iOS app Fortnite, a mass-multiplayer shooter invite-only game, grossed more than $1.5 million worldwide in the first four days after its launch, Sensor Tower reported.

“Fortnite has become a bona fide sensation on mobile, topping the iPhone download charts in more than 40 countries despite still being invite-only,” the company said.

“This places the game well ahead of its primary rivals in the burgeoning mobile survival game genre during the same post-launch period,” it added.

Fortnite, published by Epic Games, outgrossed rivals Knives Out and Rules of Survival by quite a margin: those games earned $57,000 and $39,000 respectively in their first four days of availability. The new game also managed to gross about one third as much as hit titles Pokemon Go ($4.9 million) and Clash Royale ($4.6 million) made in their first four days.

An earlier report by Sensor Tower said the game, a version of the company’s popular console and PC action game Fortnite Battle Royale, became the top app on iPhone in 13 countries within 12 hours of release.

The app also helped push up the positions of its rivals, possibly because a search for the title surfaces these games first. Rules of Survival moved up from number 14 to five in its ranking among all free iPhone apps on the US App Store; Knives Out rose from number 182 to 138.

Fortnite’s website explains players can register interest and will be sent an email invitation with a link to download the game as soon as “the servers can handle more players”.

An Android version is expected in coming months.