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The subject of this article appears in Call of Duty: Modern Warfare (2019)
"Killstreak kills count towards your next killstreaks. Only apply to killstreaks earned this life."
Create-A-Class description

Kill Chain is a Tier 1 perk in Call of Duty: Modern Warfare. It makes kills done by the user's killstreaks count towards the required kills to earn their next killstreak(s), except the Tactical Nuke. It is unlocked at level 34.

Kill Chain only affects killstreaks earned and used in the player's current life, and does not affect killstreaks earned from another player's Care Packages and Emergency Airdrops.

Up to the 1.14.0 Perk rebalancing, this was a Tier 2 perk. It traded slots with Overkill, possibly to allow for better combinations of perks.