Every year, "Big Brother" shoots during the summer, but due to COVID-19, there was lots of speculation 'BB' would be another casualty of the virus ... but we've learned the reality show is still ON.

Production sources tell TMZ ... "Big Brother" was in the middle of casting for its next season when the coronavirus outbreak hit. We're told casting was completely shut down in May, leaving execs to change the format to an All-Star edition instead.

Our sources say an All-Star cast made the most sense, because those contestants are already familiar with the show and have already been vetted by network execs ... plus it would have been difficult to cast fresh faces for a new season in the middle of a pandemic, a process we're told would have dragged on for weeks.

The coronavirus isn't preventing another season, but it is delaying things a bit ... we're told contestants were supposed to head to Los Angeles at the beginning of July to begin testing and quarantines, but the timetable is being pushed back to the end of the month.

Our sources say, contestants, who are flying in from across the country, will be tested for the virus and then quarantined for 2 weeks before being cleared to enter the "Big Brother" house.