Shares of drug giant Pfizer surged nearly 11% Friday after the company announced positive test results for an experimental pill it has developed to fight Covid-19. Pfizer\n \n (PFE) said the pill reduced the risk of hospitalization and death for high-risk patients who were part of a trial taking the drug. The news comes just a few days after Pfizer reported strong earnings and sales, thanks largely to the success of the Covid-19 vaccine it developed with BioNTech\n \n (BNTX). The US Food and Drug Administration also recently approved the use of the Pfizer-BioNTech vaccine in 5- to 11-year-old children. But shares of BioNTech, which will report earnings next week, plunged more than 20% Friday. Shares of Pfizer’s Big Pharma rival Merck\n \n (MRK), which also is developing a Covid pill, were down about 10%. Biotech Novavax\n \n (NVAX), which is applying for emergency use authorization of its Covid vaccine, fell more than 11%. Moderna\n \n (MRNA), another biotech that has its own Covid-19 vaccine, plummeted about 17% Friday. That follows a nearly 18% slide for the stock Thursday after Moderna announced weaker than expected results and a disappointing outlook due to production and shipment problems that will delay the deliveries of its vaccine.