What We Now Know About Coronavirus Transmission: Where the Virus Goes in your Body and How it is Spread
We are now more than 9 months into this pandemic in the US (has it really only been 9 months?! Seems like forever…). As we continue to learn more and more about the coronavirus (formally called SARS-CoV-2), BioSpace wanted to revisit what we now know about how the virus affects the body and how it is spread.
TL;DR: stay 6 feet away from people outside your household, wear a mask over your nose and mouth in public, avoid indoor activities (especially dining and drinking indoors because you can’t wear a mask while eating and drinking), wash your hands often (especially before touching your face or eyes), and sanitize surfaces frequently.
At this point, we can be confident that these actions successfully reduce the chance of infection, if done properly (the more time goes by, the more data we collect, the more evidence and conclusions we can come to). However, it is important to remember that this is a new virus, so researchers are constantly learning more about it every day and refining our knowledge on combating it.
Where in the body is coronavirus?
Like kids used to search the world for Carmen Sandiego (shoutout to anyone who remembers playing that game!), scientists have sleuthed out where coronavirus hides in our bodies. If we know where the virus hides (aka the types of tissues it infects), we can learn about how the virus is spread. Check out this Science article for an interactive graphic showing where the coronavirus infects the body (and see the image adapted from the article below).
How the novel coronavirus (SARS-CoV-2) affects different parts of the body. Source: Science
Since it is a respiratory virus, we know that it infects the respiratory tract (the nose and lungs), causing coughing, shortness of breath, and difficulty breathing. Some patients with coronavirus disease 2019 (COVID-19), the disease caused by the novel coronavirus, have reported loss of smell or taste, possibly due to the virus infecting and damaging nerve cells in the nose and mouth. Sicker COVID-19 patients can also have conjunctivitis (also called pink eye).
But COVID-19 patients also have some symptoms you wouldn’t readily link to a respiratory infection. The virus uses a certain human cell receptor, called angiotensin converting enzyme 2 (ACE2), to enter into the cell. The ACE2 receptor is expressed on cells throughout your body, in the lungs, nose, blood vessels, heart, intestines, and some parts of the brain. This could explain the not as straightforward COVID-19 symptoms and side effects, such as diarrhea, headache, blood clots, heart inflammation, heart attacks, strokes, seizures, confusion, and brain inflammation.
Through the power of citizen science and a phone app used by over 4 million people (called COVID Symptom Study), we now know that COVID-19 symptoms tend to appear in certain combinations. According to a preprint paper on medRxiv, there are six patient-reported symptom clusters: “flu-like” with no fever, “flu-like” with fever, mainly gastrointestinal, severe respiratory symptoms with fatigue, severe respiratory symptoms with confusion, and severe respiratory and gastrointestinal symptoms.
The most unique COVID-19 symptoms are the temporary loss of taste and smell. How would a virus cause that? Turns out, that can be traced back to which cells express the ACE2 receptor as well. Unlike originally thought, coronavirus is unable to infect sensory neurons that process smells and tastes because the neurons don’t have the ACE2 receptor. Instead, the virus infects ACE2-expressing cells that help the neurons process smells and tastes, called support cells. Disrupting the cells needed for sensing prevents you from smelling or tasting temporarily, but not permanently – your senses are restored when new support cells are regenerated within a few weeks.
Can you get coronavirus from that?!
Now that we know coronavirus infects cells in the nose, lungs, and intestines, let’s think about ways the virus can spread from one person to another.
Let’s start with the obvious. Since it is a respiratory virus, you can spread coronavirus through your nose and mouth. Sneezing, coughing, yelling, and even talking (yes, just talking!) could expel respiratory droplets, which can contain the virus if you’re infected. Picture the spit sprayed from someone’s mouth as they talk excitedly or yell – yup, those are respiratory droplets. Check out this video from a study in the New England Journal of Medicine (NEJM) to see how much spray comes out of your mouth when you just talk.
Singing can also increase the risk of transmission, as seen in a superspreading event at a choir practice in Washington state in March 2020. During the 2.5-hour choir practice, one symptomatic person spread COVID-19 to 52 of the 60 other people at practice (32 confirmed cases and 20 probable cases). The extremely high transmission was facilitated by people being close to each other indoors for an extended period of time and made worse by singing.
Respiratory droplets tend to fall to the ground quickly after being expelled, which is why they only travel 3-6 feet away. That’s also why it’s important to stay 6 feet away from other people – to prevent yourself from getting a respiratory droplet shower (yuck!).
