Stability of severe acute respiratory syndrome coronavirus 2 RNA in placenta and fetal cells

To the Editors:

We have read with great interest the systematic review and metaanalysis by Kotlyar and colleagues¹ reporting a pooled proportion of 3.2% for vertical transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The authors indicate the proportion of positive SARS-CoV-2 RNA testing in neonatal blood, urine, placental samples, and amniotic fluid, without considering the stability of viral RNA as a major limitation in the diagnosis of maternal-fetal transmission.

RNA is very susceptible to degradation, which occurs through hydrolysis and ribonuclease activity. Clinical samples are particularly vulnerable to RNA degradation by the action of host nucleases.² In the case of the diagnosis of vertical transmission of viruses, RNA is considerably less stable than DNA in the placental and fetal samples and requires more steps for detection at the laboratory level. A critical challenge for RNA preservation and detection in these samples is to prevent degradation by the nuclease during the sampling and purification processes.³ The storage and transportation of clinical samples are also at risk of RNA hydrolysis, which represents a limitation for healthcare settings with a decentralized laboratory.⁴ For example, the H5N1 RNA was undetectable if stored 24 hours at room temperature, whereas it remained detectable more than 40 days when stored in cold temperature, in RNA-safe buffer, or in dry pellet matrix, without exposure to high temperatures.⁵ The same is true for Zika virus, which rapidly degrades if not stored in RNA lather, often becoming undetectable once frozen specimens are thawed.^{6 , 7} These limitations have been well documented for reverse transcription–polymerase chain reaction (RT-PCR) assays in cases of congenital Zika virus infections.10, 8, 9

Conversely, genomic content from DNA viruses, such as Cytomegalovirus, is easily purified from whole blood or any other tissue or fluid (placenta, fetal liver and brain, amniotic fluid, urine, cerebrospinal fluid) and is less subjected to deterioration, increasing the sensitivity of PCR assays for the diagnosis of vertical transmissions.^{11 , 12} When maternal infection occurs during the first trimester of pregnancy, DNA viruses are detectable throughout the pregnancy in fetal and placental tissues. This contrasts with RNA viruses (such as Zika virus or SARS-CoV-2), which are only transiently present and detectable.⁹ Therefore, the absence of the detection of an RNA virus does not necessarily mean that the infection of the given tissue is absent. This issue should be mentioned in any study investigating the potential evidence of vertical transmission of SARS-CoV-2.