Fixes for several Intel-related flaws were included in Security Update 2017-002 for Sierra, and Security Update 2017-005 for El Capitan. Apple yesterday confirmed that "mitigations" against Meltdown were implemented in macOS 10.13.2, iOS 11.2, and tvOS 11.2. watchOS is immune to the flaw.
Spectre remains a concern in Apple's Mac and iOS Web browser, Safari. That should be patched within the next few days, possibly even later on Friday.
The company is also developing broader fixes for iOS, macOS, tvOS, and watchOS, but it's unclear when those will be released to the public.
Both Meltdown and Spectre exploit a feature in Intel and ARM processors called "speculative execution," which calculates multiple instruction branches simultaneously, predicting which one is most likely to be used. On unpatched devices, the vulnerabilities can be used to access restricted memory spaces such as a kernel.
While some reports have claimed that fixes can slow down processors, Apple said its own testing has shown little if any impact.