SciMoms COVID FAQs: How does COVID-19 spread?

In this series of COVID FAQs, the SciMoms answer your frequently asked questions about the novel coronavirus or COVID-19 pandemic. For our second FAQ, we focus on COVID-19 spread and transmission. Links to the other FAQs are at the bottom of this post.

How does SARS-CoV2 spread?

SARS-CoV2 is a respiratory virus that is easily transmitted through close personal contact. Transmission occurs when an infected person passes enough infectious virus (see next question) to another person, usually through their mouth, nose, or eyes, but even possibly inhaled into their lungs.

The virus spreads mainly from person-to-person contact and is easily spread through sneezing, coughing, and even talking. However, the virus can also spread through surfaces. When an infected person sneezes, coughs, talks, or touches a surface, they may transfer some amount of infectious virus to that surface. If someone else touches the surface while the infectious virus is still present and then subsequently touches their face, the virus might then transfer to their mouth, nose, or eyes (more on this in the section on fomites below). While both the CDC and WHO have said that this is not the major way that SARS-CoV2 spreads, under the right conditions, spread through surfaces is possible.

Luckily, there are several ways to reduce exposure. By wearing a mask, an infected person can limit how much of the virus they share with the people around them, as well as how much they leave behind on the surfaces they touch. Frequent hand-washing can also help limit exposure to virus. We can help reduce the spread of virus through surfaces with regular disinfection.


Viruses (Updated) (by the Amoeba Sisters)

Coronaviruses: An Overview of Their Replication and Pathogenesis

How Coronavirus Spreads | CDC

The three Ws of stopping the spread of COVID-19

How much virus is needed to infect someone and make them sick?

Every pathogen has an infectious dose. For a virus, this is the number of individual viruses needed to infect someone. In other words, you need to be exposed to at least this amount of the virus for it to take hold in your cells and cause you to be infected.

Scientists believe the infectious dose of SARS-CoV2 is low based on how easily it seems to spread through contact between people.

What is viral load? How does it relate to how sick someone is?

Viral load is a measure of how much virus is present in a person once they become infected. This changes over time — going up as the infection progresses and the virus replicates in the person’s cells, and then declining as, hopefully, the person begins to recover. In general, the higher your viral load, the worse your symptoms. You are also more contagious because you are shedding more whole viruses. 

The incubation period is the time from infection to symptom onset (although some infected people never develop symptoms and can still be infectious). For SARS-CoV2, this period is 2-14 days. During this time viral load increases. The infectious period is the time when someone can transmit the virus to other people. For this virus, this seems to begin about 2 days prior to symptom onset to lasts for at least 10 days. How infectious someone is during the infectious period lessens with time. During the course of infection, viral load decreases as, hopefully, the virus is cleared from the body.

Timeline of COVID-19 Infection, Image credit: Dear Pandemic Facebook Page

Scientists still have many unanswered questions about the relationship between a person’s SARS-CoV2 viral load and how contagious they are or how sick they feel (see question about asymptomatic spread).

Finding the answers to these questions becomes important when we think about the timing of testing. It is possible to test negative even after infection if the viral load has not reached a detectable level. Someone could have a negative test and no symptoms, but still be infected and even become contagious (a later FAQ will answer questions about testing). This is why surveillance testing should occur at regular intervals. (See details in our FAQ on diagnostic testing.)

However, uncertainty about these details doesn’t change what we do know about this virus — that it spreads easily through person-to-person to contact and that we can slow the spread of the disease by diligently adhering to the 3 Ws—wear a mask, wash your hands, and watch your distance (see FAQ about masks).


How much of the coronavirus does it take to make you sick? – STAT

What We Do and Do Not Know About COVID-19’s Infectious Dose and Viral Load

What are fomites and how do they contribute to COVID-19 spread?

Fomites are inanimate objects that are capable of transmitting virus. Imagine an infected person touches a doorknob. If that infected person has viral particles on their hand that they then transfer to the doorknob, they have created a fomite. If a healthy person touches that doorknob while the infectious virus is present, they may transfer the virus to their hands and from there to their nose, mouth or eyes. For that transfer to occur, however, the virus needs to be able to survive on the surface. And scientists still have questions about that point.

Several studies have examined how long SARS-CoV2 can survive on objects. Because SARS-CoV2 is an enveloped virus, it is unlikely to remain infectious on surfaces for very long. Depending on the surface—plastic, cardboard or metal—RNA from the virus can last between a few hours and a few days. But while it is possible to transmit COVID-19 through a fomite, whether it will actually happen depends on how much virus is present, how long it has been there, and whether a high enough dose of infectious virus remains.

Regardless of these questions, scientists believe that the virus is spread mostly through close human contact. Transmission through surfaces and fomites is possible and there is much scientific debate over the degree to which fomites contribute to spread. Overall, the specific level of how much spread is from surfaces or direct contact doesn’t change recommendations for diligent hygiene and masking.

