Broadcaster Sat 1 had faced a growing backlash for keeping the 14 residents in the dark while the rest of Germany comes to grips with unprecedented confinement measures to slow the virus's spread.
Host Jochen Schropp broke the news in the opening minutes of the evening show, sitting next to a doctor behind a glass screen to protect the contestants.
"Pat, I can see the fear in your eyes," Schropp said, as he tried to reassure the group their families were all well.
Michelle, a carer, burst into tears, saying she was worried about her 55-year-old mum who has a lung disease.
Most of the group sat in stunned silence, some mouthing "oh my God" to each other.
After watching news clips and an overview of how the virus has raced across the continent, the contestants were shown video messages from loved ones, prompting cheers and tears of joy from the room.
Many family members told the contestants to stay upbeat and used the opportunity to encourage them to stay put.
"It's probably the safest place in Germany right now," one woman told Pat.
Some jokingly asked contestants to bring home some toilet paper.
The latest season of the show started in early February, when the virus was still largely confined to China's Wuhan city.
However, four of the contestants joined only this month and were explicitly asked not to discuss the latest developments surrounding the virus.
Germany has in recent days closed schools and playgrounds, banned public gatherings and asked workers to work from home as it ramps up its virus response.
The contestants, most of whom are in their 20s and 30s, are staying in a glass house and adjacent log cabin-style home in Cologne, in the western state of North Rhine-Westphalia.
The state is Germany's hardest hit area, accounting for more than 2,000 of the nation's 7,000 coronavirus cases.
At least 12 people in the state have died from COVID-19.
On the show's Facebook page, comments poured in about the belated decision to finally enlighten the residents.
"Goosebumps, I feel so sorry for them right now," said one.
Many also criticised the show for broadcasting the reveal live during prime time.
Organisers have also stressed that anyone coming into contact with the residents has been taking strict hygiene precautions.
The German show, in which participants get voted out over three months until there is a winner who gets a cash prize, usually breaks the blackout rule only to inform contestants about the death or injury of a loved one.
In the US in 2001, Big Brother contestants were told about the 9/11 attacks.