Drug maker Eli Lilly joined the list on Tuesday as the third company to halt a Phase 3 trial.
The company's antibody treatment for hospitalized patients is now on hold.
Earlier this week, Johnson & Johnson halted their vaccine trial for COVID-19 after an "unexpected illness" in a volunteer.
Astra Zeneca was the first company to pause a vaccine trial in September after a volunteer in the U.K. became ill.
That trial has since resumed in the U.K., Japan, Brazil, South Africa and India.
But Dr. Luis Ostrosky, an infectious disease professor with UTHealth, tells Eyewitness News there is no reason to be alarmed.
"What we're seeing here is not necessarily bad news, we're seeing the process working," he said.
Ostrosky explained independent boards watch each trial to keep risk and safety in check.
They use these pauses to investigate data, whether it's good or bad, and the pause should reassure the public that they are keeping safety a priority.
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"This is exactly the way clinical trials work," said Ostrosky. "We don't usually hear about these things because we're never that invested in clinical trials truly."
All three of these drugs are in the final Phase 3 trial of the vaccine process.
It's unclear if any of the patients who became ill were given the placebo or the actual drug.
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