When it comes to measuring the prevalence of the coronavirus, one term frequently comes up: community spread.
According to Dr. Ted Cohen, an infectious disease epidemiologist and professor of epidemiology at the Yale School of Public Health in New Haven, Connecticut, community spread is an "informal" medical term that is used when a particular pathogen travels through a community and is not confined to a specific area. The phenomenon isn't specific to the coronavirus, it can apply to any infectious disease.
"When a pathogen is spreading in communities, it means we're seeing sort of a new phase of the epidemic," he explained.
Dr. Anne Liu, an infectious disease physician at Stanford Health Care in Palo Alto, California, said that high levels of community spread can indicate a virus is circulating in a community.
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"The likelihood of (community spread) is much higher in communities where there are more cases and where there is less mask-wearing, where there's more mingling of humans in higher-risk situations," she said.
How can you track community spread?
There's not one number or statistic that lets people easily track community spread, according to Liu, but taking a look at a variety of other metrics can help people understand what the situation looks like in their area.
"Depending on where you live, the local Department of Public Health is hopefully posting a lot of metrics online on what they're finding in terms of number of positive cases compared to the (amount) of testing done," Liu said.
It's also useful to understand the density of cases within a population: Liu recommends looking at the number of new cases diagnosed per day per 100,000 people. Again, there's not one number to watch for, Liu stressed.
"The current rates in, say, Santa Clara County where I am, being in the 5 daily new cases per 100,000 people is going in the right direction, whereas Norton, Kansas has had about 450 daily new cases per 100,000 people ... (You) usually also take the number as an average over the last 7 days," Liu explained.
Another important method of measuring community spread is by contact tracing: While there will be a "certain proportion" of cases that have "unknown sources of transmission," having a large number of cases with no known source could indicate high levels of the coronavirus in a community, according to Liu.
Hospitalization rates can also help, though Liu warned that hospitalization metrics tend to be delayed since people don't go to the hospital until they've started experiencing severe symptoms.
"There have been real efforts to aggregate data from many sources," said Cohen. "One of the challenges is the time (between when data is reported and when it is published). ... It takes a while for someone whose infectious to become symptomatic, get tested and have that reported. For COVID-19, we know that some individuals will either never be symptomatic and won't get tested, or may be infectious for some period of time before they can access testing."
Some attempts have been made to catalogue data. Cohen has been involved in an effort to estimate infections at the county and state level by using a model that combines information about reported cases, reported deaths, disease stage duration, disease severity and mortality risks. Some states are sharing more data: the Connecticut Department of Public Health recently launched an alert system for every city and town in the state, showing the average daily case rate per 100,000 population for the last two weeks. New York has launched a "Cluster Action Initiative," with new rules and restrictions directly targeting areas with the highest transmission rate.
"We don't want to focus too much on one (statistic) over the others," said Liu. "You have to take it in context. If you see the numbers in your community going up, you want to see if it is because there's more testing ... If you see the percentage of (people testing) positive is also going up, along with increased testing, then you know that there's increasing spread of COVID in your community."
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What should you do if community spread increases in your area?
If you do see infections in your area start to rise, Liu recommends going back to "the basics" of COVID-19 prevention: Masking, hand washing and social distancing.
Be sure to observe proper hand hygiene by washing your hands with soap and water for at least 20 seconds at a time. Do your best to maintain at least 6 feet of distance, and try to stay in well-ventilated or outdoor areas where disease transmission is likely to be lower.
As case counts increase, try to avoid having too many different social contacts.
"It's always very prudent to be managing your level of risk," said Cohen. "Through mask wearing, through being aware of the circle of individuals with whom we're in contact, and through hand washing and other measures, we can protect ourselves. Until we have a vaccine or better treatments available to us, these are the critical actions that individuals can take."