It’s Not Whether You Were Exposed to the Coronavirus. It’s How Much.

When experts recommend wearing masks, staying at least six feet away from others, washing your hands frequently and avoiding crowded spaces, what they’re really saying is: Try to minimize the amount of virus you encounter.

A few viral particles cannot make you sick — the immune system would vanquish the intruders before they could. But how much virus is needed for an infection to take root? What is the minimum effective dose?

A precise answer is impossible, because it’s difficult to capture the moment of infection. Scientists are studying ferrets, hamsters and mice for clues but, of course, it wouldn’t be ethical for scientists to expose people to different doses of the coronavirus, as they do with milder cold viruses.

“The truth is, we really just don’t know,” said Angela Rasmussen, a virologist at Columbia University in New York. “I don’t think we can make anything better than an educated guess.”

Common respiratory viruses, like influenza and other coronaviruses, should offer some insight. But researchers have found little consistency.

For SARS, also a coronavirus, the estimated infective dose is just a few hundred particles. For MERS, the infective dose is much higher, on the order of thousands of particles.

The new coronavirus, SARS-CoV-2, is more similar to the SARS virus and, therefore, the infectious dose may be hundreds of particles, Dr. Rasmussen said.

But the virus has a habit of defying predictions.

Generally, people who harbor high levels of pathogens — whether from influenza, H.I.V. or SARS — tend to have more severe symptoms and are more likely to pass on the pathogens to others.

But in the case of the new coronavirus, people who have no symptoms seem to have viral loads — that is, the amount of virus in their bodies — just as high as those who are seriously ill, according to some studies.

And coronavirus patients are most infectious two to three days before symptoms begin, less so after the illness really hits.

Some people are generous transmitters of the coronavirus; others are stingy. So-called super-spreaders seem to be particularly gifted in transmitting it, although it’s unclear whether that’s because of their biology or their behavior.

On the receiving end, the shape of a person’s nostrils and the amount of nose hair and mucus present — as well as the distribution of certain cellular receptors in the airway that the virus needs to latch on to — can all influence how much virus it takes to become infected.

A higher dose is clearly worse, though, and that may explain why some young health care workers have fallen victim even though the virus usually targets older people.

The crucial dose may also vary depending on whether it’s ingested or inhaled.

People may take in virus by touching a contaminated surface and then putting their hands on their nose or mouth. But “this isn’t thought to be the main way the virus spreads,” according to the Centers for Disease Control and Prevention.

That form of transmission may require millions more copies of the virus to cause an infection, compared to inhalation.

Coughing, sneezing, singing, talking and even heavy breathing can result in the expulsion of thousands of large and small respiratory droplets carrying the virus.

“It’s clear that one doesn’t have to be sick and coughing and sneezing for transmission to occur,” said Dr. Dan Barouch, a viral immunologist at Beth Israel Deaconess Medical Center in Boston.

Larger droplets are heavy and float down quickly — unless there’s a breeze or an air-conditioning blast — and can’t penetrate surgical masks. But droplets less than 5 microns in diameter, called aerosols, can linger in the air for hours.

“They travel further, last longer and have the potential of more spread than the large droplets,” Dr. Barouch said.

  • Updated June 1, 2020

    • How do we start exercising again without hurting ourselves after months of lockdown?

      Exercise researchers and physicians have some blunt advice for those of us aiming to return to regular exercise now: Start slowly and then rev up your workouts, also slowly. American adults tended to be about 12 percent less active after the stay-at-home mandates began in March than they were in January. But there are steps you can take to ease your way back into regular exercise safely. First, “start at no more than 50 percent of the exercise you were doing before Covid,” says Dr. Monica Rho, the chief of musculoskeletal medicine at the Shirley Ryan AbilityLab in Chicago. Thread in some preparatory squats, too, she advises. “When you haven’t been exercising, you lose muscle mass.” Expect some muscle twinges after these preliminary, post-lockdown sessions, especially a day or two later. But sudden or increasing pain during exercise is a clarion call to stop and return home.

    • My state is reopening. Is it safe to go out?

      States are reopening bit by bit. This means that more public spaces are available for use and more and more businesses are being allowed to open again. The federal government is largely leaving the decision up to states, and some state leaders are leaving the decision up to local authorities. Even if you aren’t being told to stay at home, it’s still a good idea to limit trips outside and your interaction with other people.

    • What’s the risk of catching coronavirus from a surface?

