Shares of Merck & Co.
MRK,
-9.86%
tumbled 9.5% in midday trading Friday, putting them on track to suffer the biggest one-day drop since January 2005, after Pfizer Inc.
PFE,
+10.86%
released data on a pill to treat COVID-19 that showed greater efficacy than Merck’s. The stock’s price decline of $8.61 was shaving about 57 points off the Dow Jones Industrial Average’s
DJIA,
+0.56%
price, while the Dow rallied 246 points, or 0.7%. The stock’s big selloff, the stock has retraced about 56% of the gain, from the Sept. 29 close of $75.11 to the Thursday’s record close of $90.54, that was triggered by Merck’s announcement that it developed an oral treatment for COVID-19 with partner Ridgeback Biotherapeutics.