Why do cases of COVID-19 continue to spread when a majority of people are in lockdown?
First, SARS-COV-2 is most unusual in that is has an incubation period of up to 12 days, compared to 5 for most other coronaviruses and influenza viruses. So even if everyone in an area enters a perfect quarantine, it would take 2 weeks to show up clearly in the infection curve.
Second, COVID-19 is disproportionately fatal the older a patient is, so many young people (all over the world) have ignored early isolation warnings, believing themselves invulnerable—and unaware that even people in their 20s and 30s can end up needing mechanical ventilation to survive with lifetime damage to their lungs.
Third, COVID-19 is also unusual among high-mortality diseases in that it seems to seriously affect a comparatively small fraction of the population, leaving lots of asymptomatic carriers to slip through traditional epidemiological surveillance measures that rely on sick people reporting to doctors with identifiable symptoms.
All this means that all over the world, officials have instituted isolation and lock-down orders later than they should have, and they have been less widely enforced and respected than they needed to be.
But once in place, lock-downs have indeed curbed infection.