As the coronavirus outbreak evolves, we answer some key questions

In this rapidly changing public health emergency, many unknowns remain

SEM image of SARS-CoV-2

A new coronavirus, called SARS-CoV-2 (round yellow objects in this scanning electron microscope image), sparked an outbreak that began in China and has since turned into a global pandemic.


As a new coronavirus that has infected more than 200,000 people around the world continues to spread, scientists and public health officials are racing to understand the virus and stop the growing public health crisis.

In this rapidly evolving situation, many unknowns remain. Here’s what we know so far about the new virus — called severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2 — and the disease that it causes. We will update these answers as more information becomes available. 

Do you have questions about the new coronavirus that you’d like answered? E-mail them to       

Some of the questions below include:

What is SARS-CoV-2?

The virus is a novel type of coronavirus, a family of viruses that typically cause colds. But three members of this viral family have caused deadly outbreaks. Severe acute respiratory syndrome coronavirus, or SARS-CoV, Middle East respiratory syndrome coronavirus, or MERS-CoV, and now SARS-CoV-2 all cause more severe disease, including pneumonia. SARS-CoV-2 got its name because it is similar to SARS-CoV. 

The disease it causes is coronavirus disease, or COVID-19. Before virologists and public health officials named the virus and its disease, it was known as 2019 novel coronavirus, or 2019-nCoV. 

Why are experts so worried about it?

There are still a lot of unknowns, including exactly how deadly the virus is. And SARS-CoV-2 is a new coronavirus and hadn’t infected people before the outbreak in China, so no one has prior immunity to it. That means everyone is susceptible to getting infected and transmitting the virus to others, so it can spread rapidly and widely. 

If there’s a sudden, big spike in U.S. cases like we’ve seen in other countries, COVID-19 patients could have to compete for hospital space with other sick people. Too big a spike could overwhelm hospitals.

Scientists and public health officials also worry about people in high-risk groups — including older adults or people with underlying health conditions — who appear more likely to develop severe disease.

But some experts also still see a chance to contain the outbreak and prevent the virus from taking root in the population, as other respiratory diseases have. Seasonal influenza, for example, causes yearly epidemics. Because flu infects millions of people, and kills about 0.1 percent of the people who get sick, that can mean tens of thousands of deaths in a single season. By tracing contacts and isolating sick patients, experts hope to prevent this from happening with COVID-19.   

So how deadly is the new virus?

Most cases have been mild. Of known cases globally, an estimated 3.4 percent die, according to the World Health Organization. But that number varies from place to place — it’s 4.2 percent in Hubei Province, the epicenter of the outbreak, for instance, and 1.6 percent outside of China — and officials say the number will probably change as the outbreak continues.

That global number is higher than a previous estimate of 2.3 percent that came from a study of more than 44,000 cases in China through February 11. As of March 18, more than 212,000 people have been reported infected globally, and more than 8,700 have died.

For comparison, the 2003 SARS outbreak killed 774 people, or nearly 10 percent of the 8,000 people it sickened (SN: 3/26/03). The virus that causes MERS, a disease that still circulates in the Middle East, has claimed about 30 percent of the people it infects, or 866 people to date (SN: 7/8/16).    

The overall deadliness of COVID-19 may not be known for some time, until researchers can determine how many people were infected, but didn’t have symptoms, or had very mild symptoms and didn’t get tested. 

Who’s most at risk? What about young children? 

That analysis of about 44,000 cases of COVID-19 from China shows that the elderly are most vulnerable. Older people, especially those with heart disease and other conditions, are more likely to die. Middle-aged and elderly adults are most likely to develop symptoms, while children and teenagers seem to rarely show symptoms or become seriously ill when they do catch the virus (SN: 2/14/20). Even though their symptoms are mild, infected children may still spread the virus.

Another study, published March 16 in Pediatrics, describes the disease in 2,143 children under 18 years old in China, about half of whom were from Hubei Province, the epicenter of the pandemic. Compared with adults, these children generally had milder cases. It’s unknown why most kids aren’t getting as sick as adults.

