Dynavax Technologies' (DVAX) shares plummeted more than 26% Monday after the company said clinical trial conditions made it impossible to tell whether its ragweed allergy treatment worked.

Interim data on its experimental immune-stimulator Tolamba showed that the allergy trial patients didn't experience any meaningful ragweed-specific allergies during the allergy season. Of three patients groups in the study, including those who received a placebo, there was a minimal change in an efficacy measure known as the total nasal symptom score.

The company said that results from the trial were not clinically significant and that efficacy measures were substantially lower in the latest trial than what has been observed in prior trials.

"We are working closely with our consultants and investigators to review the data in detail and determine the future of the program," said CEO Dino Dina.

The stock was trading $2.27 lower, at $6.27.

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