Following the emergence and rapid spread of COVID-19, several countries have succeeded in bringing local outbreaks under control. The most dramatic of these is China, where large scale restrictions on people’s movement appear to have halted domestic transmission.
South Korea, Singapore and Taiwan also had early success containing local outbreaks, using a combination of extensive contact tracing, testing, border measures and differing degrees of social distancing.
WATCH: Wuhan reopening after their eight-week lockdown. Post continues below.
However, COVID-19 is now widespread across the globe, and these countries remain at risk of a second wave of infections, sparked either by overseas arrivals or undetected pockets of infection.
As China has begun to lift travel restrictions, the world is watching to see whether they can avoid a second wave of outbreaks.
What causes a second wave of a disease outbreak?
Infectious diseases spread via contact between infectious and susceptible people. In the absence of any control measures, an outbreak will grow as long as the average number of people infected by each infectious person is greater than one.
If people who recover generate a protective immune response, the outbreak will leave a growing trail of immune people. Once enough people are immune, there are fewer susceptible people to become infected and the outbreak will die away.
When an outbreak is brought under control by social distancing and other measures, it’s possible only a small proportion of the population will have been infected and gained immunity.
If a population has not achieved herd immunity, enough susceptible people may remain to fuel a second wave if controls are relaxed and infection is reintroduced.