But children weren’t wholly protected. An estimated 5.9 percent of kids had severe or critical cases. Infants and preschoolers generally had more severe illnesses, including symptoms such as breathing trouble, than older kids, the team found. The researchers report one pediatric death, of a 14-year-old boy.

What are the symptoms?

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, so most people with those symptoms now probably don’t have COVID-19. 

Other respiratory illnesses, caused by the likes of rhinoviruses, enteroviruses and other viruses, don’t necessarily have fever, says Preeti Malani, an infectious disease specialist at the University of Michigan School of Medicine in Ann Arbor. Colds often include a runny nose, but that hasn’t been a symptom for COVID-19. Though many people infected with SARS-CoV-2 will probably experience mild symptoms, others can develop severe pneumonia.

How do people die from COVID-19?

Coronaviruses usually cause fairly mild illness, affecting just the upper airway. But the new virus, like those behind SARS and MERS, penetrates much deeper into the respiratory tract. SARS-CoV-2 leads to “a disease that causes more lung disease than sniffles,” says Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Md. And damage to the lungs can make these illnesses deadly.

Patients with the disease generally die from respiratory and multiorgan failure, partially caused by the virus but also their own immune responses. During infection, the virus that causes COVID-19 attacks cells within the respiratory tract, particularly the lungs. As these cells die, they fill the airway with fluids and debris while the virus continues to replicate — making it hard to breathe. 

The presence of dying cells and a replicating virus spark the immune system to react to the infectious intruder. Immune cells then flood the lungs to repair damaged tissues and wipe out the virus. While the immune response to the virus is generally highly controlled, it can sometimes go berserk and cause its own damage to healthy cells as well as dying ones. A flood of signals from the immune system, called a cytokine storm, can damage the lungs and cause respiratory failure, and can also harm other organs, leading to multiorgan failure.

How long is the incubation period?

The disease’s incubation period is the amount of time from being exposed to the virus to showing symptoms. Officials estimate that’s about five days, but may be as short as two and as long as 14 days. Older people may have a slightly longer incubation period. One preliminary study posted February 29 to estimates that people over 40 show symptoms after six days while those 39 and younger develop symptoms after four days. 

How long are people contagious?

Researchers are starting to get hints of just when patients are most contagious. Infected people may test positive for the virus both before and after they have symptoms. But a preprint study, posted March 8 at, of nine people who contracted the virus in Germany suggests that people are mainly contagious before they have symptoms and in the first week of the disease (SN: 3/13/20).

Patients produced thousands to millions of viruses in their noses and throats, about 1,000 times as much virus as produced in SARS patients, Clemens Wendtner, director of infectious disease and tropical medicine at Munich Clinic Schwabing, a teaching hospital, and his colleagues found. That heavy load of viruses may help explain why the new coronavirus is so infectious.

Scientists identified these nine people some time after they had been exposed to the coronavirus, so researchers don’t know for sure when exactly people begin giving off the virus.

After the eighth day of symptoms, the researchers could still detect the virus’s genetic material, RNA, in patients’ swabs or samples, but could no longer find infectious viruses. That’s an indication that antibodies that the body’s immune system makes against SARS-CoV-2 are killing viruses that get out of cells, Wendtner says.

How does the disease spread?

Coronaviruses like SARS and MERS — and now SARS-CoV-2 — probably spread between people similar to other respiratory diseases, the U.S. Centers for Disease Control and Prevention says. Respiratory droplets from an infected person’s cough or sneeze can carry virus to another person standing almost two meters away. 

Touching surfaces covered with droplets and then touching your face can also spread the virus, which has spread more widely and caused more damage than SARS. New research suggests that this isn’t because it persists in the environment longer than SARS. The new coronavirus remains viable longest on plastic and stainless steel, where it can be detected for two to three days, though infectivity drops substantially after 48 hours, similar to SARS, according to a preprint study posted March 9 at and later published in the New England Journal of Medicine. On cardboard, the virus lasts for 24 hours, and it couldn’t be detected on copper after three hours.

