system
Given the following post, respond as if you're commenting on Reddit:
human
How can a COVID-19 vaccine be ready in less time than it takes for the trials to be completed?

I'm confused about this. I've seen some articles say that the trials for the covid vaccines are supposed to last for two years, but I also have been reading that a vaccine may be ready (if not widely available) within a few months. How can a vaccine be ready that soon if the trials are supposed to last longer?
gpt
A lot of people on here are just making judgements about whether it's "too soon" for the vaccine and ignoring the logical core of the question.

The FDA has two main pathways for authorizing a drug for marketing before the Phase III is traditionally complete:

If the disease is well-understood and there are 'surrogate endpoints' that closely track a clinical endpoint, the drug might qualify for Accelerated Approval. Surrogate endpoints would usually be be a laboratory test or other physical finding, while a clinical endpoint is something like survival of the patient. For example, if you are running a trial for sickle cell disease, you might use the composition of hemoglobin in the blood as a surrogate endpoint to get Accelerated Approval while the clinical endpoint of life expectancy is pending. COVID has dire unmet needs, which is required for Accelerated Approval, but it's not well-understood. So this probably isn't the way.

The other option is Emergency Use Authorization. This one is more flexible and means that the FDA is going to make a judgement on the risks of releasing a potentially unsafe or ineffective treatment against the benefits of addressing a national medical emergency. This has already been used for a COVID drug (Remdesivir) and might be used for vaccines as well.

There are other situations where a Phase III trial might continue while the drug application is pending, or the drug is approved, for example if the trial had as 'open-label extension' at the end of the trial. But this should be thought of as a follow-up to the main trial, not a critical part of the trial itself.