(Following is a draft version, which is subject to be updated or revised. Your comments and feedbacks are welcome!)
According to one U.S. expert, about 1,800 nuclear weapons possessed by Russia and the United States are considered to be on high alert status, either Launch On Warning (LOW) or Launch Under Attack (LUA). 48 U.K. nuclear warheads and 80 French ones are also kept on alert under their continuous SSBN patrols, albeit at lower readiness levels than those of the two nuclear superpowers.
While the United States has yet to alter its alert posture, the Nuclear Employment Strategy Report implied that it would take measures to reduce alert status or maximize decision time to authorize the use of nuclear weapons in the future, stating that “[r]ecognizing the significantly diminishing possibility of a disarming surprise nuclear attack, the guidance directs DoD to examine further options to reduce the role of Launch Under Attack plays in U.S. planning, while retaining the ability to Launch Under Attack if directed.”
It is assumed that because China keeps nuclear warheads de-mated from delivery vehicles, its nuclear forces are not on a hair-trigger alert posture. The key question, however, would be whether Chinese nuclear warheads will be de-mated from the new SLBM JL-2 loaded onto the deployed Type 094 SSBN.
(Drafted by Hirofumi Tosaki, CPDNP)
 Hans M. Kristensen, “Reducing Alert Rates of Nuclear Weapons,” Presentation to NPT PrepCom Side Event, Geneva, April 24, 2013; Hans M. Kristensen and Matthew McKinzie, “Reducing Alert Rates of Nuclear Weapons,” United Nations Institute for Disarmament Research, 2012.
 US Department of Defense, Nuclear Employment Strategy Report, p. 5.