Are there two strains of the virus, one mild and one severe?

No, there are not two strains of the coronavirus going around in the United States or elsewhere. 

A study published March 3 in the National Science Review claimed that two types of the virus, which had varying severity and were called L and S, were infecting people. The researchers had studied viruses from 103 people, and found that some viruses had a particular mutation. Those viruses, the L type, were more prevalent in Wuhan, China, in the early stages of the outbreak, and produced more severe symptoms, the team concluded.

But that mutation doesn’t change any of the virus’s proteins and probably has no effect on disease severity, experts say. “The differences between [the two virus types] … can be likened with comparing two cars of identical type and color, just with a different license plate. That license plate helps you find out where the car was registered but not how fast it can go,” say researchers from GISAID, a repository that has been collecting all of the coronavirus genomes deciphered so far.

Can people who have had the virus be reinfected?

Probably not, experts say.  

While there have been some reports of patients still testing positive for the virus after they have recovered or even getting sick again, it’s possible the virus sticks around in the body longer than expected. Or people who appear to recover and then show symptoms again may have suffered a relapse of the same infection. These results could also reflect issues with the current diagnostic test, which isn’t sensitive enough to always pick up low levels of virus in an infected person. 

“I don’t think that reinfection is that likely,” says Angela Rasmussen, a virologist at Columbia University. But studying the disease in other animals, such as mice or nonhuman primates, would help determine whether the virus can reinfect a host, she says.   

One small study in rhesus macaques found that the animals couldn’t be reinfected with the coronavirus, at least in the short term, researchers report in a study posted March 14 at The monkeys developed antibody responses against the virus that likely protected them from getting infected when they were exposed again 28 days after their first exposure. It’s still unclear, however, how long immune responses against the virus last.

Is the virus spread asymptomatically?

Unlike SARS and MERS, there is evidence that the new coronavirus is spread by people with no or very mild symptoms (SN: 1/31/20). Asymptomatic transmission is common for contagious viruses such as influenza or measles, but would be a new feature for the types of coronaviruses that cause epidemics.

How big of a problem is asymptomatic spread?

Right now, it’s unclear. Researchers would need to understand how many people in affected areas have been infected overall. To investigate this, they need a test to determine who has developed antibodies against the virus, which can confirm whether someone was infected but has since cleared the virus from their body. So far only Singapore has done these tests.

Mild cases of COVID-19 that go unrecognized are fueling the coronavirus pandemic, a study in Science of the early days of the outbreak in China suggests. Undocumented cases — those occuring in people with mild or no symptoms — accounted for an estimated 86 percent of all infections, a simulation finds (SN: 3/17/20). Although those undetected cases were half as infectious as known cases, with high numbers on their side, the hidden cases became the source for almost 80 percent of the diagnosed infections.

Asymptomatic spread could make the epidemic harder to control because such patients can transmit the disease without signs that they’re sick themselves. 

How far has the disease spread? 

As of March 18, the virus has infected more than 212,000 people in at least 157 countries and territories.

From March 1 to March 2, nine times as many cases were reported outside China as inside. That’s partly because the number of daily cases reported in China is going down, a sign that containment efforts there are continuing to slow the outbreak. But cases in other countries are going up. And as of March 15, the number of COVID-19 cases in countries outside of China has surpassed the number of cases there.

“Europe has now become the epicenter of the pandemic, with more reported cases and deaths than the rest of the world combined, apart from China,” WHO director-general Tedros Adhanom Ghebreyesus said March 13 in a news conference.

Cases of COVID-19 in Italy, for instance, have skyrocketed. More than 11,000 new cases were documented there from March 6 to March 13. The entire country is now on lockdown with shops closing and extensive travel restrictions. Meanwhile, officials in South Korea have focused on aggressive monitoring and the number of new cases are declining. There, officials have set up more than 500 coronavirus testing sites around the country and are screening around 10,000 people per day.  