Coronavirus can also live for up to 3 hours in the air, so you could breathe in the virus from the air in a room where an infected person breathed/coughed/sneezed up to 3 hours before. This is similar to other contagious respiratory diseases, like flu (which can live in the air for 2-3 hours) and measles (which can live in the air for up to 2 hours).
That’s why it’s important to stay 6 feet away from other people – to prevent yourself from getting a respiratory droplet shower (yuck!).
More importantly, tinier droplets called aerosols can also come out of your mouth. Viruses can hitch a ride on these tiny, invisible droplets, being suspended in the air for hours, floating throughout a room like smoke. In fact, coronavirus can live for up to 3 hours in the air in aerosols, meaning you could breathe in the virus from the air in a room where an infected person breathed/coughed/sneezed up to 3 hours before. This is similar to other contagious respiratory diseases, like flu (which can live in the air for 2-3 hours) and measles (which can live in the air for up to 2 hours).
It took public health authorities quite awhile to publicly acknowledge COVID-19’s airborne transmission, despite mounting evidence of watching how community spread was occurring and studies isolating the virus in the air. For example, a medRxiv preprint paper found viable coronavirus in the air up to 15.7 feet (4.8 meters) around hospitalized COVID-19 patients. In fact, two aerosol scientists, backed by 237 other scientists, clinicians, and engineers, wrote a commentary in the Clinical Infectious Diseases journal in July 2020 bringing attention to COVID-19’s likely aerosol transmission, and calling on public health officials to update guidelines.
What about masks?
If it is hard to stay 6 feet away from others, especially if you’re indoors, you should wear a cloth face mask. This protects your nose and mouth from coming into contact with virus droplets from other people and virus aerosols hanging around in the air.
Earlier in the pandemic, surgical mask shortages prompted public health officials to tell the general public they did not need masks – not because masks are not effective, but because they wanted health-care workers to have access to the scarce supplies of protective equipment. While that supply has largely caught up with demand, the officials now encourage or require everyone to wear either fabric or surgical masks in social settings.
However, N95 and KN95 respirators are still not recommended for the general public as they must be fit tested for proper use and should be reserved for medical first responders. Masks with valves and wearing only face or mouth shields (as a substitute for masks) is also not recommended because they don’t properly control respiratory droplet emission from the wearer.
A perfect example showing that face masks work to limit transmission came out of a Missouri hair salon in May 2020. Two symptomatic hair stylists saw 139 clients, but no clients got sick. Why? Because the stylists and clients all wore face masks.
Another cautionary tale comes from a Starbucks in a South Korean city north of Seoul in August 2020. Within days after one infected customer visited the coffee shop, over two dozen other customers contracted COVID-19, but none of the employees. Why? The employees were all wearing face masks and the customers were not (you can’t drink a latte or eat a scone while wearing a face mask, after all).
One of the most high-profile superspreading events happened in late September 2020 at the White House, resulting in at least 34 people contracting COVID-19, including President Trump and the First Lady. Many cases can be traced back to the Supreme Court nominee announcement hosted outside in the Rose Garden on September 26. Despite being outdoors and people reportedly being tested before entering, the attendees ignored the basic safety precautions of wearing a mask and social distancing. This highlights the importance of wearing a mask (even when outdoors), keeping your distance, and not relying on coronavirus testing as protection.
Moral of the stories? Social distancing and masks work when worn properly and used consistently.
But not all fabric face masks are equally effective. Which fabrics are best for making effective cloth face masks (ones that will filter out small particles, like viruses but still let you breathe comfortably)? Each different type of fabric is woven to a different tightness, creating various “pore sizes” – the tighter the cloth is woven, the smaller the particles that can pass through.
Various household fabrics have been tested for their efficacy at filtering bacteria, viruses, and small particles (representing viruses). One study found that vacuum cleaner bags, tea towels, and a cotton mix most effectively filtered bacteria and viruses, behind surgical masks; however, vacuum cleaner bags are hard to breathe through, so they are not recommended to be used as face masks. Another study tested 30 materials on their breathability and ability to filter small particles (down to 0.3 microns, about the size of a smallpox virus particle – coronavirus is about 0.1 microns in size). The ‘Goldilocks’ materials (ones that filtered efficiently but still let the wearer breathe comfortably) were denim (10 oz), bed sheets (80-120 thread count), paper towels, canvas (0.4-0.5 mm thick), and shop towels.
