What happened

Shares of OPKO Health (OPK 0.73%) jumped 13.9% in September, according to data from S&P Global Market Intelligence, after it began its coronavirus vaccine testing program in New York City schools and again following phase 2 testing starting on a COVID-19 treatment.

So what

OPKO's stock plunged over 35% in August after Abbott Labs (ABT 0.04%) announced it would begin offering a cheap, quick COVID-19 test at the end of the month. OPKO's BioReference Labs offers a slightly slower test that takes 24 to 72 hours to see results.

Scientist testing samples.

Image source: Getty Images.

The lab, though, began its testing for all principals, teachers, and staff of the New York City school system on Sept. 2, helping to lift shares. Then, in the middle of the month, it announced its COVID-19 therapy Rayaldee would enter clinical-trials testing.

One of the markers seen with COVID-19 patients is a vitamin D deficiency, and Rayaldee is a vitamin D-based drug that has received Food & Drug Administration approval as a treatment for certain forms of kidney disease.

Now what

OPKO Health's stock ended the month strong, and that has continued into October. Shares are up over 21% over the past week, which could be caused by short sellers covering their positions. Nearly a third of the healthcare company's stock is sold short with its short interest ratio -- the theoretical time it would take for short sellers to cover their position -- sitting above 11 days, and anything over seven is considered a lot.