta Cells

The SARS-CoV-2 is the causative agent of the COVID-19 pandemic. The data available about COVID-19 during pregnancy have demonstrated placental infection; however, the mechanisms associated with intrauterine transmission of SARS-CoV-2 is still debated. Intriguingly, while canonical SARS-CoV-2 cell entry mediators are expressed at low levels in placental cells, the receptors for viruses that cause congenital infections such as the cytomegalovirus and Zika virus are highly expressed in these cells. Here we analyzed the transcriptional profile (microarray and single-cell RNA-Seq) of proteins potentially interacting with coronaviruses to identify non- canonical mediators of SARS-CoV-2 infection and replication in the placenta. Despite low levels of the canonical cell entry mediators ACE2 and TMPRSS2, we show that cells of the syncytiotrophoblast, villous cytotrophoblast, and extravillous trophoblast co-express high levels of the potential non-canonical cell-entry mediators DPP4 and CTSL. We also found changes in the expression of DAAM1 and PAICS genes during pregnancy, which are translated into proteins also predicted to interact with coronaviruses proteins. These results provide new insight into the interaction between SARS-CoV-2 and host proteins that may act as non-canonical routes for SARS-CoV-2 infection and replication in the placenta cells.

Coronavirus Disease 2019 Infection among Children: Pathogenesis, Treatment, and Outcome

Coronavirus disease 2019 (COVID-19) is a contagious disease that may lead to respiratory distress syndrome and even death. Neonates and children are most vulnerable population to COVID-19 infection; however, the infection is usually milder and has a better prognosis in pediatric patients compared with adults. It remains unclear why pediatric population is less symptomatic than adults. Children frequently experience respiratory infections and their immune system is in developing stage. However, large proportion of the asymptomatic pediatric population may contribute to transmission. This review explored several aspects of COVID-19 infection such as its epidemiology, its molecular pathogenesis with respect to angiotensin-converting enzyme 2 receptor and inflammatory mediators, intrauterine vertical transmission, imaging findings, and complications like cytokine release syndrome (multisystem inflammatory syndrome in children). We also looked at prognostic factors and treatment modalities like corticosteroids, RNA replicate inhibitors, protease inhibitors, Bruton tyrosine kinase inhibitor, that is, acalabrutinib and convalescent plasma therapy. Since there is no strong evidence for the intrauterine transmission, early isolation should be performed to protect a neonate from a COVID-19 infected mother. Development of vaccine and an effective antiviral drug are the need of the hour.

COVID-19 and fertility-at the crossroads of autoimmunity and thrombosis

The SARS-CoV-2 virus is known to mediate attack via ACE-2 Receptor, thus having adverse effects on cardiovascular, respiratory, digestive and reproductive systems, the latter being an area of emerging concern, due to the associated impact on fertility, with potential for an outsized effect on population distribution and socioeconomic road map in subsequent years. This narrative review aims to put forth the current evidence of effect of SARS-CoV-2 on human fertility from a multipronged immunologic, haematologic, and gynaecologic perspective; highlighting the areas of contradiction and potential future measures. A literature search was conducted through the MEDLINE and SCOPUS databases to identify articles on the subject in English. Relevant information was extracted from around 300 articles for this review. The existing data give non-conclusive evidence about the impact of SARS-CoV-2 infection on fertility; however, a greater impact on male fertility as compared to females merits further exploration. However, reproduction and fertility is a key concern and considering the pandemic is prolonged, natural conception or ART require extra precautions.

Clinical and in Vitro Evidence against Placenta Infection at Term by Severe Acute Respiratory Syndrome Coronavirus 2

Despite occasional reports of vertical transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) during pregnancy, the question of placental infection and its consequences for the newborn remain unanswered. Herein, we analyzed the placentas of 31 coronavirus disease 2019–positive mothers by reverse transcriptase PCR, immunohistochemistry, and in situ hybridization. Only one case of placental infection was detected, which was associated with intrauterine demise of the fetus. Differentiated primary trophoblasts were then isolated from nonpathologic human placentas at term, differentiated, and exposed to SARS-CoV-2 virions. Unlike for positive control cells Vero E6, the virus inside cytotrophoblasts and syncytiotrophoblasts or in the supernatant 4 days after infection was undetectable. As a mechanism of defense, we hypothesized that trophoblasts at term do not express angiotensin-converting enzyme 2 and transmembrane protease serine 2 (TMPRSS2), the two main host membrane receptors for SARS-CoV-2 entry. The quantification of these proteins in the placenta during pregnancy confirmed the absence of TMPRSS2 at the surface of the syncytium. Surprisingly, a transiently induced experimental expression of TMPRSS2 did not allow the entry or replication of the virus in differentiated trophoblasts. Altogether, these results underline that trophoblasts are not likely to be infected by SARS-CoV-2 at term, but raise concern about preterm infection.

