People with Covid-19, the disease caused by the coronavirus, are most contagious about two days before symptoms start and five days after, according to new analysis of previous research.

A small number of people who are extremely sick or have impaired immune systems can keep shedding or “shedding” the virus for up to 20 days, as other studies have suggested. Even in mild cases, some patients can shed live viruses for about a week, according to the new analysis.

The data collected poses a dilemma: should public health officials reduce the recommended isolation time if it means more infected people will work together? Or should officials opt for longer periods of time to prevent transmission in virtually all cases, even if it puts more strain on the economy?

The Centers for Disease Control and Prevention recommend that infected individuals isolate at least 10 days after their illness began. The agency is considering shortening the recommended isolation period and, according to two federal officials aware of the discussions, could issue new guidelines as early as next week.

In September France reduced the required isolation period from 14 days to seven days, and Germany is considering reducing it to five days. (Isolation refers to people who are sick; quarantine refers to people who have been exposed to the virus and may get sick.)

Setting the isolation time to five days is likely a lot tastier and can result in more infected people complying with regulations, said Dr. Muge Cevik, an infectious disease expert at the University of St. Andrews in Scotland, who led the new analysis published in The Lancet Microbe.

A recent UK survey found that only one in five people managed to isolate for 10 days after having symptoms. “Even if we do more testing and we can’t make sure that people are self-isolating, I don’t think we can control the spread,” said Dr. Cevik.

In the United States, many people don’t get tested for the infection until a day or two after they feel sick. With the current delays, many get results two or three days later, towards the end of the period in which they are infectious.

“Even if you did the PCR test on the very first day you could get the results back, 90 percent of your dandruff would be complete,” said Dr. Michael Mina, a virologist at Harvard TH Chan School of Public Health. “This meta-analysis shows how short your transmission window is.”

Dr. Cevik and her colleagues set out to analyze the so-called kinetics of the coronavirus in the course of an infection and to compare the pathogen with the closely related SARS and MERS viruses.

The researchers looked at nearly 1,500 studies published from 2003 to June 2020 at the time of infection in thousands of people, most of whom were sick enough to be hospitalized. The team pulled data from 79 studies on the new coronavirus, 11 studies on MERS, and eight studies on SARS.

People who never develop symptoms appear to be carrying roughly the same amount of the new coronavirus as symptomatic patients, found Dr. Cevik and her colleagues out. But asymptomatic people seem to clear the virus from their bodies faster.

People with Covid-19 are usually most contagious a day or two before symptoms appear until about five days afterward, the analysis concluded. However, patients can carry genetic fragments of the virus in their nose and throat for an average of 17 days, and in some cases up to three months.

Some patients, unlike the nose and throat, may have infectious virus in their lungs for up to eight days after symptoms begin, Dr. Megan Ranney, an emergency doctor at Brown University. For these patients at least, the isolation periods should likely be longer than five days if only they could be identified.

“The problem is who has Covid pneumonia and who doesn’t, is not always fully apparent just from the physical exam,” she said. “You wouldn’t know alone.”

Older people tend to be infectious longer than younger people, but no study in the analysis has found live viruses nine days after symptoms appeared. The results suggest that positive tests after this point find only genetic fragments, rather than all of the live virus, said Dr. Cevik.

Because the period of infection seems to peak relatively quickly as the disease progresses, health professionals in community clinics may be at greater risk of infection than those in intensive care units where patients are in later stages, Dr. Cevik added.

The analysis underscores the data collected since March. Based on similar evidence, in July the CDC cut its isolation recommendation from 14 days to 10 days.

But even after 10 days, the isolation time could be too long for many people, experts said. Patients may not be financially able to isolate for that long, or they may not feel sick enough to want to do so.

“If you could make this shorter for people, it would really help people adhere to public health guidelines,” said Angela Rasmussen, a virologist with the Center for Global Health Science and Safety at Georgetown University, referring to the recommended isolation period.

However, the new analysis is limited by the fact that few of the studies included looked at live viruses, she added.

Some people who are elderly or very sick can be contagious for more than a week. However, if a shorter recommended length of time encourages more people to isolate, the benefits will more than offset the risk to the community from the small amount of virus some patients may still carry after five days, said Dr. Stefan Baral, epidemiologist at Johns Hopkins University.