These precautions can also help reduce risk of spread, even with the scientific questions that remain: wash your hands well after coming into contact with something that may have been contaminated, wear a mask so you don’t inadvertently contaminate a surface, and disinfect shared surfaces.

Fortunately, taking these precautions gets around all the scientific uncertainty about how long SARS-CoV2 can survive on surfaces. We need to take these precautions and always wash your hands well after coming into contact with something that may be contaminated


CDC coronavirus risk guidance: Why surfaces are less of a risk than close contact

Cleaning and Disinfection for Households

General questions about COVID-19 transmission

What is the difference between aerosols and droplets?

Droplets and aerosols refer to the respiratory particles we produce every time we talk, cough, sneeze, or sing. These can range from fine aerosols (less than 5 microns in size, also sometimes called droplet nuclei or small droplets) to medium or large droplets (more than 5 microns in size). Keep in mind that these particles exist on a continuum of sizes, so categories like droplets and fine aerosols aren’t quite as strict and separate as you might imagine. These distinctions can sometimes create a false dichotomy of ‘either/or’ when the reality is more like ‘yes and’.

Research indicates that medium and large droplets tend to fall to the ground or nearest surface. Diseases transmitted through droplets are spread from person-to-person, usually within 6 feet. For example, droplets from an infected person’s mouth can land in another person’s mouth or on their hands that they then use to touch their face. Smaller aerosols, on the other hand, can stay suspended in the air for longer periods of time and travel more than 6 feet.

While these questions are very important for scientists, for most people, focusing on whether the virus is spread through droplets or fine aerosols isn’t particularly helpful. These details won’t change the recommendations to wash your hands, wear your mask, and watch your distance.


General question about aerosol transmission

Does COVID-19 spread by airborne transmission?

Airborne transmission occurs when viruses in fine aerosols remain infectious as they spread over distance and linger over time. Viruses in fine aerosols have the potential to be spread by airborne transmission. However, particle size is only one factor that contributes to potential airborne transmission.

The question of whether SARS-CoV2 is transmitted through the larger droplets or airborne transmission of fine aerosols is mostly important for determining what precautions to take. Wearing surgical or cloth masks can slow the spread of droplets. On the other hand, to prevent long-range airborne transmission, a respirator with a tight seal and better filtering than a cloth mask, like an N95, may be necessary. The modes of transmission are not mutually exclusive. This means that even if SARS-CoV2 spreads primarily via droplets, there could still be airborne spread in certain situations.

Based on evidence from the previous SARS epidemic and the current pandemic, SARS-CoV2 appears to be transmitted through droplets exchanged through close human contact. There is also evidence that some level of airborne transmission. If the virus is also spreading partly through airborne transmission, we may need to implement additional precautions to reduce risk in scenarios where airborne spread is possible (generally crowded indoor spaces). This seems particularly important given the current conversation about reopening schools.

For now, the current recommendations are still the best way for most of us to reduce our risk. Those recommendations — wear a mask, wash your hands, watch your distance, don’t gather indoors, and limit the number of people gathering — will continue to slow the spread of SARS-CoV2 whether it is by droplets, personal contact or airborne transmission. As scientists learn more about how the virus is spread, they can continue to fine-tune their advice.


The debate over “airborne” coronavirus spread, explained (This is the single best detailed explanation of this issue I have seen)

WHO Brief – July 9 2020: Transmission of SARS-CoV-2: implications for infection prevention precautions

How COVID-19 spreads | CDC

Coronavirus: The Difference Between Airborne vs. Aerosol vs. Droplets

Is COVID-19 transmitted by airborne aerosols?

A tiresome SPAT

Protecting ourselves from aerosol transmission

Can asymptomatic people spread COVID-19?

Yes, people can spread the disease even if they do not feel sick, because they are infected and still able to shed the virus. This is one of the characteristics of this virus that contributed to it becoming a pandemic. Multiple studies of COVID-19 have documented spread from people without symptoms. In fact, estimates are that as much as half of the cases of COVID-19 were from people who did not feel sick. 

The use of the words asymptomatic and presymptomatic has caused a lot of confusion. Regardless of which word is correct, the important point to keep in mind as we make our way through this pandemic is that we cannot rely on our perception of health as an indicator that we are not infected. 

Health screening and temperature checks will only catch a portion of active cases. We cannot know that the true level of asymptomatic or presymptomatic spread without more widespread surveillance testing and follow-up with people who get tested.

All of this underscores why it is important to wear masks, wash your hands, and watch your distance even if you feel healthy.


The Silent Spreaders Of COVID-19: Asymptomatic, Presymptomatic, Mildly Symptomatic Cases : Goats and Soda

Can people spread the coronavirus if they don’t have symptoms? 5 questions answered about asymptomatic COVID-19


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