Inovio Pharmaceuticals, a Pennsylvania firm whose chief executive boasted to President Trump in March that it was the world’s leader in coronavirus vaccines, said on Monday that it will delay the next stage of testing for its vaccine until addressing questions from the Food and Drug Administration.

The company said the pause is not related to any serious side effects from the first phase of testing of its vaccine, which uses a DNA technology that stimulates a person’s immune system with some of the coronavirus’s genes. No DNA-based vaccine has ever made it to market for any human disease. The company is not able to start Phase ⅔ testing but its Phase 1 trial will continue, it said.

Inovio’s pitch has long been that this genetic approach would allow for faster design and manufacturing than its competitors. But it’s now far behind the pack. More than two dozen coronavirus vaccines have made it to later testing phases than Inovio’s has.

The F.D.A. had questions about the vaccine’s “Cellectra 2000 delivery device,” which uses an electrical pulse to open skin pores in order to more consistently deliver the vaccine dose. Regulatory questions about a similar device held up clinical trials for an Inovio cancer vaccine in 2016.

The company said it plans to respond to the F.D.A.’s questions in October. After that, the agency will have 30 days to decide whether to allow the trials to proceed.

Most of the coronavirus vaccine trials have not run into safety issues. The exception is AstraZeneca’s, which was recently halted when two volunteers became seriously ill after getting its experimental vaccine. That trial is still on pause in the United States.



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