Here’s how health officials trace disease transmission — and why it could be no match for the coronavirus outbreak

When Maryland health officials confirmed the state’s first positive tests for the novel coronavirus last week, tied to a cruise on Egypt’s Nile River, they quickly found half a dozen other residents who recently stayed aboard the same ship. Within days, it turned up at least two more cases of COVID-19.

The practice known as contact tracing is key in early stages of an epidemic, a trusted public health strategy to contain the spread of any pathogen.


In places like Baltimore and Howard County, where no cases have been confirmed yet, health officials say they are ready to investigate exposure to any infection that does arise.

But public health experts say it may be too late — and undiagnosed cases of COVID-19 too numerous — for coronavirus transmission to be tracked.


“This is an absolutely key containment strategy,” said Dr. Gabor Kelen, a professor of emergency medicine at Johns Hopkins University. “We’re sort of on the cusp, and maybe have already moved away from containment to mitigation.”

Maryland has so far reported only a single case of what is known as “community transmission,” when the disease is passed among people with no known exposure through travel or a person known to be infected. It did not disclose any details about the six new cases confirmed Friday aside from their counties.

But with urgent measures announced Thursday, including closure of public schools for weeks, officials acknowledged the coronavirus is spreading faster than it can be tracked.

Ohio officials, for example, said Thursday they estimated there could be 100,000 cases of COVID-19, the infection caused by the coronavirus, within that state — even though there are only a handful of confirmed cases there.

With communicable diseases that are required to be reported to public health officials, any positive test triggers an investigation into possible exposure.

Health officials ask the person when their symptoms started to determine when they may have become infectious to others, said Dr. Adena Greenbaum, assistant commissioner for clinical services for the Baltimore City Health Department. And then they ask the patient to trace their movement and contacts with as much detail as possible — sometimes using bank or credit card records and phone and text message histories.

Officials ask the patients for names and contact information for anyone they know they’ve had contact with, and health departments often seek assistance from other authorities to help track the contacts down, Greenbaum said.

When officials reach those people, they typically don’t share the identity of the infected person, and always seek to balance public risk with patient privacy, she said. But in extreme situations, Kelen said, there can be greater latitude on privacy issues.

In hospitals, similar reviews occur when infectious patients arrive, said Dr. Faheem Younus, chief of infectious diseases at University of Maryland Upper Chesapeake Health. To estimate exposure risk, anyone in contact with a patient must be interviewed about exactly how close they got and what protective equipment they were wearing.

Dr. Jacqueline Douge, medical director in the Howard County Health Department’s health services bureau, said it’s hard to say the extent to which contact tracing could be conducted if and when any COVID-19 cases arise in the county.

Scientists and health officials are still learning and sharing lessons about how contagious COVID-19 patients are, and when.

“I think that’s evolving,” Douge said. “There are going to be a lot of factors determining the extent of contact tracing.”


With sexually transmitted infections, for example, contacts can be easier to track. But with diseases such as the measles, risk to the community can be significantly harder to track. For example, amid a measles outbreak last year, health officials could not determine everyone who visited a Baltimore grocery store where an infected person shopped, so they had to issue a broad public warning, Greenbaum said.

With the novel coronavirus, contact tracing could be effective at containment, if it’s started early, Younus said. As with so many other aspects of medicine, “it’s a race against time,” he said.

Maryland health officials did not respond this week to questions about the resources the state is putting toward contact tracing.

Typically, health departments have core staff focused on contact tracing for communicable diseases, but can pull in other staff to assist in the efforts during an emergency. Doctors said there is a limited window during which those resources can be effective, though.

“The farther this goes, there just isn’t going to be the manpower for contact tracing," Kelen said. “Whatever they have, it’s not enough.”

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