How COVID-19 spreads
COVID-19 can spread in many different ways. It can transmit from person to person both directly and indirectly. It can also transmit from animals to humans and from humans to animals.
SARS-CoV-2 (the virus that causes COVID-19) was likely first transmitted to humans either directly or indirectly from Rhinolophus horseshoe bats. Research has shown that a virus present in these bats shares close genetic similarity with SARS-CoV-2.
Regardless of the exact origins, we know that an ancestral virus evolved over time into SARS-CoV-2, a virus capable of infecting human hosts. From the initial spread from animals to humans, SARS-CoV-2 has continued to spread from person to person both directly and indirectly.
COVID-19 can spread from person to person directly through respiratory droplets. SARS-CoV-2 is enveloped in a droplet in a person’s respiratory tract and expelled in various sizes into the air from the infected person’s mouth or nose. Both symptomatic and asymptomatic people can produce these droplets when they sneeze, cough, talk or even breathe. If these droplets land on someone else, they can transmit the infection directly. Some of these droplets are small enough that when they are released, they evaporate and convert to particles. These small particles can travel through the air and remain viable for approximately three hours, during which time the particles can be inhaled and cause infection in the lungs and other organs. The droplets and particles have been found to travel up to about 25 feet after a sneeze and 6 feet after a cough, which is why physical distancing is so important.
Spending time with others outdoors instead of indoors is also an effective measure to prevent infection. Airflow varies based on wind conditions outside and the degree of ventilation inside, and tends to be greater outside. Increased airflow more quickly disperses air, decreasing the concentration of viral particles and reducing the risk of infection.
Face masks are also critical measures to prevent infection since they can block most of the respiratory droplets and particles emitted by an infected person. Face masks can also protect the wearer from inhaling respiratory droplets and particles. Overall, physical distancing, being outdoors and face masks can all reduce the risk of direct COVID-19 transmission.
In addition to the direct spread of the virus, COVID-19 can be transmitted via shared surface contact. For example, infected individuals can contaminate nearby surfaces by sneezing or coughing on them. Respiratory particles can remain in the air for hours, but on surfaces for up to three days.
The amount of time the virus is able to survive on these surfaces is dependent on the surface type. SARS-CoV-2 has been found to survive the longest on plastic and stainless steel, lasting for up to 72 hours after initial contact. Conversely, the virus can typically last up to 24 hours on cardboard.
Contaminated surfaces can be touched by healthy individuals, resulting in the spread of the viral particles. When an individual touches an infected surface, the germs can spread to their hands and fingers. This individual can then become infected if and when they touch their face with their infected hand, transmitting the virus into their mouth, nose or eyes. Good hand hygiene and intentional avoidance of surface contact are some protection mechanisms against the indirect transmission of COVID-19.
As we continue to learn more about COVID-19 and the way in which the virus spreads, researchers have begun investigating the possibility of direct or indirect transmission from humans back to animals. This human-to-animal transmission has recently been confirmed in mink farms across the globe.
In November, over 10 million mink were killed in Denmark after human COVID cases were discovered that seemed to result from direct or indirect contact with infected mink. One reason these mink were killed is that SARS-CoV-2 appears to evolve in mink into a variant with a mutated spike protein.
Spreading among humans of viral variants with alterations in the spike protein could result in less effective vaccination because the leading vaccines specifically prepare our immune system to recognize the spike protein of SARS-CoV-2 on infection. If the spike protein changes, our immune system might not recognize it as well.
In many countries, including the United State, COVID outbreaks in farmed mink have resulted in strict quarantines of the animals as well as a halted distribution of the animal products. It is suspected that infected workers are transmitting the virus to the mink, which then spread it to the other animals they come into contact with and also back to humans. More research, however, is needed to better understand the ways in which COVID is being transmitted across these different groups.
As for household pets, only a small number worldwide have tested positive for COVID.
Continued investigation will provide a more complete understanding of the frequency of and implications of animal-to-human transmission of COVID. Although its transmission is not fully understood, COVID-19 can spread between and amongst both humans and animals, directly and indirectly. The best way to avoid contracting COVID from a carrier is to practice the safety measures of social distancing, mask wearing, and hand sanitizing.
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