The zombies keep getting stronger.

AMC’s hit “The Walking Dead” set a series record Sunday night with 12.3 million viewers. The show was returning from a hiatus, and the Nielsen Co. said it beats the series’ previous record of 10.9 million for its third season debut last October.

More than half of Sunday’s viewers, or 7.7 million, were in the youthful demographic of people ages 18 to 49. Nielsen says a cable series has never reached that big an audience in that age group.

That’s impressive by broadcast standards, too. While “NCIS” on CBS was the most popular drama on the air with 22 million viewers the previous week, fewer than 5 million of those viewers were under 50.