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.

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

Why do we care about undetected cases?

Undetected cases matter because they may seed outbreaks when travelers carry them to other countries, says infectious disease dynamics researcher Katelyn Gostic of the University of Chicago. But even the best efforts to screen airline passengers for COVID-19 infections will miss about half of cases, she 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.

Is containment of the virus still possible?

Despite the rising case numbers outside China, “containment of COVID-19 is feasible and must remain the top priority for all countries,” WHO’s Ghebreyesus said March 2 during a news conference. “With early, aggressive measures, countries can stop transmission and save lives.”

Restricting travel, closing public transportation and banning mass gatherings like concerts can affect virus spread. Also, “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.

It’s unclear how long such measures would have to be in place, and would likely depend on what is happening in the affected area. But some regions have closed down schools, including countries like China, Japan and France, as well as several states in the United States. 

What’s the situation in the United States?

As of March 18, U.S. health officials have confirmed the new coronavirus in 7,708 people across all 50 states and territories, including 120 deaths. The cases include three people who were repatriated from China and 46 who were passengers on the Diamond Princess cruise ship that had been quarantined off the coast of Japan.

Officials announced the first COVID-19 case in the United States linked to travel on January 21 (SN: 1/21/20). On February 26 and 28, U.S. health officials announced that two women in California had been infected (SN: 2/28/20). Neither woman had traveled to affected areas and were not exposed to someone known to have the disease. Officials have since identified a growing number of infected people in Washington, suggesting the virus is spreading locally there. As of March 18, at least 1,012 patients there had tested positive for the virus — including 52 of the U.S. deaths. 

The virus may have been circulating in Washington state since at least mid-January, according to a genetic analysis of virus samples from two patients in the state conducted by Trevor Bedford, an evolutionary biologist at the Fred Hutchinson Cancer Research Center and the University of Washington in Seattle, and his colleagues. As a result, an estimated few hundred people in Washington may currently be infected, Bedford said March 2 on Twitter. The size of the outbreak may also double every seven days in the absence of containment measures, he said. 

In the wake of steadily rising case numbers, health officials have implemented social distancing measures, including advising people to avoid gatherings of more than 10 people for the next 15 days. Some states have shut down bars and restaurants. And on March 17, a shelter-in-place order affecting close to 7 million people in the San Francisco Bay area went into effect. 

What can I do to prepare?

Practicing good hygiene is the most important way to protect yourself. Tips from the WHO include washing hands with soap and water or alcohol-based sanitizers, disinfecting surfaces, and coughing or sneezing into your sleeve or using a tissue (SN: 2/28/20). If you think you are sick, stay home and avoid traveling. 

The CDC also recommends having a plan in place for what will happen if you and your family have to stay home from work or school.  

Does hand sanitizer actually work?

Though washing your hands with soap and water is best, hand sanitizers will also work.

Coronaviruses are enveloped viruses, meaning that when a single viral particle leaves an infected cell, it takes part of the cell’s membrane with it. This membrane forms a protective coating around the virus. But alcohol in hand sanitizer can disrupt this envelope and essentially kills the virus.     

Why aren’t masks recommended for protection?

Surgical masks are designed to keep germs in, not keep them out. 

If a sick patient is wearing a surgical mask, the fabric will catch respiratory droplets and prevent viral particles from getting on surfaces other people might touch. But these types of masks are not designed to protect healthy people from viruses in the environment. Surgical masks don’t fit perfectly around the face, leaving gaps on the sides. Many people also don’t wear them properly — like leaving their nose exposed while covering their mouth, for instance. 

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

If you have a fever and respiratory symptoms, call your medical provider ahead of time, infectious disease specialist 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’s important to remember that the risk of getting severely ill appears to be fairly low for many people. But “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. 

How do doctors test for the virus?