So, what is the best kind of mask to wear? For the general public, a multi-layer mask made of tightly woven fabric most effectively blocks respiratory particles while still allowing easy breathing. The most effective design is a 3-layer hybrid mask made with an inner layer that absorbs (like cotton), a middle layer that filters (like polypropylene fabric), and a nonabsorbent outer layer (like polyester). A hybrid mask (2 layers of cotton with an outer layer of polyester, silk, chiffon, or flannel), 2-layer cotton mask, or tea towel/dishcloth mask are other solid options.
Not sure if your mask is sufficiently protective? Try blowing out a candle while wearing it. If you can’t blow out the flame, the mask is good.
More (less common) ways coronavirus can be spread
Okay, back to how the virus invades your body. Contact with an infected person can transfer virus from them to you. An infected person may touch their nose or mouth, picking up some virus on their hands – the same hands they use to shake yours. Then you touch your nose or face – boom, you’ve infected yourself.
Virus-containing respiratory droplets from someone’s nose or mouth can land on nearby surfaces, such as doorknobs, handrails, and tables, creating contaminated objects (called fomites). The virus can live on surfaces for hours or days, up to 5 days on certain objects. If you touch an infected surface then touch your face, nose, or eyes, you could indirectly infect yourself. While everyone was sanitizing everything from packages to groceries in the beginning, transmission through respiratory droplets or aerosols is much more common than through contaminated surfaces.
This is why hand washing and sanitizing is so important – it removes any virus particles you may have picked up while touching infected surfaces.
Another possible way to be exposed to the virus is through fecal-oral transmission – going from an infected person’s poop into your mouth (ew!). If an infected person goes to the bathroom and doesn’t wash their hands, they can infect everything (and everyone) they touch. If you touch the infected item then touch your face, voila, you’ve infected yourself from someone else’s poop. Now, it’s important to note that, while researchers did find the virus in infected people’s poop, they don’t yet know if someone can get sick after being exposed to infected poop.
Detecting the coronavirus in wastewater is actually a useful tool to track how widespread virus transmission is in communities. Wastewater surveillance can even be an early warning sign of an impending rise in COVID-19 cases because coronavirus genetic information can be detected in poop before people become symptomatic. In fact, colleges are using wastewater surveillance to keep tabs on coronavirus spread on campus and stifle potential outbreaks before they flare up.
Questions have been raised about other possible transmission routes:
- Can farts spread the coronavirus? Although bacteria do travel on farts, farts are not likely culprits to spread coronavirus.
- Can coronavirus be spread by flushing the toilet? Gross, but not impossible. We know that bits of toilet contents are aerosolized and sprayed into the air when we flush (ew!) and coronavirus has been detected in poop samples. Therefore, it is possible that the virus could be aerosolized into the air around the toilet, lurking undenounced to the next person to use the toilet. Just something to keep in mind when using (or deciding not to use) a public restroom. And a good reminder to always put the seat down before you flush!
- Can I get coronavirus through my eyes? Yes! Respiratory droplets from an infected person’s cough/sneeze/yell can get into your eyes (just like they can get into your nose and mouth). The virus can also make its way into your eyes from your hands (if you touch an infected surface then touch your eyes). In fact, one study found that eye swabs from 2 of 33 COVID-19 patients (6 percent) tested positive for the virus, even though they didn’t have any eye symptoms (red, irritated, itchy eyes) that some COVID-19 patients have. So some infected people may spread coronavirus by touching their eyes then contaminating a surface by touching it.
- Is coronavirus spread through sex? New York City guidelines state that while coronavirus can be spread through direct contact with saliva and mucus, it has not been found in semen or vaginal fluid. Other coronaviruses are not efficiently spread through sex. However, a study published in the Journal of the American Medical Association (JAMA) in May 2020 showed that 4 of 15 men actively sick with COVID-19 (27 percent) and 2 of 23 men who recovered from COVID-19 (9 percent) had SARS-CoV-2 in their semen. This doesn’t mean the virus is infectious and can be transmitted through sex – more research is needed to fully understand if coronavirus is spread through sex. (As always, it wouldn’t hurt to use condoms if you do choose to have sex.)
- Are pregnant women more likely to get sick with coronavirus? The Centers for Disease Control and Prevention (CDC) say that “pregnant people seem to have the same risk [of getting sick from COVID-19] as adults who are not pregnant.” However, they also note that pregnant people may be at a higher risk of severe illness from respiratory viruses, such as the coronavirus, because they can have a slightly suppressed immune system while pregnant.
