Reply

We thank Pomar and colleagues¹ for their interest in our article and the important point they raised in their letter. We acknowledge that RNA degrades more easily than DNA and that sampling, purification processes, storage, and transport conditions can influence RNA stability, which may impact the ability to detect severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA in placental and certain fetal samples using molecular assays. However, this seems to be much less of a concern when assessing the presence of SARS-CoV-2 RNA in neonatal nasopharyngeal (NP) specimens. Although our systematic review summarized the evidence of SARS-CoV-2 detection in various fetal and neonatal clinical sites, the vertical transmission rate estimate of 3.2% of SARS-CoV-2 infections occurring in the third trimester of pregnancy is based on our metaanalysis, which was focused specifically on neonatal NP swab testing, the gold standard to detect SARS-CoV-2 RNA, performed within 48 hours of birth.²

Established methods of viral NP specimen collection and transport seem to preserve discernible amounts of SARS-CoV-2 for an extended period. This exact question was recently addressed by Rogers et al.³ In a prospective study, the authors assessed the effects of various media, temperatures, and storage times on the stability of SARS-CoV-2 RNA specimens obtained using NP swabs. Samples were defined as “positive” if their polymerase chain reaction (PCR) amplification cycle number (C_t) was <40 and “stable” if their mean C_t value did not increase by more than 3 amplification cycles of the initial C_t value. Importantly, all samples stored at room temperature over 7 days exhibited increasing C_t values over time but had C_t value variation <3, thus not impacting the qualitative interpretation of positive results. Such stability was maintained for all 5 media types tested. Similar stability levels were seen in refrigerated and frozen samples even after 14 days of storage in 5 different media.³ RNA stability was assessed in this study for a much longer duration than is customary for routine clinical testing, further reducing the likelihood of clinical impact. These stability assessments complement previous studies evaluating the strong survival and persistence of the closely related SARS-CoV-1 in various different human specimens and environments, as it was shown to survive in serum and feces at infectious levels for at least 96 hours and in urine for at least 72 hours.⁴

The goal of our metaanalysis was to summarize existing data and arrive at preliminary estimates for the likelihood of vertical transmission of SARS-CoV-2 rather than exact viral transmission rates. Although we agree with the letter authors that viral RNA stability within placental and fetal specimens may lead to underestimation of viral detection by quantitative reverse transcription–polymerase chain reaction (qRT-PCR), the case for vertical transmission is all the more strengthened with the relative consistency of SARS-CoV-2 detection rates in all of the clinical specimen sources reported in our systematic review. Our analysis indicated that the rate of vertical transmission of 3.2% based on NP swabs is consistent with the neonatal anti–coronavirus disease 2019 (COVID-19) immunoglobulin M (IgM) serology positivity rate of 3.7%. If placental samples were more susceptible to nuclease-induced degradation, then we would have expected a lower positivity rate. In contrast, this rate was higher at 7.7%. Further meticulous research and large cohort studies are needed to establish the dynamics of SARS-CoV-2 infection in pregnancy and more accurately characterize vertical transmission rates. These should include consistent testing of multiple biologic samples immediately after delivery (cord blood, placental samples, amniotic fluid, urine, NP swab correlated with maternal samples) utilizing multiple methods to detect evidence of SARS-CoV-2 infection (RT-PCR, IgM serology, immunohistopathology, etc.). These efforts should be coupled with close monitoring of pregnant women with COVID-19 for fetal adverse outcomes and long-term neonatal sequelae.