New evidences that discard the possible vertical transmission of SARS-CoV-2 during pregnancy

To the Editor:

On 31st December 2019, the Wuhan Municipal Health Commission (Hubei province, China) reported on a group of 27 cases of pneumonia of unknown aetiology, with a common exposure to a wholesale market. On 7th January 2020, a new coronavirus, SARS-CoV-2, was identified as the causal agent of the outbreak. The disease caused by this new germ has been called COVID-19 by international consensus.

Given its recent occurrence, our current knowledge about the possible implications of COVID-19 disease during pregnancy is very limited.¹ From the data available, it does not appear that pregnant women are more susceptible to infection, nor that in the event of infection, respiratory complications are more severe than in the general population.¹ On the other hand, in the few cases reported so far of COVID-19 in the peripartum, no evidence of intrauterine transmission of SARS-CoV-2 has been found.² The objective of this series of cases is to provide objective information on this matter.

Using real-time polymerase chain reaction techniques for SARS-CoV-2 nucleic acids, the possible presence of this germ in the vaginal fluid and amniotic fluid of 3 pregnant Caucasian patients affected by acute mild cases of COVID-19 was studied. All cases corresponded to pregnant women in their second trimester of pregnancy who underwent an amniocentesis for the diagnosis of chromosomal diseases and who suffered at the time of the invasive technique a confirmed active SARS-CoV-2 infection of less than 10 days of progression. The passage of SARS-CoV-2 from the infected mother to the amniotic fluid was in no case observed. Its presence in vaginal secretions could not be proved either.

The clinical course was favourable in all cases, with none of the pregnant women requiring hospitalization or drug therapy other than the administration of antipyretics and analgesics. At present, 3 weeks after amniocentesis, pregnancies are within normal conditions, with persistent PCR positive results for SARS-CoV-2 in the nasopharyngeal swab in one of the cases.

Although the presence of type 2 angiotensin converting enzyme in placental cells has been described,³ at the moment, no increased risk of pregnancy loss, preeclampsia, or premature rupture of membranes has been found in pregnant women affected by COVID-19.⁴ Cases of premature birth^1,4 associated with the disease have been described, but most were secondary to the need to end the pregnancy early in the mother's interest. A case of delayed intrauterine growth has also been reported, but this occurred in a pregnant woman suffering from a very severe systemic condition, which makes it difficult to discern whether the pregnancy complication was secondary to infection or to maternal deterioration itself.⁵

Before SARS-CoV-2, another 6 species of coronavirus capable of causing infection in humans had already been described. Of these, the best known in their interaction with pregnancy are SARS and MERS, which show an important structural analogy with SARS-CoV-2.¹ So far, observational studies with coronaviruses have not identified a single undisputed case of vertical transmission.^1–5

The best evidence for intrauterine transmission of SARS-CoV-2 would be confirmation of its presence and replication in foetal lung tissue, but this is technically unfeasible. For this reason, viral isolates made from placenta, amniotic fluid and cord blood are considered indirect but reliable indicators of congenital transmission, provided that these samples are collected during pregnancy or under conditions that limit the risk of contamination.

The main contribution of our series is to evaluate the possible vertical transmission of SARS-CoV-2 during the second trimester of pregnancy, which is still unknown.

Although the sample studied is limited and comes exclusively from mild COVID-19 conditions, at the moment there is no evidence of vertical transmission of SARS-CoV-2 in the second trimester of pregnancy. In any case, it will be time and rigorous observation of the cases that clarify the real influence that SARS-CoV-2 exerts on pregnant women and their offspring, as well as the factors that modulate the disease.