Grassley, 87, R-Iowa, said he never experienced any symptoms.
He said on Nov. 17 he tested positive for the virus, the sixth U.S. senator to do so. Sen. Rand Paul, R-Ky., had the virus in March; Sen. Bill Cassidy, R-La., in August; and Sens. Mike Lee, R-Utah, Thom Tillis, R-N.C., and Ron Johnson, R-Wis., in October.
Twenty-one members of the U.S. House of Representatives also have tested positive. People who are 85 or older account for 32.3% of all U.S. COVID-19 deaths, the most of all age groups, according to the U.S. Centers for Disease Control and Prevention.
"While I continued working from home during my quarantine, I'm glad to be back in the office working for Iowans," Grassley said in a statement. "I'm thankful for their prayers and well wishes. This disease affects people differently.
"We all have to do our part to help protect our friends, families and fellow Americans," Grassley said. "I will continue to wear a mask and practice social distancing. Promising vaccine news means there is light at the end of the tunnel. That makes staying vigilant in the coming months all the more important."
Moderna announced Monday it will seek an emergency use authorization from U.S. regulators for its COVID-19 vaccine candidate after further clinical trials confirmed it is 94% effective. Pfizer and BioNTech sought the same authorization for their vaccine on Nov. 20.
Grassley called for the passage of "long overdue relief legislation" to help communities during the ongoing pandemic.
The United States reported nearly 140,000 new COVID-19 cases on Sunday. It marked the 27th consecutive day of at least 100,000 new infections.