Central role of the placenta during viral infection: Immuno-competences and miRNA defensive responses

Pregnancy is a unique immunological condition in which an "immune-diplomatic" dialogue between trophoblasts and maternal immune cells is established to protect the fetus from rejection, to create a privileged environment in the uterus and to simultaneously be alert to any infectious challenge. The maternal-placental-fetal interface (MPFI) performs an essential role in this immunological defense. In this review, we will address the MPFI as an active immuno-mechanical barrier that protects against viral infections. We will describe the main viral infections affecting the placenta and trophoblasts and present their structure, mechanisms of immunocompetence and defensive responses to viral infections in pregnancy. In particular, we will analyze infection routes in the placenta and trophoblasts and the maternal-fetal outcomes in both. Finally, we will focus on the cellular targets of the antiviral microRNAs from the C19MC cluster, and their effects at both the intra- and extracellular level.

Conflicting and ambiguous names of overlapping ORFs in the SARS-CoV-2 genome: A homology-based resolution

At least six small alternative-frame open reading frames (ORFs) overlapping well-characterized SARS-CoV-2 genes have been hypothesized to encode accessory proteins. Researchers have used different names for the same ORF or the same name for different ORFs, resulting in erroneous homological and functional inferences. We propose standard names for these ORFs and their shorter isoforms, developed in consultation with the Coronaviridae Study Group of the International Committee on Taxonomy of Viruses. We recommend calling the 39 codon Spike-overlapping ORF ORF2b; the 41, 57, and 22 codon ORF3a-overlapping ORFs ORF3c, ORF3d, and ORF3b; the 33 codon ORF3d isoform ORF3d-2; and the 97 and 73 codon Nucleocapsid-overlapping ORFs ORF9b and ORF9c. Finally, we document conflicting usage of the name ORF3b in 32 studies, and consequent erroneous inferences, stressing the importance of reserving identical names for homologs. We recommend that authors referring to these ORFs provide lengths and coordinates to minimize ambiguity caused by prior usage of alternative names.

Transitory Fetal Skin Edema in a Pregnant Patient with a Mild SARS-CoV-2 Infection

Background

Vertical transmission of the Coronavirus Disease 2019 (COVID-19) is still controversial. Additionally, the consequences of the infection during pregnancy in the offspring also are unknown.

Case

A transitory fetal skin edema and polyhydramnios have been demonstrated by ultrasound in a pregnant patient with COVID-19 after a negative RT-PCR for SARS-CoV-2. The fetal findings presented a spontaneous resolution in utero, and abnormal findings were not found in the newborn.

Conclusion

Women who have undergone SARS-CoV-2 infection during pregnancy should receive a subsequent appropriate follow-up in order to clarify the fetal consequences of the novel coronavirus, if any.

Maternal Coronavirus Infections and Neonates Born to Mothers with SARS-CoV-2: A Systematic Review

The coronavirus disease 2019 (COVID-19) pandemic is continuously affecting the lives of all people. Understanding the impact of COVID-19 on pregnancy in terms of morbidity, mortality, and perinatal maternal and fetal outcomes is essential to propose strategies for prevention and infection control. Here, we conducted a systematic review to investigate pregnant women infected with COVID-19 in terms of signs and symptoms, type of delivery, comorbidities, maternal and neonatal outcomes, and the possibility of vertical transmission. A search on Embase and PubMed databases was performed on 31 October 2020. Observational studies and case reports on pregnant women infected with COVID-19 were included without language restrictions. The 70 selected studies included a total of 1457 pregnant women diagnosed with COVID-19 in the first, second, and third trimesters of pregnancy. The most common signs and symptoms were fever, cough, and nausea. The most frequent comorbidities were obesity, hypertensive disorders, and gestational diabetes. Among maternal and fetal outcomes, premature birth (n = 64), maternal death (n = 15), intrauterine fetal death or neonatal death (n = 16), cases of intrauterine fetal distress (n = 28), miscarriage (n = 7), decreased fetal movements (n = 19), and severe neonatal asphyxia (n = 5) were the most frequent. Thirty-nine newborns tested positive for SARS-CoV-2. Additionally, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA was detected in the placenta (n = 13) and breast milk (n = 6). This review indicates that COVID-19 during pregnancy can result in maternal, fetal, and neonatal complications. In addition, SARS-CoV-2 viral exposure of neonates during pregnancy and delivery cannot be ruled out. Thus, we highlight the need for long-term follow-up of newborns from mothers diagnosed with COVID-19 to establish the full implications of SARS-CoV-2 infection in these children.

Histopathologic evaluation of placentas after diagnosis of maternal severe acute respiratory syndrome coronavirus 2 infection

Background

The impact of maternal severe acute respiratory syndrome coronavirus 2 infection on placental histopathology is not well known.

Objective

To determine if any significant placental histopathologic changes occur after the diagnosis of severe acute respiratory syndrome coronavirus 2 infection during pregnancy and whether these changes are correlated with the presence or absence of symptoms associated with the infection.

