(Bloomberg) -- Inovio Pharmaceuticals Inc. put a partial hold on a phase 2-3 study of its Covid-19 vaccine after U.S. regulators raised questions about a delivery device used in the inoculation.

The U.S. vaccine maker is working to answer questions from the U.S. Food and Drug Administration about the Cellectra 2000 device used in the trial, Plymouth Meeting, Pennsylvania-based Inovio said Monday in a statement. The hold is not due to any adverse events from the vaccine, Inovio said.

The company plans to respond in October, after which the FDA will have 30 days to notify Inovio with a decision as to whether the trial can proceed. The shares fell 35% in early trading before U.S. markets opened.

Cellectra 2000 is an electroporation device designed to improve the delivery of vaccine particles. Inovio’s inoculation is based on messenger RNA, an approach similar to that used by rival Moderna Inc., which instructs the body’s cells to produce proteins that spark a protective response.

Inovio said in June that it’s early-stage study of 40 healthy volunteers found that 94% of trial participants demonstrated overall immune responses at week six after two doses of the vaccine.

©2020 Bloomberg L.P.

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