What the new phase of the coronavirus outbreak in the U.S. means for you

Health experts warn there are probably many undetected COVID-19 cases already here

central government-sanctioned horse race in Japan

Japan, which is experiencing sustained community spread of COVID-19, has banned spectators from all central government-sanctioned horse races, such as at this one held February 27 in Tokyo, in an effort to contain the disease. The United States now faces the threat of community spread, too.

Yuta Omori/Kyodo News via AP

As U.S. public health officials are working to figure out how two California women contracted a novel coronavirus that’s spreading widely around the world, experts say the cases mark a troubling new phase of the outbreak in the United States.

A 50-year-old woman from Solano County tested positive for the virus on February 26. Her case appears to be the first in the United States of what’s known as community spread, meaning she had no history of travel to affected areas and was not exposed to someone known to have the COVID-19 illness. On February 28, a couple counties south, Santa Clara County officials announced a second instance of COVID-19 with no known origin. The patient, an older woman with underlying health conditions, was diagnosed after she was hospitalized with a respiratory disease.

The announcement suggests the virus may be circulating in at least two U.S. communities.

Since the start of the outbreak, there have been more than 83,000 cases of the disease in at least 57 countries. A few regions — including Italy, Iran, South Korea and Japan — have reported sustained community spread, meaning the virus is circulating among people outside China’s borders, where the outbreak first began. The World Health Organization on February 28 upgraded the risk of global spread to “very high,” but stopped short of calling it a pandemic (SN: 2/25/20).  “We don’t see evidence yet that the virus is spreading freely in communities. As long as that is the case, we still have a chance of containing this virus,” WHO director-general Tedros Adhanom Ghebreyesus said February 28 during a news briefing.

Here’s what the California cases mean, what to expect in the coming days and months, and what to do if you think you are infected. 

What does the first discovery of suspected community spread in California mean?

Public health officials aren’t sure how the first California woman, who has severe symptoms, became infected with the virus that causes COVID-19. Without a clear idea of where the infection came from, she probably wasn’t the first person to get infected in that area, says Aubree Gordon, an epidemiologist at the University of Michigan in Ann Arbor.  

“It [probably] means there’s an unknown number of other cases” in Northern California, Gordon says. “It’s probably not a super large number at this point, but there may be a large number of people who are infected but haven’t started to show symptoms yet.”

One reason some infections may be going unnoticed is because it’s the season for respiratory diseases. Influenza and the common cold have similar symptoms as COVID-19 — and are the likely culprit in the majority of respiratory disease cases in the United States, where the coronavirus risk is still relatively low — which can make the new coronavirus harder to detect.   

If health officials conducted more tests, they’d probably find more cases, says Michael Osterholm, an epidemiologist at the University of Minnesota in Minneapolis. “Absence of evidence is not evidence of absence.”

When will COVID-19 become more widespread in the United States?

That’s hard to say at this point. But experts have been expecting community spread (SN: 2/25/20). That’s because computer simulations tracking where the virus might spread from China indicated it had probably been introduced in the United States. The California cases hint that there may be additional undetected infections across the country.

People need to “prepare themselves for the possibility that there will be multiple outbreaks or even widespread transmission in the United States,” Gordon says. That could happen “in the coming months to a year. [Or] it could be days, it’s really hard to say.”   

Infectious disease dynamics researcher Katelyn Gostic of the University of Chicago agrees. “We should definitely be prepared for the possibility that the outbreak is going to grow in the United States,” she says.  That doesn’t mean people should panic, she says. “The vast majority of people are going to be just fine even if they get sick.” But people should be prepared to change their behavior and daily routines to benefit people in their communities who are at high risk.

How many undetected cases are out there?

No one knows for sure how many people have been infected with the coronavirus. That’s partly because there aren’t enough test kits to test everyone, and partly because people may be infected with the virus but have no symptoms or very mild symptoms. Those people may, nevertheless, unwittingly infect others.

For instance, a woman from China passed the virus to colleagues in Germany before she knew she was ill (SN: 1/31/20). That case was controversial, but researchers have found other evidence of people with very mild or no symptoms transmitting the virus to others, including a woman from Wuhan who gave the virus to five relatives in Anyang, China. The woman never had symptoms, but she tested positive for the virus, researchers report February 21 in JAMA. Two of her relatives developed severe disease and the other three became moderately ill.

Health officials in Nanjing, China, traced contacts of people with COVID-19 and found 24 people who were asymptomatic when they were tested for the virus, the researchers report in a preprint posted at medRxiv.org on February 25. Five of the 24 went on to develop symptoms and 12 had chest X-rays that indicated they carried the virus, but seven people never showed signs of the illness. People with symptoms were infectious for one to 21 days, with a median of 9.5 days. Asymptomatic people, who tended to be younger, had detectable virus for a median four days. But one asymptomatic man who passed the virus to his wife, son and daughter-in-law may have been infectious for up to 29 days, the researchers report. Those results have not been vetted by other scientists in peer review yet.