Study Design

A retrospective cohort study of women diagnosed as having severe acute respiratory syndrome coronavirus 2 infection who delivered at a single center from April 9, 2020 to April 27, 2020, and had placental specimens reviewed by the Department of Pathology. Women with singleton gestations and laboratory-confirmed severe acute respiratory syndrome coronavirus 2 infection were eligible for inclusion. Historical controls selected from a cohort of women who delivered 6 months before the study period were matched in a 1:1 fashion by weeks of gestation at delivery. Histopathologic characteristics were evaluated in each placenta, and the incidence of these findings was compared between placentas of those who received a diagnosis of maternal severe acute respiratory syndrome coronavirus 2 infection and historical controls, and between placentas from patients with or without typical symptoms related to the infection. Statistical analyses included the use of Wilcoxon rank-sum test and Fisher's exact test for the comparison of categorical and continuous variables. Statistical significance was defined as a P value of <.05.

Results

A total of 50 placentas after the diagnosis of maternal severe acute respiratory syndrome coronavirus 2 infection and 50 historical controls were analyzed. Among the placentas from patients diagnosed with severe acute respiratory syndrome coronavirus 2 infection, 3 (6%) were preterm (33 3/7, 34 6/7, and 36 6/7 weeks of gestation), 16 (32%) were from patients with typical symptoms related to the infection, and 34 (68%) were from patients without typical symptoms related to the infection. All patients had received a diagnosis of severe acute respiratory syndrome coronavirus 2 infection in the third trimester. Decidual vasculopathy was not visualized in any of the placentas from patients diagnosed as having severe acute respiratory syndrome coronavirus 2 infection. There was no statistically significant difference in placental histopathologic characteristics between the groups. Severe acute respiratory syndrome coronavirus 2 test results for all neonates at 24 hours of life were negative.

Conclusion

Based on the results of this study, there are no significant placental histopathologic changes that occur after the diagnosis of severe acute respiratory syndrome coronavirus 2 infection in women during the third trimester of pregnancy compared with a gestational age-matched historical control group. Similar incidences of histopathologic findings were also discovered when comparing placentas from patients with severe acute respiratory syndrome coronavirus 2 infection with or without the presence of symptoms typically related to the infection.

A structured review of placental morphology and histopathological lesions associated with SARS-CoV-2 infection

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a novel coronavirus, was first identified after a cluster of cases in Wuhan, China in December 2019. Whether vertical transmission or placental pathology might occur following maternal infection during pregnancy remains unknown. This review aimed to summarise all studies that examined the placenta or neonates following infection with SARS-CoV-2, or closely related highly pathogenic coronavirus (SARS-CoV-1, or the Middle East respiratory syndrome coronavirus (MERS-CoV)). Structured literature searches found 50 studies that met the inclusion criteria. Twenty studies reported placental histopathology findings in third trimester placentas following maternal SARS-CoV-2 infection. Using the Amsterdam Consensus criteria to categorise the histopathology results, evidence of both fetal vascular malperfusion (35.3% of cases; 95% Confidence Interval (CI) 27.7-43.0%) and maternal vascular malperfusion (46% of cases; 95% CI 38.0-54.0%) were reported, along with evidence of inflammation in the placentas (villitis 8.7% cases, intervillositis 5.3% of cases, chorioamnionitis 6% of cases). The placental pathologies observed in SARS-CoV-2 were consistent with findings following maternal SARS-CoV-1 infection. Of those tested, a minority of neonates (2%) and placental samples tested positive for SARS-CoV-2 infection (21%). Limited conclusions can be drawn about the effect of maternal SARS-CoV-2 infection on placental pathology as most lack control groups and the majority of reports followed third trimester infection. Collaboration to maximise the number of samples examined will increase the reliability and generalisability of findings. A better understanding of the association between maternal SARS-CoV-2 infection and placental pathology will inform maternity care during the coronavirus pandemic.

A Message from the Human Placenta: Structural and Immunomodulatory Defense against SARS-CoV-2

The outbreak of the coronavirus disease 2019 (COVID-19) pandemic has caused a global public health crisis. Viral infections may predispose pregnant women to a higher rate of pregnancy complications, including preterm births, miscarriage, and stillbirth. Despite reports of neonatal COVID-19, definitive proof of vertical transmission is still lacking. In this review, we summarize studies regarding the potential evidence for transplacental transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), characterize the expression of its receptors and proteases, describe the placental pathology and analyze virus-host interactions at the maternal-fetal interface. We focus on the syncytium, the barrier between mother and fetus, and describe in detail its physical and structural defense against viral infections. We further discuss the potential molecular mechanisms, whereby the placenta serves as a defense front against pathogens by regulating the interferon type III signaling, microRNA-triggered autophagy and the nuclear factor-κB pathway. Based on these data, we conclude that vertical transmission may occur but rare, ascribed to the potent physical barrier, the fine-regulated placental immune defense and modulation strategies. Particularly, immunomodulatory mechanisms employed by the placenta may mitigate violent immune response, maybe soften cytokine storm tightly associated with severely ill COVID-19 patients, possibly minimizing cell and tissue damages, and potentially reducing SARS-CoV-2 transmission.