By the time Labor Day weekend wraps, summer box-office revenue in North America will end up being down nearly 16 percent over last year, the steepest decline in modern times and eclipsing the 14.6 percent dip in 2014. It will also be the first time since 2006 that summer didn't clear $4 billion.

That's according to comScore, which is predicting that revenue will come in at roughly $3.78 billion (a 15.7 percent decline). Attendance also plummeted, and is almost assured of hitting a 25-year low in terms of the number of tickets sold, according to Box Office Mojo.

The sequelitis virus that first invaded Hollywood last year only grew worse this summer. A number of franchise installments underperformed domestically, including Transformers: The Last Knight ($132 million), The Mummy ($80.1 million) and Pirates of the Caribbean: Dead Men Tell No Tales ($172 million). While Pirates 5 certainly fared the best, it paled in comparison to the previous installments.

The good news for a worried Hollywood? The international box office — which is up more than 3 percent year-to-date — helped save a number of summer event films that underperformed in the U.S. Pirates 5 has grossed $618 million overseas for a global total of $790 million, while Transformers 5 stands at $604 million globally after earning $474 million offshore. And The Mummy scared up $328 million abroad for a worldwide cume of $407.8 million.

Summer titles that all-out bombed domestically include King Arthur: Legend of the Sword, Valerian and the City of a Thousand Planets and The Dark Tower, which kicked off an especially brutal August.

Another sore point was R-rated comedies. Baywatch, The House and Rough Night all bombed despite impressive star wattage. The only film to break the R-rated curse was Malcolm D. Lee's Girls Trip, from Universal and producer Will Packer, which has earned $108.1 million to date.

"The lesson for Hollywood this summer is that every movie counts when it comes to box office and there are no 'throwaway' titles," says Paul Dergarabedian of comScore. "At least three tentpoles missed the mark in North America as well as a handful of R-rated comedies that left audiences frowning, and the missing revenue from those failures could arguably have left a $500 million-plus void in the marketplace — enough to turn a potentially strong $4 billion-plus summer season heavyweight into a 98-pound weakling."

The tentpole winners of summer were Guardians of the Galaxy Vol. 2, which earned $389.4 million domestically and $862.8 million globally; Wonder Woman ($406.2 million/$806.2 million); Despicable Me 3 ($254.5 million/$971.7 million); and Spider-Man: Homecoming ($318.8 million/$737 million). Smaller gems include Baby Driver and Annabelle: Creation.

Year-to-date, domestic revenue is down 5.7 percent, while international revenue is up nearly 4 percent.