A: In short, yes. At this point, it’s all community transmission—except in rare cases when contact tracers can pinpoint one specific point of exposure and identify the exact chain of transmission from person-to-person.
Community transmission is a technical epidemiology term that means new cases can’t be linked back to a particular point of origin through the applied science of outbreak investigation. When community transmission is happening, this is an indicator that the disease is circulating widely—that an outbreak is not contained. You might think of it analogous to a wildfire that is currently uncontained. Basically, the outbreak has progressed through primary and secondary and tertiary cases and beyond, until specific chains of infection can no longer be traced. Rather, the disease is circulating through a local area freely.
We’ve also noticed a less formal usage of the term. Sometimes when we say community transmission of COVID, we are trying to describe whether there’s a lot of disease in an area (or not). This would be more accurately referred to as ‘burden of disease.’ Burden of disease is measured by statistics such as the daily incidence, 7-day rolling average incidence, incidence per 100,000 population (aka incidence rate), positivity rate, deaths, and perhaps the prevalence for a given time period. (And if you want to know more about those terms, stay tuned! We have a post queued up about them.)
The epi term “community transmission” is related to several other infectious disease concepts, including endemic disease and eradication.
A disease is endemic when community transmission is ongoing, so that it’s just kind of always around. It may go through seasons, but it’s never truly gone and outbreaks tend to be large enough that outbreak investigations do not identify to their specific source. In the United States, we have some endemic diseases: seasonal influenza is an example.
A disease is considered “eradicated” when we’ve managed to do such a great job of prevention that there are no new cases stemming from community transmission over some (relatively long) period of time. In the US, we *eradicated* measles in the year 2000 through vaccination. We still have measles outbreaks, but they are ultimately traceable back to a case that was imported after travel abroad—typically to areas where measles is still endemic. This is like a spark that starts a new fire. In 2018-2019, we had a really big outbreak of measles. A case was imported from outside the U.S., and that sick person infected others, who infected others, who infected others… until we had community transmission of measles occurring. The outbreak went on for just a couple of weeks short of 1 year. If community transmission of measles had continued past that one year mark, the US would have lost its measles eradication status.
COVID-19 hasn’t been around anywhere in the world for long enough to be considered endemic yet. But if we rewind a few months, we can see how we’ve shifted from imported cases to community transmission. Remember when you couldn’t get a test unless you had traveled to mainland China? That stemmed from the assumption that we were not experiencing community transmission in most places (in retrospect, we should have abandoned this assumption sooner than we did).
Early in 2020, the United States’ very first cases of COVID were arriving via air travel from areas with community transmission like mainland China, Italy, South Korea, and Iran. Those individual patients’ specific point of exposure may not have been known. But anyone who got sick following an exposure to those index cases would be traceable back to them. This is what’s called an imported case. By late February and early March, cases were popping up all around the US among people who had certainly not traveled to mainland China, Italy, Iran, or South Korea. This was a sign that the disease had transitioned to community transmission.
So in short, yes. At this point, it’s all community transmission—except in rare cases when contact tracers can pinpoint one specific point of exposure and identify the exact chain of transmission from person-to-person.
