Review 1: "SARS-CoV-2 Can Infect Human Embryos"

RR:C19 Evidence Scale rating by reviewer:

• Potentially informative. The main claims made are not strongly justified by the methods and data, but may yield some insight. The results and conclusions of the study may resemble those from the hypothetical ideal study, but there is substantial room for doubt. Decision-makers should consider this evidence only with a thorough understanding of its weaknesses, alongside other evidence and theory. Decision-makers should not consider this actionable, unless the weaknesses are clearly understood and there is other theory and evidence to further support it.

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Review:

This manuscript by Montano et al. seeks to define the impact of SARS-CoV-2 infection on early embryogenesis. The goal is accomplished using both S-pseudotyped reporter viruses (HIV and  VSV) and the full SARS-CoV-2 virus expressing a reporter gene to infect blastocyst-stage preimplantation human embryos in vitro. The primary finding is the infection of trophectoderm cells, making the external layer of the blastocyst immediately below the zona pellucida and representing the precursor of the placenta, which express the viral receptor ACE-2 and the other proteins necessary for SARS-CoV-2 entry and replication. The infection was associated with distinct cytopathic effects and appeared to be more severe in fully hatched blastocysts without the protection offered by the zona pellucida. The paper is well written, and the experimental procedures and interpretation of data are sound. However, the practical relevance is – in my opinion – somewhat limited because of the profound differences between the experimental model and the real-life circumstances of a natural pregnancy. Indeed, the infection of preimplantation embryos can occur only with viruses contained in the semen of infected males or vaginal secretions of infected females. While this has been shown to be possible in principle,  the viral load is likely to be relatively low and further diluted by the secretions in the uterine microenvironment, and certainly much smaller than the inoculum applied directly to the blastocysts during the experiments outlined in the paper. The logical conclusion is that - while theoretically possible – preimplantation vertical infection by COVID-19 is a sporadic event in natural pregnancies, as confirmed by most epidemiologic data published so far. In addition, the full extent of the infection and its structural and functional consequences on the embryo and fetus would require longitudinal analysis through the successive phases of gestation that cannot be performed in vitro. Finally, the in vitro system does not include the many effectors of innate,  trained, and adaptive immunity in the uterine microenvironment that could curb the infection before it reaches the embryo. Nevertheless, this work provides credible proof of concept and highlights the importance of strict infection control during extracorporeal embryo manipulations in the setting of IVF procedures. The most important information we need to fully understand the impact of maternal COVID-19 infection on fetal development and plan effective strategies of active or passive prophylaxis remains the long-term, post-natal consequences of mild, subclinical  COVID-19 acquired in pregnancy and whether chronic pathology in one or more organ systems can become apparent months or years after the birth of newborns vertically infected or even just exposed to virus carried by the mother. This ultimate goal will require complex studies in vivo that are difficult to carry out but undoubtedly worth the investment of adequate resources to prevent the long-COVID cases originating even before birth.