What’s more, people may also still give off the virus after they have recovered from the illness. Four health care workers from Wuhan still had positive test results five to 13 days after their symptoms cleared, researchers report February 27 in JAMA. Researchers don’t yet know whether viruses that are present after symptoms disappear are infectious.

“There’s really no doubt that there are many undetected cases,” says Erik Volz, a mathematical epidemiologist at Imperial College London in England. 

Undetected cases matter because they may seed outbreaks when travelers carry them to other countries, says Gostic, the University of Chicago researcher. But even the best efforts to screen airline passengers for COVID-19 infections will miss about half of cases, Gostic and colleagues report February 25 in eLife.

“Not only does screening at airports miss over 50 percent of travelers, but those failures are not due to correctable mistakes,” Gostic says. It’s not because sick travelers are trying to avoid detection or screeners aren’t good at their jobs. “It’s just a biological reality that a majority of infected travelers are fundamentally undetectable, because they don’t realize they’ve been exposed and they don’t yet show symptoms at the time when they pass through screening.”

That is true of almost every pathogen, but the coronavirus’s prevalence of mild and undetectable cases and airborne transmission are bigger challenges. People may catch the virus without ever knowing they were exposed and may develop mild cases that wouldn’t cause them to seek medical attention and get tested. Those people may unwittingly start epidemics in new places. “We just see this as inevitable,” Gostic says.

coronavirus test samples
After the first suspected case of community spread of COVID-19 in the United States, the Centers for Disease Control and Prevention has issued updated guidance to health care workers for testing patients for coronavirus. Previous CDC criteria had limited testing to people who had traveled to China or had close contact with someone who had. The new guidelines now include those who had traveled to other regions with potential local spread or patients with severe symptoms.narvikk/iStock/Getty Images Plus

How widely will the coronavirus spread?

Already, the virus has infected more than 83,000 people in 57 countries.

Because the COVID-19 virus hadn’t infected people before the outbreak in China, no one has prior immunity to it. So the coronavirus spread might be similar to pandemic influenza spread, Volz says. Although seasonal flu circulates around the world each year, pandemic influenza is caused by new viruses that haven’t previously infected humans. Examples include the 1918 “Spanish flu”, the “Asian flu” of 1957 and 1958, the 1968 “Hong Kong flu, and H1N1 influenza in 2009. Depending on the country, the 2009 H1N1 flu infected 5 percent to 60 percent of people. The 1918 pandemic infected an estimated third to half of the worldwide population, Volz says.

There is still opportunity to contain the virus. On February 26, the number of new cases reported outside China surpassed those within China for the first time, the WHO reported.  “China has at least partial control of their epidemic,” Volz says.

“I think it’s good to be aware of the hypothetical worst case, with the knowledge that the worst case is unlikely,” because of public health efforts to contain the epidemic, he says.

“There are no-brainers like school closure,” that might help limit spread of the virus, Volz says. Children aren’t suffering much severe illness, but they may still get infected and transmit the virus. Restricting travel, closing public transportation and banning mass gatherings like concerts can affect virus spread.

The rest of the world probably won’t see the explosive growth of cases that Wuhan experienced, Gostic says. “The first emergence of a virus is always a worst-case scenario,” she says, “because no one is prepared for it and people who are getting infected at first have no idea that they have a novel pathogen.”

So how can I tell if I’m infected?

People with COVID-19 often have a dry cough and sometimes shortness of breath. And the vast majority of patients with this illness have fever, according to reports characterizing patients from China. 

One tricky thing is that these symptoms also apply to the flu and it’s still flu season in the United States. Most people with those symptoms now probably don’t have COVID-19. “February was a bad month in a lot of communities” for the flu, says Preeti Malani, an infectious disease specialist at the University of Michigan School of Medicine in Ann Arbor. “If people haven’t gotten flu shots, it’s not too late.”

Other respiratory illnesses, caused by the likes of rhinoviruses, enteroviruses and other viruses, don’t necessarily have fever, she says. Colds often include a runny nose, but that hasn’t been a symptom for COVID-19.

What should I do if I think I do have COVID-19?

If you have a fever and respiratory symptoms, call your medical provider ahead of time, Malani says, so they can let you know what the next step is. “This is not something that you can just walk into an urgent care and easily get tested,” she says. 

Local health departments, with help from physicians, determine whether someone should be tested for coronavirus. Testing initially occurred at the Centers for Disease Control and Prevention in Atlanta, but the Association of Public Health Laboratories expects more labs will be able to begin testing soon.

It appears that the risk of getting severely ill is fairly low for many people. About 80 percent of COVID-19 cases are mild, according to a report on more than 44,000 confirmed cases in China (SN: 2/25/20)

But the virus is estimated to kill about 2 percent of people it infects, especially older people and those with other health conditions such as diabetes and cardiovascular disease. “Even though individual risk may be low, there’s still a need to take the situation seriously and do what you can to limit spread if it does start circulating in your community,” Gostic says.

People should stay home from work and school when they are ill, cover their cough and wash their hands often. If soap and water aren’t available, use alcohol-based hand sanitizers. Start practicing those measures now to limit the spread of other respiratory illnesses, and to be prepared if coronavirus does start spreading in the community, Gostic advises.

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