Is it too late for lockdowns to be effective?

The countries that seem to have entirely stopped the spread of the virus, like Australia and South Korea, had strict lockdowns at the beginning of the pandemic when it was easier to contain. Australia has now had days with 0 new cases, a far cry from the U.S. where there are thousands of new cases every day.

Lockdowns here seem to be a non-stop back-and-forth between closing and re-opening, the moment cases rise everything closes down again, then re-opens when things calm, then there’s a new rise and everything closes down again. How long will this back-and-forth go on? Can we actually reach a state of 0 new cases like Australia has without a vaccine?