The coronavirus outbreak>

Things to know about testing

Confused by Coronavirus Testing Conditions? Let us help:

    • antibody: A protein produced by the immune system that can recognize and attach to certain types of viruses, bacteria or other invaders.
    • Antibody test / serology test: A test that detects antibodies specific to the coronavirus. About a week after the coronavirus infects the body, antibodies start appearing in the blood. Because antibodies take so long to develop, an antibody test cannot reliably diagnose an ongoing infection. However, it can identify people who have been exposed to the coronavirus in the past.
    • Antigen test: This test detects parts of coronavirus proteins called antigens. Antigen tests are quick and only take five minutes. However, they are less accurate than tests that detect genetic material from the virus.
    • Coronavirus: Any virus that belongs to the Orthocoronavirinae virus family. The coronavirus that causes Covid-19 is known as SARS-CoV-2.
    • Covid19: The disease caused by the new coronavirus. The name stands for Coronavirus Disease 2019.
    • Isolation and quarantine: Isolation is separating people who know they have a contagious disease from those who are not sick. Quarantine refers to restricting the movement of people who have been exposed to a virus.
    • Nasopharyngeal smear: A long, flexible stick with a soft swab that is inserted deep into the nose to collect samples from the space where the nasal cavity meets the throat. Samples for coronavirus tests can also be obtained with swabs that do not go as deep into the nose – sometimes called nasal swabs – or with mouth or throat swabs.
    • Polymerase chain reaction (PCR): Scientists use PCR to make millions of copies of genetic material in a sample. With the help of PCR tests, researchers can detect the coronavirus even when it is scarce.
    • Viral load: The amount of virus in a person’s body. In people infected with the coronavirus, viral loads can peak before symptoms, if any.

However, some doctors said they were not convinced by the analysis that five days of isolation would prevent transmission by the majority of people.

“There’s a sweet spot there, I imagine, but I haven’t figured out where that is,” said Dr. Taison Bell, an intensive care and infectious disease doctor at the University of Virginia.

Dr. Cevik and other experts suggest that people can isolate as soon as they notice mild symptoms such as sore throats or headaches and body aches – without venturing out for a PCR test right when they are most contagious.

Dr. However, Bell said he wasn’t sure how this would work in practice because these early symptoms were similar to other viral infections, including the common cold.

Dr. Cevik said that once the isolation is complete, a PCR test could be done to confirm the diagnosis. Alternatively, it may be useful to perform a rapid antigen test during isolation, which can detect large amounts of virus to confirm active coronavirus infection.

Other experts also advocated the use of rapid tests at home. “I think that’s a nice solution,” said Dr. Ranney. “If you have symptoms and have a reliable test to do at home, stay home, test at home, and isolate for five days.”

Overall, the new analysis underscores how quickly the coronavirus blooms in the body and at what speed both patients and doctors must react to keep it in check, said Dr. Baral. MERS virus levels peak seven to 10 days after symptoms appear and SARS virus peak 10 to 14 days.

In contrast, “the new coronavirus is moving fast,” said Dr. Baral. “It’s a very difficult virus to control compared to SARS.”

Home isolation is safe for most people newly infected with coronavirus, he added – essentially the model of care doctors use for patients suspected of having influenza.

Some countries have already adopted guidelines to make it easier for people to isolate themselves. Vietnam provides income support for people who need to take time off. By May, the Japanese government asked patients who were young with mild symptoms to stay home for four days before seeking tests.

Japan’s guidelines now require patients to call their doctors and only get tests if they are likely to be infected. Anyone who tests positive will be sent to a hospital or hotel for isolation. In the United States, New York City and Vermont have provided similar accommodations to infected patients.

Even if the rest of the country doesn’t implement such guidelines, isolating patients at home for five days is more practical – while wearing a mask, keeping windows open, cleaning touch-sensitive surfaces, and staying away from other household members, 10 said Dr. Baral.

“I think the returns have been going down in the last four or five days,” he said. “Intensive isolation during those first five to seven days would avert a ton of infections – a ton.”

Makiko Inoue contributed to coverage from Tokyo.

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