Assymptomatic carriers have been observed to carry the virus and spread it to those around them for periods that range between a few weeks up to a couple of months.

https://www.thelancet.com/journals/lanepe/article/PIIS2666-7...

All it takes is a single spreader moving from outside of the metro area to negate the effect of the lockdown, specially if a considerable portion of the population is quite militant in their refusal to follow even the most basic health and safety precautions.