Hi MD, Vaccines for covid19 were first developed a year ago and finally they're here. What took them so long? The answer to this question points to the most significant A/B test conducted in recent history: the clinical trials for making sure vaccines are safe and effective against the covid19 virus. From Vaccine Clinical Trials 101 These clinical trials take so long because they're divided into three phases, with each phase testing a different aspect of the vaccine. This 3 stage design has taken shape over multiple decades with thousands of scientists informing how it should be run. Because human lives are at stake, clinical trials of vaccines are done very carefully. Be too hasty, and an unsafe vaccine can get released. But be too careful, and many more lives can be lost from the unchecked pandemic. This tradeoff leads to thoughtful design of clinical trials of vaccines and hence for the conversion optimization industry, there'