 Everyone wants an effective and safe treatment or vaccine for COVID-19 as soon as possible. To reach that point, therapies and vaccines need to go through a series of clinical trials, says Mindy Aysen, clinical professor of neurology at the University of Southern California. The process can take several months or in some cases many years. Pre-clinical trials are conducted on cells or non-human subjects such as rats or primates. The purpose is to see whether the intervention has any potential and whether it's safe for testing on humans. Phase 1 trials involve around 20 to 100 healthy volunteers or infected people. Around 70% of drugs pass phase 1 trials and these can be considered safe but not necessarily effective. Phase 2 studies use hundreds of subjects and are focused on applying successful doses based on phase 1 results. About 33% of drugs that make it to phase 2 trials proceed to phase 3. At this point, scientists focus on the efficacy and side effects, testing groups of 300 to 3000 participants. If researchers determine the treatment is medically important, it moves ahead. Only 25 to 30% of drugs that undergo phase 3 trials receive FDA approval. Phase 4 occurs after FDA approval. It provides information about real-world use of medications outside controlled environments. One example is testing whether a vaccine approved for adults is also safe and useful for children.