1. Moderna only took 2 days to develop this COVID vaccine. [1] That was thanks to major advancements in vaccine development over many, many years. It wasn't just Moderna, other vaccine candidates were developed extra quick too thanks to advancements in science.
2. This vaccine was developed taking advantage of research done on very similar human coronaviruses over decades, starting from SARS in 2002.
3. Most diseases just aren't very prevalent in the population. For instance, if you created an ebola vaccine, there were a total of 14,000 cases globally between 2014 and 2016. Determining the efficacy would take years. On the other hand there have been 82 million cases of COVID since March. 19.2M in the US alone -- that's a prevalence of 5% (!!). It doesn't take long to prove the vaccine works when 1 in 20 people in the US have or have had COVID.
4. Most of the time human and animal trials are done back-to-back to reduce risk. With the government footing the bill and high confidence you can do them both at the same time.
All in all the result is a vaccine much faster but with all the usual rigor.
[1] https://www.businessinsider.com/moderna-designed-coronavirus...