Novavax Inc. shares plunged after the company said its vaccine for a potentially lethal respiratory virus failed to meet its main goals in a final-stage study in elderly patients, hurting prospects for the company’s most anticipated medicine.

The shares dropped 84 percent to $1.35 in late trading at 4:58 p.m. New York time.

Slightly more patients on the company’s RSV-F vaccine had symptoms associated with respiratory syncytial virus than those on placebo, the Gaithersburg, Maryland-based company said Thursday in a statement.

There are currently no approved vaccines to fight respiratory syncytial virus, which kills about 14,000 American adults over 65 every year, according to the U.S. Centers for Disease Control and Prevention. The study looked at 11,856 people over 60 years old who were randomly selected to take either the vaccine or a placebo.

Novavax shares surged last year after the company said a mid-stage trial of 1,600 elderly patients had showed that the vaccine was able to fend off the virus, compared with placebo. Chief Executive Officer Stan Erck said at the time that he thought the vaccine would be the world’s best-selling vaccine.

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