The gasping for breath and desperate hacking of whooping cough. The iron lungs and braces of polio.
Birth defects from rubella.
For many people today, those signs of terrible diseases are the stuff of history books, thanks to vaccines. The first step to licensing a new vaccine is safety testing in animals. If the laboratory animals immunized with the vaccine don't have serious reactions, FDA consults with the vaccine manufacturer or sponsor on further refining of the manufacturing process. Only after those studies have been done does testing in people begin. Phase 1 trials evaluate basic safety and identify only very serious or very common adverse events.