PROS1 and COVID-19: In addition, anti-N protein Abs can be used as a marker of vaccine breakthrough infection, as most COVID-19 vaccines listed for emergency use by the World Health Organization (WHO) are S protein-encoding vaccines such as mRNA vaccines, viral vector vaccines, and protein subunit vaccines that can induce neutralizing Abs against S protein, but not anti-N protein Abs due to the absence of the N protein/mRNA/gene in these vaccines.