The ductus arteriousus is a blood vessel that connects the pulmonary artery to the descending aorta. Otherwise known as the ductus Botalli, this structure pumps blood into the fetus' non-functioning lungs from the right ventricle. This blood vessel will close at birth and becomes the ligamentum arteriousum.
Viruses can multiply very quickly, and so they can go through natural selection at a much quicker rate, allowing certain vaccine resistant mutations to take over a population.