- Can a pregnant mom give her baby coronavirus? The jury is still out on this one. However, the CDC says “mother-to-child transmission of coronavirus during pregnancy is unlikely, but after birth a newborn is susceptible to person-to-person spread.” They also recommend that new moms take precautionary measures, such as wearing a face mask while breastfeeding and washing your hands before holding or breastfeeding their baby.
- Can coronavirus be spread through blood transfusion? No, coronavirus is not likely spread through the blood. AABB, an international not-for-profit association for transfusion medicine and cellular therapies, issued a statement saying that SARS and MERS (the other two new coronaviruses that emerged over the past 20 years) weren’t transmitted through blood transfusions. There haven’t been any reports of the novel coronavirus spreading via blood transfusions. Some blood donation centers, especially those in China when the outbreak began, put extra precautions in place, such as taking a donor’s temperature before they donate blood and asking more questions about a donor’s recent travel history. If you’re healthy and able to donate blood, please consider donating because there is always a need, especially right now during this coronavirus-exacerbated blood shortage. Find a local blood donation center here.
- Should I worry about getting coronavirus from food or groceries? No, coronavirus doesn’t seem to be spread through food or water.
- Can I get coronavirus from my mail/packages/newspaper? The now famous NEJM study tested how long coronavirus could live on various surfaces. While coronavirus can live on cardboard for 24 hours, the risk of getting sick from a contaminated box is really low and there haven’t been any documented cases of that yet.
- How long does coronavirus live on clothes? A study reported in The Lancet showed that coronavirus could live on cloth for 2 days.
- What about other surfaces, like paper, tissue paper, wood, glass, paper money, stainless steel, and plastic? The same Lancet study reported coronavirus living on paper and tissue paper for 3 hours, wood for 2 days, glass and paper money for 4 days, and stainless steel and plastic for 7 days. (It is important to remember that how long a virus lives is highly dependent on the temperature and presence of moisture (such as humidity). If you change the temperature and humidity of the testing conditions, you’ll get slightly different times for how long the virus can live on surfaces.)
- Can I get COVID-19 from my pet? Or give my pet COVID-19? Although a handful of domestic dogs and cats (and even a tiger in the Bronx Zoo) have tested positive for COVID-19, pets are not known to give humans COVID-19. In fact, it seems to be the other way around – the confirmed COVID-19 cases in animals occurred after a known COVID-19 patients (their owner or a caretaker at the zoo) came into contact with the animals. Treat pets like another family member – keep them away from people outside the household and stay away from them if you become sick.
There are also many, many myths circulating about how coronavirus spreads or how to prevent getting sick. Lists of coronavirus myth busters and frequently asked questions can be found on the World Health Organization (WHO), CDC, and Healthline websites.
Who can spread coronavirus?
When you think of an ‘infected person,’ a visibly sick, coughing, sniffling person may come to mind. While the virus can definitely be spread by actively sick people, it can also be spread by people without any symptoms.
Someone may be infected and be totally unaware because they have no symptoms (called asymptomatic). Estimates of how many people with coronavirus who don’t have symptoms have ranged widely, from 17.9 percent to almost 50 percent (CDC director Dr. Robert Redfield estimated 25 percent and National Institute of Allergy and Infectious Disease (NIAID) director Dr. Anthony Fauci estimated 25-50 percent). Even though they aren’t sick, they can still spread the virus to others, who can become infected and get sick.
An infected person can also spread the virus 2-3 days before they start having symptoms and know they are sick (called presymptomatic). There is a lag time (called the incubation period) between when someone is infected with a virus and when they start showing symptoms. For coronavirus, the incubation period ranges from a few days to almost two weeks. As the virus replicates in their body, they can spread the virus a few days before symptoms appear.
What should I do?
Remember (as if you haven’t heard this enough already), to protect yourself and others from coronavirus:
- Minimize person-to-person contact (stay at home as much as possible)
- Avoid crowded spaces, especially indoors
- Practice social distancing by staying 6 feet away from people outside your household
- Wear a fabric or surgical mask over your nose and mouth when out in public (even outdoors)
- Wash your hands for 20 seconds (or use alcohol-based hand sanitizer) often
- Try not to touch your face (I know, it’s hard)
- Cover your cough and sneeze (using your elbow is best)
- Clean frequently touched surfaces often