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Coronavirus: Planes may not be the sick factories we think.

When an airline passenger is diagnosed with COVID-19, how many people should know about it? And should they worry?

Coronavirus cases have been confirmed in the Tampa Bay area. For the latest news on the outbreak, go to our coronavirus page, which we are updating regularly. You also can sign up for our DayStarter newsletter to have the day’s news sent right to your inbox each morning for free.

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A standard Boeing 717 seats about 110 passengers. But when health officials determined a woman with coronavirus had been aboard such a plane during a flight to Tampa, they focused only on those people sitting within a couple rows of her.

Travel is blamed for much of the spread of the contagious, flu-like disease, which originated in China and has triggered outbreaks in several countries, killing nearly 3,500 people as of late Friday.

Although travelers could act as unwitting couriers for coronavirus, spreading it from one country to another, directives from the Centers for Disease Control and Prevention suggest the risk of transmitting the illness while on a plane is relatively low.

Related: 'Unacceptable.' Woman on Tampa flight with coronavirus patient blasts Florida officials.

The center says only passengers seated 6 feet or closer to an infected person are at “medium risk,” which in Florida is enough to warrant a two-week home quarantine.

A study published in 2018 from researchers at Emory University and the Georgia Institute of Technology, with support from Boeing, said that although the 6-foot boundary accounts for 2 rows in front of, or behind, an infected passenger, the greatest area of risk may be even smaller — only a 1-row buffer. The researchers were considering respiratory diseases passed in “droplets,” or particles of water big enough to fall from a person’s mouth to the ground when they speak, sneeze or cough. Coronavirus is thought to be such an illness, passed primarily from person-to-person.

“The highest risk is to those seated within a few seats of the sick person,” Lisa Elon, a biostatistician at Emory, wrote in an email to the Tampa Bay Times. “Others have a much lower risk; those in window seats have the lowest risk.”

Those findings fall in line with guidance from the World Health Organization, which says planes aren’t any more risky than a movie theater or subway car. Modern planes generally use advanced air filters, according to the organization, and inside the cabin, air might change out 20 to 30 times an hour.

The relatively low risk has as much to do with how people behave on planes as how the virus is spread, the researchers said.

They made notes on 10 flights, tracking how often passengers and attendants moved around. They found that more than a third of passengers never left their seats, with those in the aisle leaving far more often than those who sat by the window. Passengers along the aisle, naturally, also came into contact with more people walking past.

Flight attendants, strolling up and down the cabin, had far more contacts than passengers. (It remains unknown whether the crew of the Delta flight that carried an infected Hillsborough County woman has been notified; Delta did not respond to requests for comment Friday.)

The Georgia researchers studied flights between about 3½ and 5½ hours long. That makes their examples imperfect comparisons for the flight the Hillsborough coronavirus patient took, Delta 2794, which took 3 hours and 4 minutes to fly from New York City to Tampa on Feb. 26.

Related: Tampa Bay’s first-responders plan, prepare for coronavirus fears

People on longer flights obviously move around more. The researchers noted that in five previous instances of viruses spreading on planes, either SARS or the flu, 40 percent of those who fell ill sat outside of the 2-row zone around the first infected patient. Three of those cases were flights more than 9½ hours long, and “some transmissions may have occurred while waiting in the airport, while boarding, or while deplaning ... [or] by other sources before or after the flight.”

Movement is key because droplets, which are thought to be the primary means of transmission, don’t go far after leaving a person’s mouth. Dr. Sally Alrabaa, an infectious disease specialist at the University of South Florida, said the water globs carrying the virus can fly about 6 feet, consistent with the Centers for Disease Control and Prevention’s guidelines.

For a virus to infect another person, it generally has to make contact with a “mucosal membrane,” said Dr. Derek Cummings, a biology professor in the University of Florida Emerging Pathogens Institute. Think of slimy surfaces on a person’s face: the mouth, nostrils or eyes.

A droplet can also land on a surface, planting a virus there for hours or days, the doctors said, or until someone wipes it with soap or alcohol. Another person might touch that surface, pick up the virus and touch their face — thus infecting themselves.

Scientists are still learning just how contagious coronavirus is, Alrabaa said. Some viruses, like measles, are so light that they can move on air, meaning even people further away from a sick person can get infected.

Health officials have not said whether the coronavirus patient who flew to Tampa showed symptoms of being sick while on the plane. Her travel companion to Italy has also been diagnosed with coronavirus, they said, though they have not disclosed whether that woman was on the same flight. The general rule: The sicker a person is, the more virus they are carrying and could spread, Alrabaa said.

So much remains unknown about coronavirus and exactly who the Hillsborough patient came into contact with that Ken Qualls, a Boca Raton aviation consultant, said health officials should notify everyone who was aboard the Feb. 26 Delta flight about her condition.

The Centers for Disease Control and Prevention and Florida Department of Health have declined to discuss who they have notified or how they have tracked down who the woman sat near or came into contact with. Nor have they publicly confirmed what flight she was on.

Alrabaa said it’s better to err on the side of more information. What if a fellow passenger was undergoing chemotherapy or had a lung condition that made them especially vulnerable?

“Knowledge is always important,” she said.

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Tampa Bay Times coronavirus guide

Q&A: The latest and all your questions answered.

PROTECT YOURSELF: Household cleaners can kill the virus on most surfaces, including your phone screen.

BE PREPARED: Guidelines for essentials to keep in your home should you have to stay inside.

FACE MASKS: They offer some protection, but studies debate their effectiveness.

WORKPLACE RISK: A list of five things employers could be doing to help curb the spread of the disease.

READER BEWARE: Look out for bad information as false claims are spreading online.

We’re working hard to bring you the latest news on coronavirus in Florida. This effort takes a lot of resources to gather and update. If you haven’t already subscribed, please consider buying a print or digital subscription.

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