A phenomenon of disease outbreaks where the number of people infected by a virus may appear to decrease, and then later increase again in the same population or a different one.
There is no official definition of when a “wave” begins or ends but in general, it requires a peak in infections followed by a substantial reduction. A new rise and peak would signal the start of another wave. There may also only be a series of smaller peaks rather than one large wave.
The WHO has warned against using the term ‘wave’ to describe a resurgence of COVID-19 cases saying the idea comes from a flawed comparison with the seasonality of the flu virus, which suggests it’s out of human control. Rather than experiencing a second (or even third) wave, countries are according to the WHO, experiencing ebbs and flows of transmission, which can be due to adherence or non-adherence to health prevention strategies such as physical distancing and wearing masks.
SARS-CoV-2 has not affected all parts of any particular country in the same way, or at the same time. Some cities in the United States, for example, went into lockdown and quarantine at different times, and many other countries are easing restrictions in phases and at their own pace. Some health experts have warned the lack of a unified in-country reopening plan might help spread the coronavirus and could actually fuel a second wave as people travel from the hardest-hit areas to places with far fewer infections.
Epidemiology Infectious Disease Transmission