Is there a Meltdown vulnerability fix already available for macOS? While Apple has yet to comment on the flaw, Alex Ionescu, Windows
  security expert, noted a fix was present in a new 10.13.3 update to
  macOS.


source: How to Protect Yourself From Meltdown and Spectre, the Latest Processor Security Flaws
Meltdown

macOS patched December 6, 2017 in macOS 10.13.2
iOS patched December 2, 2017 in iOS 11.2

Apple patched CVE-2017-5754 (Meltdown) in macOS High Sierra 10.13.2, Security Update 2017-002 Sierra, and Security Update 2017-005 El Capitan. (Support Article HT208331)

Spectre

As of January 8th, Apple has released updates for Safari on macOS and iOS to minimize the effectiveness of Spectre. (Support Article HT208394) Note that Spectre cannot be "patched", only more difficult to execute.