It’s now more important than ever to keep your Macs and iOS devices fully updated. Apple has addressed the Meltdown vulnerability in High Sierra 10.13.2. A supplemental update to 10.13.2 addressed the Spectre vulnerability in Safari and Webkit. Apple has also released Safari 11.0.2 for Sierra and El Capitan. Note that Apple has not provided Meltdown mitigations in older Mac operating systems. Should this change, I will update the post. I recommend enabling the “Install OS X updates” or “Install macOS updates” option in your App Store System Preferences. This option will automatically install updates.
Update 2/9/18: Apple has addressed Meltdown in Sierra and El Capitan via Security Update 2018-001, released on January 23. High Sierra 10.13.3 also includes security updates.