Investors watching the COVID-19 vaccine development process could be forgiven for thinking it’s not so hard. The effort has moved extraordinarily quickly so far, and with few hiccups.

That isn’t how things usually go, especially for new diseases. The world got a reminder Tuesday as AstraZeneca Plc paused the trial of its leading candidate, developed with Oxford University, to investigate a single volunteer’s illness. It’s easy to panic; a setback like this could mean big trouble for AstraZeneca’s efforts, and it raises worries about whether any vaccine will ultimately succeed, or if one does, how long the process will take. That’s the wrong reaction. This pause shows the system is working as it should. The possibility of setbacks is why the world is testing many candidates in many people.

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