      Touching contaminated objects and then infecting ourselves with the germs is not typically how the virus spreads. But it can happen. A number of studies of flu, rhinovirus, coronavirus and other microbes have shown that respiratory illnesses, including the new coronavirus, can spread by touching contaminated surfaces, particularly in places like day care centers, offices and hospitals. But a long chain of events has to happen for the disease to spread that way. The best way to protect yourself from coronavirus — whether it’s surface transmission or close human contact — is still social distancing, washing your hands, not touching your face and wearing masks.

    • What are the symptoms of coronavirus?

      Common symptoms include fever, a dry cough, fatigue and difficulty breathing or shortness of breath. Some of these symptoms overlap with those of the flu, making detection difficult, but runny noses and stuffy sinuses are less common. The C.D.C. has also added chills, muscle pain, sore throat, headache and a new loss of the sense of taste or smell as symptoms to look out for. Most people fall ill five to seven days after exposure, but symptoms may appear in as few as two days or as many as 14 days.

    • How can I protect myself while flying?

      If air travel is unavoidable, there are some steps you can take to protect yourself. Most important: Wash your hands often, and stop touching your face. If possible, choose a window seat. A study from Emory University found that during flu season, the safest place to sit on a plane is by a window, as people sitting in window seats had less contact with potentially sick people. Disinfect hard surfaces. When you get to your seat and your hands are clean, use disinfecting wipes to clean the hard surfaces at your seat like the head and arm rest, the seatbelt buckle, the remote, screen, seat back pocket and the tray table. If the seat is hard and nonporous or leather or pleather, you can wipe that down, too. (Using wipes on upholstered seats could lead to a wet seat and spreading of germs rather than killing them.)

    • How many people have lost their jobs due to coronavirus in the U.S.?

      More than 40 million people — the equivalent of 1 in 4 U.S. workers — have filed for unemployment benefits since the pandemic took hold. One in five who were working in February reported losing a job or being furloughed in March or the beginning of April, data from a Federal Reserve survey released on May 14 showed, and that pain was highly concentrated among low earners. Fully 39 percent of former workers living in a household earning $40,000 or less lost work, compared with 13 percent in those making more than $100,000, a Fed official said.

    • Should I wear a mask?

      The C.D.C. has recommended that all Americans wear cloth masks if they go out in public. This is a shift in federal guidance reflecting new concerns that the coronavirus is being spread by infected people who have no symptoms. Until now, the C.D.C., like the W.H.O., has advised that ordinary people don’t need to wear masks unless they are sick and coughing. Part of the reason was to preserve medical-grade masks for health care workers who desperately need them at a time when they are in continuously short supply. Masks don’t replace hand washing and social distancing.

    • What should I do if I feel sick?

      If you’ve been exposed to the coronavirus or think you have, and have a fever or symptoms like a cough or difficulty breathing, call a doctor. They should give you advice on whether you should be tested, how to get tested, and how to seek medical treatment without potentially infecting or exposing others.

    • How can I help?

      Charity Navigator, which evaluates charities using a numbers-based system, has a running list of nonprofits working in communities affected by the outbreak. You can give blood through the American Red Cross, and World Central Kitchen has stepped in to distribute meals in major cities.


Three factors seem to be particularly important for aerosol transmission: proximity to the infected person, air flow and timing.

A windowless public bathroom with high foot traffic is riskier than a bathroom with a window, or a bathroom that’s rarely used. A short outdoor conversation with a masked neighbor is much safer than either of those scenarios.

Recently, Dutch researchers used a special spray nozzle to simulate the expulsion of saliva droplets and then tracked their movement. The scientists found that just cracking open a door or a window can banish aerosols.

“Even the smallest breeze will do something,” said Daniel Bonn, a physicist at the University of Amsterdam who led the study.

Observations from two hospitals in Wuhan, China, published in April in the journal Nature, determined much the same thing: more aerosolized particles were found in unventilated toilet areas than in airier patient rooms or crowded public areas.

This makes intuitive sense, experts said. But they noted that aerosols, because they are smaller than 5 microns, would also contain much less, perhaps millions-fold less, virus than droplets of 500 microns.

“It really takes a lot of these single-digit size droplets to change the risk for you,” said Dr. Joshua Rabinowitz, a quantitative biologist at Princeton University.

Apart from avoiding crowded indoor spaces, the most effective thing people can do is wear masks, all of the experts said. Even if masks don’t fully shield you from droplets loaded with virus, they can cut down the amount you receive, and perhaps bring it below the infectious dose.

“This is not a virus for which hand washing seems like it will be enough,” Dr. Rabinowitz said. “We have to limit crowds, we have to wear masks.”

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