COVID-19 spreads primarily through the air when an infected person breathes, talks, coughs, or sneezes. Tiny virus-carrying particles leave the nose and mouth, float in the air, and enter another person’s airways. This is by far the dominant route of transmission, though the virus can also spread through eye exposure and, very rarely, contaminated surfaces.
Airborne Particles Are the Main Route
When someone with COVID exhales, they release a range of respiratory particles. Larger droplets (bigger than about 5 micrometers across) are heavier and tend to fall to the ground within a few feet. Smaller particles, called aerosols, are light enough to stay suspended in the air for extended periods and travel much farther. Both sizes can carry infectious virus, but the aerosols are what make COVID so contagious indoors. A cough or sneeze can send virus-laden particles 7 to 8 meters (roughly 25 feet) from the source, well beyond the old “6 feet apart” guideline.
This is why indoor spaces with poor ventilation pose the highest risk. Aerosols accumulate in still air the same way cigarette smoke fills a closed room. Outdoors, the risk drops dramatically. Quantitative modeling published in Environmental Research found that outdoor transmission risk is typically orders of magnitude lower than indoor risk, because wind disperses particles quickly. The only outdoor scenarios where risk approaches indoor levels involve very calm air, very stable atmospheric conditions, and dense crowds.
When an Infected Person Is Most Contagious
Not every stage of a COVID infection carries the same transmission risk. Viral shedding peaks between one day before symptoms start and about five days after. During that window, roughly 44% to 50% of nasal or throat samples from infected people contain live, culturable virus. After that, infectiousness drops sharply: by day 7, only about 28% of samples still grow virus in the lab, and by day 9, that falls to 11%. After day 10, the rate hovers between 0% and 8%. Live virus has been detected as early as four days before symptoms and as late as 18 days after, but those extremes are uncommon.
The practical takeaway: people are most likely to spread COVID in the days right around symptom onset, including the day or two before they feel sick. This is one reason the virus spread so effectively through populations, since people were contagious before they had any reason to stay home.
Spread Without Symptoms
A significant share of people infected with SARS-CoV-2 never develop symptoms at all. A global meta-analysis covering nearly 15,000 people found that about 44% remained asymptomatic throughout their entire infection. These individuals still carry and shed the virus, though typically at lower levels than people who become symptomatic.
The asymptomatic rate varies by age. It peaks around 36% in teenagers (around age 13 to 14), gradually decreases through middle age, and drops to roughly 8% in people over 90. This age pattern helps explain why schools and workplaces became such common transmission settings: younger people were more likely to carry the virus silently while interacting with others all day.
Surface Transmission Is Extremely Rare
Early in the pandemic, people wiped down groceries and left packages outside for days. That level of caution turned out to be unnecessary. The CDC concluded that each contact with a contaminated surface carries less than a 1 in 10,000 chance of causing an infection. Surface transmission does happen in theory, because the virus can survive on certain materials for hours, but it is not a meaningful driver of spread compared to breathing in airborne particles.
Eyes as an Entry Point
The virus can enter your body through your eyes, though this route is far less common than inhalation. The cells lining the surface of the eye (the conjunctiva and cornea) carry the same receptor the virus uses to get into respiratory cells. A review of 172 studies across 16 countries found that wearing eye protection made COVID transmission about three times less likely among healthcare workers. That said, studies testing tear samples from infected patients found little to no virus, suggesting the eyes are a possible but low-probability entry point for most everyday situations.
How the Virus Gets Into Your Cells
Once virus particles reach your airways (or, less commonly, your eyes), they latch onto a specific protein on the surface of your cells called ACE2. Think of ACE2 as a lock, and a spike on the virus surface as the key. After the spike protein binds to ACE2, enzymes on your cell’s surface cut the spike in a way that lets the virus fuse with the cell membrane and slip inside. Once in, the virus hijacks the cell’s machinery to make copies of itself, which then spread to neighboring cells and repeat the cycle.
ACE2 receptors are abundant in the nose, throat, and lungs, which is why the respiratory tract is the virus’s primary target. They also appear in the heart, kidneys, and intestines, which helps explain why COVID can sometimes affect organs beyond the lungs.
Omicron and Increased Contagiousness
The original strain of SARS-CoV-2 was already quite transmissible, but the Omicron variant, which became dominant in late 2021, took contagiousness to another level. Omicron’s basic reproduction number (the average number of people one infected person spreads the virus to in a fully susceptible population) averaged around 9.5, with estimates ranging from 5.5 to 24 depending on the setting. That made it roughly 2.5 times more transmissible than the Delta variant in terms of basic reproduction and 3.8 times higher in real-world spread. For context, seasonal influenza has a reproduction number around 1.3, and the original COVID strain was estimated at 2 to 3.
Omicron’s increased contagiousness came from mutations in its spike protein that made it better at binding to cells and better at evading immunity from prior infection or vaccination. The fundamental transmission route stayed the same: airborne particles, inhaled by another person.
How Ventilation and Airflow Reduce Risk
Because COVID spreads through the air, the single most effective environmental measure is improving ventilation. The CDC recommends aiming for at least 5 air changes per hour in public indoor spaces, either through outdoor air ventilation alone or through a combination of ventilation, filtration (such as HEPA filters), and other air cleaning methods. Each air change replaces a portion of the room’s air, diluting any floating virus particles.
Opening windows, running exhaust fans, and using portable air purifiers with HEPA filters all contribute to this goal. Spaces with high ceilings and good cross-ventilation naturally achieve more air mixing. Small, enclosed rooms with recirculated air (think waiting rooms, break rooms, or poorly ventilated bars) pose the greatest indoor risk.
Vaccination and Transmission
COVID vaccines were designed primarily to prevent severe illness and death, which they do very effectively. Their impact on transmission has been more complicated and has shifted with new variants. During the Alpha wave in early 2021, modeling estimated that full vaccination reduced overall transmission risk by about 88.5%, combining both a lower chance of getting infected and a lower chance of spreading the virus if infected. In household studies from that period, the infection rate within a pair was 10% if at least one person was vaccinated, compared to 30% if both were unvaccinated.
That picture changed with the Delta variant. One household study found that fully vaccinated people who got breakthrough infections spread the virus to close contacts at nearly the same rate as unvaccinated people (25% versus 23%). Vaccination still reduced the likelihood of getting infected in the first place, but once a breakthrough infection occurred, the person was similarly contagious. This pattern has continued with Omicron subvariants, reinforcing that vaccination is most valuable for protecting you from serious illness rather than preventing you from spreading the virus to others.