COVID-19 is caused by infection with SARS-CoV-2, a coronavirus that primarily spreads through respiratory particles released by an infected person. When an infected person breathes, talks, coughs, sneezes or otherwise exhales, they can release droplets and much smaller airborne particles containing virus. Another person can become infected by inhaling these particles or when infectious material reaches the mucous membranes of the eyes, nose or mouth. Transmission is therefore primarily a respiratory process rather than something that requires direct physical contact. The risk is affected by how much virus an infected person is releasing, how close people are, how long they spend together, the quality of ventilation and whether infectious particles accumulate in the surrounding air.
Airborne transmission and close-range respiratory exposure are particularly important because very small respiratory particles can remain suspended in air and move through indoor spaces, especially where ventilation is poor. The concentration of infectious particles tends to be greatest close to the infected person, but particles can also accumulate in enclosed areas. Crowded indoor environments, prolonged conversations, shared accommodation, workplaces, classrooms and other settings where many people occupy the same air can therefore facilitate transmission.
Cleaner indoor air, effective ventilation and appropriate filtration reduce the concentration of respiratory particles and consequently reduce exposure. The precise contribution of different particle sizes varies with circumstances, but SARS-CoV-2 should not be regarded simply as a virus transmitted only by large droplets that immediately fall to nearby surfaces.
A person can spread SARS-CoV-2 before symptoms appear, while symptomatic, and sometimes when experiencing no noticeable symptoms. This makes transmission control more difficult than diseases in which people become infectious only after becoming visibly ill. A person who feels completely well can therefore participate in transmission without knowing that they are infected. Infectiousness varies between individuals and over the course of infection.
People with more severe or prolonged infection can sometimes remain infectious for longer, while people with weakened immune systems may shed virus for an extended period. A positive test can provide useful information about the likelihood of current infectiousness, although the relationship between test results, viable virus and transmission is not identical for every type of test.
Close personal contact increases the probability of infection because proximity increases exposure to respiratory particles released by an infected person. Face-to-face interaction is particularly relevant because breathing, speaking and coughing occur close to another person's nose and mouth. Physical contact itself is not necessarily the main mechanism; a handshake does not automatically transmit COVID-19 simply because two hands touch.
The greater concern is that close interaction can place someone in the breathing zone of an infected person or lead to subsequent touching of the face. Surface transmission is biologically possible, but current prevention strategies place much greater emphasis on respiratory exposure, cleaner air, staying away from others when ill, vaccination, hygiene and appropriate masking than on treating every surface as a major source of infection.
The probability of transmission depends on several interacting variables rather than a single universal infectious dose or distance. Duration of exposure, proximity, ventilation, crowding, viral load, infectiousness of the source, susceptibility of the exposed person and the presence of protective measures all influence risk. A brief encounter outdoors in moving air is generally a very different exposure from prolonged close contact in a crowded poorly ventilated room. This is why ventilation is a major public-health intervention. Increasing the supply of outdoor air, improving filtration and reducing accumulation of exhaled air can lower the concentration of infectious respiratory particles. Masking can provide another layer of protection by reducing inhalation, exhalation or both, depending on the type and fit of the mask.
Vaccination does not make infection impossible, because vaccinated people can still become infected and transmit SARS-CoV-2. Its major public-health value is reducing the risk of severe COVID-19, hospitalisation and death, while vaccination can also influence the overall consequences of infection. Consequently, prevention involves multiple layers rather than assuming that one intervention provides complete protection.
Current CDC guidance identifies staying up to date with recommended COVID-19 vaccination, good hygiene and cleaner air as core prevention strategies, with masking, physical distancing and testing providing additional measures in appropriate circumstances. Testing can be particularly useful when someone has symptoms, has been exposed, or needs to make decisions about treatment and contact with vulnerable people.
When someone develops a respiratory illness, reducing contact with other people while infectious can substantially reduce onward transmission. Current CDC respiratory-virus guidance recommends staying home and away from others when respiratory symptoms are not better explained by another cause, returning to normal activities when symptoms are improving overall and the person has been free of fever for at least 24 hours without fever-reducing medicine, and then taking additional precautions for the following five days.
Those additional precautions can include cleaner air, good hygiene, a well-fitting mask, physical distancing and testing when appropriate. The guidance recognises that people can remain capable of transmitting a respiratory virus after they begin feeling better, so improvement in symptoms does not necessarily correspond to an instantaneous end to infectiousness.
COVID-19 transmission is best understood as a dynamic respiratory infection process rather than a simple event caused by touching an infected object. SARS-CoV-2 must reach susceptible tissues and establish infection, after which infected cells produce additional virus and the immune response develops. Transmission can occur from people with obvious symptoms, mild symptoms or no symptoms, and the probability of transmission changes throughout infection.
Practical prevention therefore combines vaccination, cleaner indoor air, staying away from others while acutely ill, hand and respiratory hygiene, appropriate masking, physical distancing and testing when useful. These measures are complementary: ventilation reduces the concentration of airborne particles, masks can reduce exposure, distancing reduces proximity, staying home reduces opportunities for contact, and vaccination primarily reduces the risk of severe consequences.
