Clinical trials for Inovio’s COVID-19 vaccine were put on hold Monday by the U.S. Food and Drug Administration, a delay that rattled Wall Street investors and sent its stock tumbling.

The Plymouth Meeting-based biotechnology company’s stock closed trading Monday at $12.14 a share, down 28%.

The “partial clinical hold” on Inovio’s planned Phase 2/3 trial is not due to adverse events in the vaccine’s ongoing Phase 1 trial, but rather because the FDA has questions about the next phase of the trial and the company’s Cellectra 2000 delivery device, which would be used to administer the vaccine, the company said in a statement.

Inovio has been considered among the front-runners in developing a COVID-19 vaccine, alongside much larger pharmaceutical competitors, including Pfizer and AstraZeneca.

In early September, AstraZeneca’s vaccine development was put on hold after a late-stage trial participant became ill.

Inovio is developing the only DNA vaccine candidate for COVID-19, which uses a special hand-held device, the Celectra 2000, to administer the vaccine directly into the skin using electrical pulses. Traditional vaccines are made using a weakened or inactive virus. A human DNA vaccine has never made it to market anywhere in the world, and the technology is rapidly evolving.

Inovio said it planned to respond to the FDA’s questions in October, and the agency will have 30 days to notify the company whether the trial will be allowed to proceed.