As the entire world awaits a vaccine against Covid-19, the U.S. Food and Drug Administration is explicitly rejecting one of the fastest possible paths to releasing one. That’s a good thing.

On Tuesday, the agency issued guidance on the standards it will use in evaluating and approving a vaccine. Its benchmarks are nonbinding, but have teeth nonetheless. The FDA has let vaccine developers know that it won’t be enough to show evidence that their candidate can merely provoke an immune response in a blood test — the kind of data that could be available relatively quickly. Vaccine hopefuls will have to demonstrate real-world safety and effectiveness in large trials.

The agency also set an explicit threshold for success: Candidates should perform at least 50% better than a placebo in blunting the risk of infection or illness.

The guidance suggests that approval may take longer than some hope. However, demanding real data and setting a transparent standard is the best way to ensure that we get an effective vaccine.

The FDA often approves drugs based on so-called surrogate endpoints. These are measures of the effect of a treatment that probably correlate to, but don’t guarantee, favorable clinical outcomes. Such metrics can be well-validated, and using them gets medicines to needy patients faster. The process is imperfect, however, and has in the past led to the approval of drugs that, on further examination, turned out to be ineffective.

Such an outcome would be more likely — and particularly catastrophic — in the case of a Covid-19 vaccine, because scientists are only beginning to understand immunity to the novel coronavirus and how potential vaccines might work. The FDA cites this uncertainty to explain its recommendation that drug makers focus on real-world performance.

If using a surrogate endpoint led the FDA to approve an ineffective shot, it could not only put inoculated people at risk of contracting Covid-19 but also weaken public trust in the process, reducing the acceptance of future vaccines.

The FDA has made it clear that even emergency approval will be granted only after it sees data from extensive studies that show a reasonable degree of effectiveness.

The new guidance sets out a broad set of potential trial goals, ranging from preventing severe disease to stopping infection outright. This could mean that the agency initially approves a vaccine that provides imperfect protection — and that Covid-19 will plague the world for longer than hoped. But the targets give drug makers reason to run robust trials.

The path the FDA outlines is a long one. It’s going to take a while to recruit and enroll 30,000 people in a trial and give half of them two shots in the arm — as Moderna Therapeutics Inc. intends to do to test its candidate. And until a sufficient number of subjects in the placebo arm of such a trial contract Covid-19, there won’t be any firm results. Any number of variables could cause further delays: bad luck, a poor vaccine performance, or slowing case growth.

The FDA is by no means ignoring the urgency of the moment. Its guidance includes a variety of concessions on safety data and other issues that are meant to speed the process. But the world can be grateful the agency is willing to bend only so far.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Max Nisen is a Bloomberg Opinion columnist covering biotech, pharma and health care. He previously wrote about management and corporate strategy for Quartz and Business Insider.

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