More children than ever are vaccinated.

【翻译】现在，比以往任何时候都有更多的儿童接种疫苗。

像than before, than usual, than ever 等这些表达，都是在than 之后省略成分的结果。语言交际，长此以往，约定俗成，不宜细分。

More children than ever are vaccinated.

这句话，如果一定要补充完整，那就是：

More children are vaccinated than were ever vaccinated.

“比较对应项”是children，在than 从句必须省去，这样，理解为省去的children 由than 担任即可。