A new survey from the Centers for Disease Control found that 54% of people who tested positive for Covid-19 couldn't pinpoint how they contracted the virus. The remaining 46% of respondents could recall having close contact with a person — commonly a family member or coworker — who was also diagnosed with Covid-19.
These findings are "very concerning," and suggest that people are likely contracting Covid-19 from people in their community who are asymptomatic, Dr. Joshua Barocas, infectious disease physician and assistant professor at Boston University School of Medicine, said in an Infectious Disease Society of America briefing Wednesday.
"We've seen that asymptomatic a disease is incredibly common, not just in what we would classify as low-risk populations, but also in high-risk populations," such as people who live in shared settings or those with underlying health conditions, Barocas said.
And if there is a high level of asymptomatic spread in a community it also makes contact-tracing "incredibly difficult," Barocas said, which is an important tool in fighting the pandemic.
So what does that mean for you?
Estimates suggest that 25 to 45% of people are asymptomatic Covid-19 carriers. "Our best estimate right now is that for every case that's reported, there actually are 10 other infections," Dr. Robert Redfield, director of the CDC, said Thursday.
Given that, experts say that you should take prevention measures as seriously as you would if you and everyone you interacted with were infected. Doctors are "very strongly urging" people to follow behaviors, including wearing a mask whenever in public, avoiding large gatherings, limiting socialization with people outside of your home and maintaining social distance from other people, Barocas said.
Keep track of anyone who you did have sustained contact with, such as someone who visited your home or a person who was with you in a car. (Quick interactions, like saying "hi" on the sidewalk, are not relevant.) Also keep reminders, like receipts, of anywhere you've shopped, eaten out or spent time.
That's to help with contact tracing, which is the process of identifying people who have been exposed to someone who is infectious, Dr. Emily Gurley, an infectious disease epidemiologist and associate scientist at the Johns Hopkins Bloomberg School of Public Health previously told CNBC Make It. "You're trying to let them know so that they can change their behavior and not unknowingly or inadvertently infect anybody else," she says.
A trained contact tracer will ask an infected person questions about where they went, and who they interacted with, and relying simply on memory leaves more room for error. The contact tracer will then contact those people and inform them that they may have been exposed to the virus.
But contact tracing in the U.S. is "not going well," White House health advisor Dr. Anthony Fauci told CNBC's Meg Tirrell in an interview June 26. There are about 27,000 or 28,000 contact tracers working in the U.S. currently, but an estimated 100,000 are necessary, Redfield testified last week.
Even when people can identify a contact, that person may not answer the call from a contact tracer, Dr. Ricardo Franco, assistant professor of medicine at the University of Alabama at Birmingham, said in the briefing. And in some cases, "your contact is going to be your anonymous door handle or gas pump handle," he said.
Contact tracing apps that have been successful in other countries "could mitigate some of the challenges" that the U.S. faces, he added.
In the absence of effective contact-tracing measures, the best thing you can do is limit the amount of exposure you have to others. Part of a contact tracer's job is to help you remember who you've spent time with and where you've been, Gurley said. Of course, the fewer people you're exposed to and places you visit, the easier it is to do that.
The survey authors say that this finding highlights the need for increased screening, case investigation, contact tracing, and isolation of infected persons during periods of community transmission.
In discussing contact-tracing flaws, Fauci said it's time to rethink "the idea of many more tests getting into the community and even pooling tests." Pooling tests is a strategy that involves testing samples from multiple people in a single batch.
"The difficult thing is we're always behind the virus," because it can take up to 14 days for an infected person to display symptoms, Dr. Ricardo Franco, assistant professor of medicine at the University of Alabama at Birmingham, said in the briefing Wednesday. However, more widespread testing and active surveillance (such as temperature checks and group testing) of the virus would give more insight into how many people are infected, including those who are asymptomatic.
If you have symptoms of Covid-19 (a fever, cough or shortness of breath), or if you don't have symptoms but you've been in close contact with someone who was infected with the virus, the Centers for Disease Control suggests you should get tested for the Covid-19 virus.
Antibody tests, which look for levels of Covid-19 antibodies in your blood, can also be "very useful" because they can help illustrate how many people in a specific community were infected, Dr. Saad Omer, director of Yale Institute for Global Health and professor of medicine in infectious diseases at Yale School of Medicine, previously told CNBC Make It.
However, given the dynamic nature of the Covid-19 virus, and the unreliability of antibody test results, it's important to remember that a test only provides a snapshot of your status at one point in time. There's no harm in getting tested, "as long as you interpret [the results] very sufficiently, and you don't relax the other restrictions," Omer said.
The CDC survey took place between April 15 and May 24, and respondents included 350 randomly selected Covid-19 patients whose median age was 52.