What Really Causes the Spread of the Coronavirus?

After close six months living under "the new normal" of the coronavirus pandemic, many questions still remain, as economic and public activity begins to pick back up again in many states. While much remains a mystery or is contested by different experts, like exactly how many people have been infected and why some get sicker than others, experts have begun to reach a consensus on certain points.

At the forefront of everyone's mind remains the question: How does one become infected? According to the Wall Street Journal, scientists say that close-up, face-to-face interactions over a long period of time are mainly responsible for infection.

This is why crowded events, small spaces or areas with bad ventilation, and places where people are talking loud and close are all major risks. Brief interactions with others outdoors and contaminated surfaces are no longer considered major sources for infection. John Brooks, the Centers for Disease Control and Prevention's chief medical officer for the coronavirus response, said that prolonged exposure can be defined as 15 minutes or more of unprotected contact with someone else less than six feet away. But infection can also occur in a matter of minutes, when respiratory droplets are emitted into the air.

Now, health agencies have identified respiratory-droplet contact as a main viral transmission mode. This occurs when your speaking and breathing produces respiratory bits that are dispersed along air currents and can infect those around you if they happen to land on the eyes, nose, or mouth, which is not so common.

Other researchers say that aerosols can also play a role in coronavirus transmission, as these smaller droplets can float in the air for a long time and be inhaled directly. Scientists are studying a situation like this at a restaurant in Guangzhou, China, in which an asymptomatic infected person transmitted the virus to five others at other tables. This study has not been peer-reviewed yet, but the researchers hypothesize that the respiratory droplets from the infected person could have built up as aerosol to be re-circulated in the space by the air-conditioning unit. This points out the major issue of the importance of good ventilation in places where people gather, as more fresh air will lower the risk of infection.

Those who have been following coronavirus news may be familiar with the term "superspreader," or events when a small number of people can infect many others. This is what is likely to have occurred in the March 10 incident in which 87 percent of attendees at a choir practice in Washington state were infected. Of the 61 attendees, 53 were infected, two of whom died. There were many factors that experts feel were responsible for this frightening "superspreader" incident, including choir members moving around and changing places four times, the confined space of the room, the heavy breathing required with singing, and the fact that most of the members were older and more vulnerable to infection, according to The Wall Street Journal.

At events like this, as well as at gyms, weddings, conferences, and musical performances, the risks are amplified by so many people in close, extended contact. This makes the "attack rate," or the percentage of people infected at a given place or time, very high in these crowded spaces. The attack rate for COVID-19 in households ranges between 4.6 percent and 19.3 percent, while higher for spouses, at 27.8 percent.

According to a study in Wellcome Open Research, an estimated 10 percent of people with COVID-19 are responsible for about 80 percent of transmissions. Some may be more likely than others to infect because they may produce an amplified number of droplets when speaking or breathing, be in a confined crowded space at their most infectious point, or just have a higher viral load. Overall, though, the infection risk is low, and these superspreader events are not common.

While being outside is safer because particles dilute more quickly in fresh air, being in close, prolonged contact with others outside can still be risky because of the airborne droplet transmission.

Although it's still a mystery how much virus it takes for someone to get infected, a new study in the journal Nature showed that researchers could not culture live coronavirus if the patient's throat swab had less than one million copies of viral RNA. It takes much lower levels of the virus to infect someone else than is found in an infected and sick patient.

"Based on our experiment, I would assume that something above that number would be required for infectivity," said Clemens Wendtner, one of the study's lead authors and head of the department of infectious diseases and tropical medicine at München Klinik Schwabing, a teaching hospital at the Ludwig Maximilian University of Munich, according to The Wall Street Journal.

As studies continue to provide more information about the infection and spread of the disease, we may see more changes in policies for protecting health and safety. While the standard is for those who test positive to stay at home, some cities are starting to give free housing and services temporarily for those who are infected, to avoid infecting family members they may live with. Precautions like social distancing and wearing protective masks are expected to continue for the foreseeable future as states reopen.

Some expect that if economic reopening brings a resurgence of COVID-19 cases, local leaders may have to decide to return to the more stringent stay-at-home policies from March. Meanwhile, the CDC has put out guidelines for employers to ensure the safety of returning workers, including masks, limited use of public transit and elevators, replacing communal snacks with single-serve products, social distancing of desks, and protective plastic dividers. However, many of these guidelines do not include precautions about the distribution of aerosols, which may be the biggest risk factor in an office's infection.

"Aerosol transmission is a scary thing," said Lisa Brosseau, a respiratory-protection consultant for the University of Minnesota's Center for Infectious Disease Research and Policy, according to The Wall Street Journal. "That's an exposure that's hard to manage, and it's invisible." She said it's important that infected individuals stay home, which is why it is important that testing is more readily available, but other protocols like social distancing and N95 respirators can also help mitigate infection.

On the other hand, aerosol transmission doesn't completely explain most infections.

"If this were transmitted mainly like measles or tuberculosis, where infectious virus lingered in the airspace for a long time or spread across large airspaces or through air-handling systems, I think you would be seeing a lot more people infected," said the CDC's Dr. Brooks. Some say employers should sample air in high-traffic areas of the office to figure out which employees need to be tested.

Experts are currently advising everyone, from businesses to court systems to therapists, about how to safely conduct business while still preventing the spread of COVID-19. These recommendations include wiping down high-touch surfaces and cleaning, but those aren't as important as decreasing close-range person-to-person prolonged interactions and limiting being in an enclosed space for a long time.

Still, these recommendations for workers' safety cannot reach across all points and are not standardized yet. As businesses and states continue to open, it is essential for employers to consider the discoveries of studies about how the virus is transmitted in order to protect workers.