What are hotspots and why are they happening?

Hotspot isn’t a well-defined term, but it is often used to describe an area with many cases, rapid growth in the number of cases, or both. The term “cluster” can have a similar meaning.

Hotspots happen because of differences in the people who live somewhere, their behaviors, the local environment, public health policies, or medical practices there. Personal factors include those that may make people more susceptible to infection, like being older or being immunocompromised. Personal behaviors include the number of people you physically interact with every day, how often you wash your hands, and whether people around you cover their mouths when coughing. 

Environmental factors include whether housing keeps people close together (like high-rise apartments or single-family homes) and whether transportation options allow for physical distancing (like using the bus or subway versus a private car). Public health and medical practices that may matter include closing businesses, issuing stay-at-home orders, instituting travel restrictions, or the availability of testing. 

To illustrate how these factors influence the spread of infectious disease, consider a nursing home, where older people who are often immunosuppressed live in close proximity and interact frequently with many people every day. These factors make it easier for coronavirus to spread quickly in a nursing home and for it to become a hotspot

In some instances, hotspots are the result of random chance or because of biases in case detection. Random chance is often ‘the cause’ of what seems like clustering. This is known as the clustering illusion. False hotspots can also appear when there are differences in case detection between regions. For example, if only one of the hospitals in a major city has a test for COVID-19, that will be the only hospital that reports COVID-19 cases. While it might appear that this hospital is a hotspot, there are probably uncounted cases at other hospitals too.

Last updates: March 30, 2020 at 4:53pm ET
Science review: ERS, JAB, GSN