It worked the same as it worked on personal level too. At the very beginning you can just lock yourself inside and there is basically no chance to get sick. But what then?

If you could just lock everybody in the world for a month you could basically get rid of all the infectious diseases. However you can't. So lock down only helps to win time.

After you win your time you really need to know what to do with it - e.g. effective vaccine is created in this time.

Otherwise you just delay inevitable and that is it.