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Coronavirus is so contagious it warrants two-week quarantine, CDC and Johns Hopkins experts say

How contagious are people? When are they spreading the virus? These are the questions scientists and public health officials are asking as the novel coronavirus outbreak spreads further every day.

Definitive answers remain unknown, but the virus appears to be easily passed and the two-week quarantine period that has become standard is reasonable, according to researchers from the U.S. Centers for Disease Control and Prevention and the Johns Hopkins University looking at available data.

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“China describes this as being highly contagious, and there essentially is no immunity against this virus in the population because it’s a new virus,” Dr. Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, said during a Monday news conference.

“It’s fair to say that as the trajectory of the outbreak continues, many people in the United States either this year or next year will be exposed to this,” she said. “And there is a good chance many will become sick.”

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People are potentially contagious longer than those infected with the flu or common cold, according to a Johns Hopkins study published Monday afternoon.

The median time, or halfway point, between when people are exposed and develop symptoms is just over five days, and most people are back to normal after two weeks, said Justin Lessler, an associate professor in the Bloomberg School of Public Health’s department of epidemiology and the study’s senior author.

Most likely won’t become seriously ill from the respiratory disease caused by the virus, named COVID-19.

Symptoms include fever, cough and shortness of breath. Messonnier said 80 percent appear to have mild illness. Another 15 percent to 20 percent become more seriously sick. At highest risk are people over age 80 and with underlying health conditions such as diabetes or high blood pressure, though younger people with health issues also are at greater risk. The virus doesn’t appear to be a significant threat to children.

People likely are passing the virus before they show symptoms, Lessler said. They are certainly more contagious while they are actively sick because they are coughing out droplets that contain virus onto their hands and surfaces where others can pick them up.

That is why officials recommend people wash their hands and avoid touching their face. And for COVID-19, that’s why the CDC recommends they quarantine themselves for two weeks.

“Based on our analysis of publicly available data, the current recommendation of 14 days for active monitoring or quarantine is reasonable, although with that period some cases would be missed over the long term,” Lessler said.

There will be people who develop symptoms and remain contagious after being released from the 14-day quarantine. Lessler said the number would be about 101 people for every 10,000 cases.

He said extending the quarantine period likely wouldn’t be practical because two weeks already is a significant amount of time for people to leave behind their normal lives and their day jobs.

Lessler said health care workers could end up quarantined in larger numbers because of their exposure, and taking so many off the front lines could be detrimental to fighting the virus.

Quarantines have consequences. An absence of firefighters or police could endanger public health in other ways. Teachers keep schools operating, which in turn allows parents to go to work. And so on, he said.

“Particularly at this point, it’s not necessarily the goal of quarantines or active monitoring to make sure every case develops in that period,” he said. “You have to balance costs and benefits.”

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About 97.5 percent of people who develop symptoms of COVID-19 do so within 11.5 days after exposure, the Hopkins study found.

The incubation period is similar to that of other serious coronaviruses, including SARS and MERS. It’s longer than the coronaviruses that cause the common cold, which is about three days, and the flu, which is about two days.

The study, published Monday in the journal Annals of Internal Medicine, looked at 181 cases from China and other countries where exposure dates and development of symptoms were known. Lessler said studies will continue as more data becomes available.

There are more than 113,000 cases of COVID-19 globally, more than 600 in the United States in 34 states. There are six cases reported in Maryland.

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