Polio is slow, and it has to go through the blood to cause severe disease. RSV and COVID-19 (or really SARS-CoV-2) don't. IPV can't stop transmission, and OPV h

Polio is slow, and it has to go through the blood to cause severe disease. RSV and COVID-19 (or really SARS-CoV-2) don't. IPV can't stop transmission, and OPV has dubious long term protection from spread.