It could be a matter of fearing a public scare. A lot of countries still have difficulty getting vaccines, and North Korea would reasonably stand to have an even greater difficulty considering their isolation and lack of sea trade routes. They'd have to get their vaccines from China, and considering their low trade capacity I doubt they could pay for them.

So unless they ask for help from China, they aren't getting vaccines anytime soon. Whether they've done so and failed, or are simply too proud to do so, I don't know, but they don't have any other way to get those vaccines.