WHO laboratory-testing guidelines suggest doctors take multiple samples, including nose and throat swabs, blood and sputum from the lower respiratory tract.

In the lab, researchers look for genetic evidence of the virus, using a method called reverse transcription polymerase chain reaction, or RT-PCR. If the virus is present, the technique produces copies of RNA — the virus’s genetic code — that is unique to SARS-like coronaviruses. For positive tests, researchers do further genetic analyses to pin down whether SARS-CoV-2 is the cause. The method relies on patients being sufficiently sick that they have high amounts of the virus for it to detect. Not everyone who is infected will have a positive test. 

Although initial testing in the United States was limited to people with a travel history to Wuhan or contact with someone who had, the CDC expanded their testing criteria in the wake of the first cases of community spread. Now patients who have a doctor’s order can supposedly be tested. But many who are sick and whose doctors want them tested are still being turned away. 

The first CDC diagnostic kits were also flawed, restricting the ability of local and state labs to screen patients. But officials have since allowed commercial and academic labs to develop their own tests, which will help increase testing capacity. Those test results must then be confirmed at a state lab. Still, the number of tests and testing capabilities remain limited in the country.

Where did the virus come from?

Coronaviruses are zoonotic, meaning they originate in animals and sometimes leap to humans. Bats are often thought of as a source of coronaviruses, but in most cases they don’t pass the virus directly on to humans. SARS probably first jumped from bats into raccoon dogs or palm civets before making the leap to humans.

MERS, meanwhile, went from bats to camels before leaping to humans (SN: 2/25/14). A paper published January 22 in the Journal of Medical Virology suggests that the new coronavirus has components from bat coronaviruses, but that snakes may have passed the virus to humans. Many virologists, however, are skeptical that snakes are behind the epidemic (SN: 1/24/20). Other analyses have proposed pangolins as the source of the virus, but coronaviruses from pangolins are not as closely related to SARS-CoV-2 as bat viruses are, meaning the mystery is far from solved. 

Current data suggest that the virus made the leap from animals to humans just once and has been passing from person to person ever since. Based on how closely related the patient viruses are, animals from the seafood market probably didn’t give people the virus multiple times as researchers originally thought, Bedford says. If the virus leaped from animals to humans more than once, the researchers would expect a greater number of mutations.

Why does knowing the virus’s origin matter at this point?

Pinpointing the source of the virus is a step toward protecting people from coming into contact with more infected animals, and possibly starting another outbreak. 

Can pets get sick?

There is one report of a dog in Hong Kong having a low-level infection with the new coronavirus. The animal, whose owner was infected too, has not shown any signs of disease related to the illness (SN: 3/5/20). There is currently no evidence that pets can actually get sick or transmit the virus to people or other animals.

Several types of coronaviruses can infect animals and in some cases make them ill. So the CDC advises avoiding contact with pets and wearing a face mask if you are sick. Researchers reported in 2003 in Nature that cats could be infected with the SARS virus and transmit it to other cats in the same cage, but they didn’t show any symptoms. The same was true for ferrets, although the ferrets became sick.

While the CDC recommends that people traveling to China avoid animals, the agency says there is no reason to believe that animals or pets in the United States can transmit the virus that causes COVID-19.

When will it end?

It’s a tough question for experts to answer, and right now, no one knows. On March 11, the WHO announced that it is now calling the coronavirus outbreak a pandemic, given its spread and rapidly growing impact (SN: 3/11/20). 

It’s possible that the virus could begin circulating permanently in humans, like influenza or common colds. It’s unknown at this point if the virus might become seasonal like the flu.

For now, the situation is likely to get worse before it improves. “We expect to see the number of cases, the number of deaths, and the number of affected countries climb even higher,” WHO’s Ghebreyesus said March 11 in a news conference.

Erin Garcia de Jesus, Tina Hesman Saey, Jonathan Lambert and Aimee Cunningham contributed to reporting of this story.   

More Stories from Science News on Health & Medicine

From the Nature Index

Paid Content