A few principles that help me understand the approach to containing the current outbreaks of COVID-19. 1. Outbreaks can grow explosively in some settings due to 'super-spreader' events, and have many cases by the time one case is detected.

2. A large outbreak means it's likely there are latent or asymptomatic cases in the community. It is vital at that point to limit congregation and contact between households to limit onward transmission

3. Test-trace-isolate is a tool used by public health to manage and control outbreaks, not a magic wand to prevent outbreaks in the first place. It’s an important tool, but just one tool, in the complex process of case and outbreak management

4. The ways to interrupt viral transmission work in combination: limit contacts, avoid crowded indoor settings, hand and respiratory hygiene, physical distance, face coverings, self-isolation at the first sign of symptoms, test-trace-isolate

5. Where a community is at high risk these measures may have to be reinforced with specific regional restrictions on movement and congregation. This is a difficult decision, and must be made quickly. Speed saves lives

6. ‘Population screening’ is an attractive idea, but there are huge technical difficulties, because testing for screening is fundamentally different from testing for diagnosis. We are working on this, but it’s not a quick or simple solution /END