The current prognosis for a COVID-19 vaccine to become available is "12-18 months" and a big part of the reason for such a long timeline is the fact that scientists want to ensure that the vaccine is safe enough. But did this ever happen in practice? An article on CNN mentions the fiasco of the 1976 vaccine:
In 1976, President Gerald Ford's administration reacted at speed to a novel swine flu outbreak, ignoring the World Health Organization's words of caution and vowing to vaccinate "every man, woman and child in the United States." After 45 million people were vaccinated, the flu turned out to be mild. Worse, researchers discovered that a disproportionately high number of the vaccinated -- roughly 450 in all -- had developed Guillain-Barré syndrome, a rare disorder in which the body's immune system attacks the nerves, leading to paralysis. At least 30 people died. Upon discovery of the risk, the program was terminated in late 1976. A crush of lawsuits against the federal government followed.
However this looks like a very rare complication, only causing problems for 1 in 100,000 patients. And even that complication could've been detected quickly with enough volunteers:
About a week after getting the swine flu shot, she recalled, “I was so weak I couldn’t push down the toaster button.” She spent a month in the hospital, paralyzed from the neck down, before gradually recovering.
So has there ever been a vaccine trial satisfying the following conditions?
- Significant side effects were detected
- Those side effects appeared in otherwise healthy candidates after more than 3 months since they were injected with the vaccine
- These side effects were something other than birth defects (those obviously take up to 9 months to show up)
If not, what causes scientists to be so cautious about testing a new vaccine quickly?