On 4 March 2020, public health officials from Placer County, California reported that an elderly resident with underlying health conditions who had died earlier that day had tested positive for SARS-CoV-2 after returning from a cruise from San Francisco to Mexico and back on Grand Princess between 11 and 21 February.
This marked the first death known at the time in California attributable to the virus.
The source of the new case's infection appeared to be the same as that of a resident of Sonoma County who tested positive on 2 March and who was also aboard Grand Princess on the same dates.

Consequently, Princess Cruises, the owner and operator of Grand Princess, working with the Centers for Disease Control and Prevention (CDC), the government of California, and public health officials in San Francisco, terminated a port call in Ensenada, Mexico planned for 5 March and ordered the ship to return to San Francisco over concerns about the potential for an outbreak on board.

After Grand Princess docked in San Francisco on 21 February, 62 passengers who had been on the previous cruise to Mexico reboarded the ship as it set sail for Hawaii, making stops at Kauai, Oahu, Maui, and Hilo (on the Big Island) between 26 and 29 February.
These passengers, who may have made been exposed to the same environment as were the Placer or Sonoma County cases during the previous cruise, were quarantined in their own on-board staterooms on 4 March by order of the CDC.
In addition, eleven passengers and ten crew members were exhibiting potential symptoms, and Grand Princess was ordered by the government of California to remain offshore while the California National Guard's 129th Rescue Wing airlifted test kits by helicopter to the ship.

On 5 March, Princess Cruises confirmed that there were 3,533 people on board the ship—2,422 passengers and 1,111 crew members—representing 54 nationalities in total.

On 6 March, U.S. Vice President Mike Pence announced that of the 46 tests run on selected passengers and crew members on Grand Princess, 19 crew members and two passengers had tested positive, 24 had tested negative, and one test was inconclusive.
Pence announced at a White House Coronavirus Task Force briefing that the ship would be brought to a non-commercial port, and that everyone on the ship would be tested and quarantined as necessary.
On 6 March, President Trump said that, despite what experts told him about limiting the spread of the virus, he wanted those on board Grand Princess to stay on the ship so that they would not be counted as American cases, which would otherwise "double because of one ship that wasn't our fault."

On 7 March, Princess Cruises confirmed that Grand Princess was still at sea roughly 50 miles (80 km) from San Francisco, and that it was scheduled to dock in Oakland on 9 March, with its passengers to be transferred to facilities on land while the crew would be quarantined and treated on board.
 The United States Coast Guard had airlifted supplies, including personal protective equipment, to the ship, and had medivacked a critically ill passenger and his travel companion for treatment unrelated to the virus.

On 9 March, the ship docked at the Port of Oakland and passengers started disembarking.
More than 3,000 people on board were to be quarantined, with passengers at land facilities and the crew on board.
By 22:00 that evening, 407 people had disembarked, by 10 March, 1,406 people had disembarked, and by 12 March, 2,042 people had disembarked.
 On 16 March, Grand Princess undocked from the Port of Oakland and anchored in San Francisco Bay with 6 foreign passengers, 340 crew members, and 75 medical workers.

By 25 March, the Department of Health and Human Services (HHS) said that, of the 1,103 passengers who elected to be tested, 103 tested positive, 699 tested negative, and the remaining results were pending.
 Nearly two-thirds of the passengers quarantined at Travis Air Force Base refused to take the test, with some passengers stating that federal officials had discouraged them from doing so, and others admitting that the
On 4 March 2020, public health officials from Placer County, California reported that an elderly resident with underlying health conditions who had died earlier that day had tested positive for SARS-CoV-2 after returning from a cruise from San Francisco to Mexico and back on Grand Princess between 11 and 21 February. This marked the first death known at the time in California attributable to the virus. The source of the new case's infection appeared to be the same as that of a resident of Sonoma County who tested positive on 2 March and who was also aboard Grand Princess on the same dates.
Consequently, Princess Cruises, the owner and operator of Grand Princess, working with the Centers for Disease Control and Prevention (CDC), the government of California, and public health officials in San Francisco, terminated a port call in Ensenada, Mexico planned for 5 March and ordered the ship to return to San Francisco over concerns about the potential for an outbreak on board.
After Grand Princess docked in San Francisco on 21 February, 62 passengers who had been on the previous cruise to Mexico reboarded the ship as it set sail for Hawaii, making stops at Kauai, Oahu, Maui, and Hilo (on the Big Island) between 26 and 29 February. These passengers, who may have made been exposed to the same environment as were the Placer or Sonoma County cases during the previous cruise, were quarantined in their own on-board staterooms on 4 March by order of the CDC. In addition, eleven passengers and ten crew members were exhibiting potential symptoms, and Grand Princess was ordered by the government of California to remain offshore while the California National Guard's 129th Rescue Wing airlifted test kits by helicopter to the ship.
On 5 March, Princess Cruises confirmed that there were 3,533 people on board the ship—2,422 passengers and 1,111 crew members—representing 54 nationalities in total.
On 6 March, U.S. Vice President Mike Pence announced that of the 46 tests run on selected passengers and crew members on Grand Princess, 19 crew members and two passengers had tested positive, 24 had tested negative, and one test was inconclusive. Pence announced at a White House Coronavirus Task Force briefing that the ship would be brought to a non-commercial port, and that everyone on the ship would be tested and quarantined as necessary. On 6 March, President Trump said that, despite what experts told him about limiting the spread of the virus, he wanted those on board Grand Princess to stay on the ship so that they would not be counted as American cases, which would otherwise "double because of one ship that wasn't our fault."
On 7 March, Princess Cruises confirmed that Grand Princess was still at sea roughly 50 miles (80 km) from San Francisco, and that it was scheduled to dock in Oakland on 9 March, with its passengers to be transferred to facilities on land while the crew would be quarantined and treated on board.  The United States Coast Guard had airlifted supplies, including personal protective equipment, to the ship, and had medivacked a critically ill passenger and his travel companion for treatment unrelated to the virus.
On 9 March, the ship docked at the Port of Oakland and passengers started disembarking. More than 3,000 people on board were to be quarantined, with passengers at land facilities and the crew on board. By 22:00 that evening, 407 people had disembarked, by 10 March, 1,406 people had disembarked, and by 12 March, 2,042 people had disembarked.  On 16 March, Grand Princess undocked from the Port of Oakland and anchored in San Francisco Bay with 6 foreign passengers, 340 crew members, and 75 medical workers.
By 25 March, the Department of Health and Human Services (HHS) said that, of the 1,103 passengers who elected to be tested, 103 tested positive, 699 tested negative, and the remaining results were pending.  Nearly two-thirds of the passengers quarantined at Travis Air Force Base refused to take the test, with some passengers stating that federal officials had discouraged them from doing so, and others admitting that the
COVID-19 pandemic on Grand Princess