Is "it is" skipped in this sentence? It seems like "Because it is critical thing..." sounds grammatically right.
You're right, it sounds like he did skip "it is" in this sentence. If he would have said "the critical thing we must do to stop the disease spreading between households is to stay at home" then it would have been grammatically correct. However, he left out the reason of staying at home at the end of the sentence so it is not grammatically correct and he should have used "it is" here.