CORRECTED - (OFFICIAL)UPDATE 1-Celldex:brain cancer vaccine results on track

* 70 pct of patients progression-free at 5.5 months

* Shares fall 20 pct after hours (Removes erroneous references to previous studies in lead and third paragraph, after analyst corrects statement)

LOS ANGELES, May 20 (Reuters) - Celldex Therapeutics Inc's CLDX.O experimental brain cancer vaccine helped some patients in a mid-stage trial, and shares fell 20 percent after hours.

The interim results from 40 glioblastoma patients showed that 28, or 70 percent, were alive with no signs of their cancer worsening 5.5 months after treatment, according to research submitted to the American Society of Clinical Oncology.

“It is a little difficult to read into this,” said Joseph Pantginis, an analyst at Roth Capital Partners. “If 70 percent holds true, it is still pretty promising for this patient population in newly diagnosed glioblastoma.”

Thomas Davis, chief medical officer at Celldex, said the 70 percent rate is about 40 percent better than would be seen with patients receiving standard care.

He noted that patients in the trial had to have completed chemotherapy treatment without their tumor growing back -- a period about two or three months beyond the time patients would typically be enrolled in a clinical trial.

Patients in the trial were injected with the vaccine after initial treatment with Temodar, known generically as temozolomide.

Davis said the results were “very much the same” as those reported last year from the two smaller trials.

“You can extrapolate to a median progression-free survival of around 14 months,” he said.

Results from the trial’s total of 81 patients will be available later this year, according to Celldex.

The company has partnered with Pfizer Inc PFE.N to develop CDX-110, and the pharmaceutical company is planning a randomized international Phase 3 study, Davis said.

Shares of Celldex, up 64 percent so far this year as of Thursday’s close at $7.68, fell 20 percent after hours to $6.17. The stock finished regular trading down $1.25, or 14 percent on Thursday, as the broad market sold off sharply. (Reporting by Deena Beasley, editing by Matthew Lewis and Carol Bishopric)