Mukul Prasad, Jia Le Lin, Yue Gu, Rashi Gupta, Paul Macary & Herbert Schwarz

The spike protein of SARS-CoV-2, which is the main target of all vaccination schemes, shares homology to Syncytin-1. Syncytin-1 enables the fusion of trophoblasts to syncytiotrophoblasts, cells that are essential for placenta formation, an early step in the establishment of a pregnancy. To provide a scientific basis for a rational discussion about this question, we investigated the crossreactivity of anti-spike protein antibodies with Syncytin-1. Based on an EMBOSS water pairwise analysis, the overall amino acid identity and similarity between Syncytin-1 and the SARS-CoV-2 spike protein are only 8.8% and 15.8%, respectively. We did not detect any crossreactivity with Syncytin-1 in the plasma of vaccinated individuals, in either the pre- or postvaccination samples, despite an increase in anti-spike protein titers. To determine whether SARS-CoV-2 infection induces antibodies against Syncytin-1, we tested the plasma of 37 COVID-19 patients collected at 28 days post recovery, and no anti-Syncytin-1 antibodies were detected in the plasma of these recovalescent patients. These results lead us to conclude that there is no - or at least no major - danger of female infertility due to the induction of Syncytin-1 crossreacting antibodies.