[1][5] The transmission of the virus is carried out through virus-laden fluid particles, or droplets, which are created in the respiratory tract, and they are expelled by the mouth and the nose.

[6] Droplets that are below a certain critical size, generally thought to be <100μm diameter, evaporate faster than they settle; due to that fact, they form respiratory aerosol particles that remain airborne for a long period of time over extensive distances.

[13][14] The largest droplets of respiratory fluid do not travel far, but can be inhaled or land on mucous membranes on the eyes, nose, or mouth to infect.

[12] Aerosols are highest in concentration when people are in close proximity, which leads to easier viral transmission when people are physically close,[12][13][14] but airborne transmission can occur at longer distances, mainly in locations that are poorly ventilated;[12] in those conditions small particles can remain suspended in the air for minutes to hours.

[7] People are most infectious shortly before and after their symptoms begin[7]—even if mild or non-specific—as the viral load peaks at this time.

[10][34][35][36][37][2][11][38] Spread occurs when the particles are emitted from the mouth or nose of an infected person when they breathe, cough, sneeze, talk, or sing.

[10][3] But breath contains many droplets that smaller than 100 micrometres in size, and these can stay suspended in the air for at least minutes and move across a room.

[5][46][47] This has led to statements that transmission occurs most easily in the "three C's": crowded places, close contact settings, and confined and enclosed spaces.

The turbulent multiphase cloud contributes critically to increasing the range of the pathogen-bearing drops originating in human coughs and sneezes.

[51] Smaller droplets travel a considerable distance as freely suspended tracers and may still get reflected and follow the turbulent cloud.

[53] The risk of transmission from all size droplets and aerosols is lower in indoor spaces with good ventilation.

[55][56] Transmission events occur in workplaces, schools, conferences, sporting venues, dormitories, prisons, shopping facilities, and ships,[57] as well as restaurants,[47] passenger vehicles,[58] religious buildings and choir practices,[59] and hospitals and other healthcare settings.

[46] Airborne transmission also occurs in healthcare settings; long-distance dispersal of virus particles has been detected in ventilation systems of a hospital.

[65][11] There is concern that some medical procedures that affect the mouth and lungs can also generate aerosols, and that this may increase the infection risk.

[71][72][73] There is a research that suggests that variation in airway resistance, as measured by CFD (Computational Fluid Dynamics), may be a useful tool for predicting the forecast of critically ill COVID-19 patients.

[36][11][16][19][24][34] As of July 2020, "no specific reports which have directly demonstrated fomite transmission" although "People who come into contact with potentially infectious surfaces often also have close contact with the infectious person, making the distinction between respiratory droplet and fomite transmission difficult to discern.

[19][36] For example, temperature, humidity, and ultraviolet radiation (sunlight) all influence reductions in viral viability and infectiousness on surfaces.

[36] The initial amount of virus on the surface (i.e., the viral load in respiratory droplets) also affects fomite transmission risk.

[76] Experimental research in laboratory settings also shows that other types of mammals (e.g., voles, rabbits, hamsters, pigs, macaques, baboons) can become infected.

[78][79] Noting the benefits of breastfeeding, the WHO recommends that mothers with suspected or confirmed COVID-19 should be encouraged to initiate or continue to breastfeed, while taking proper infection prevention and control measures.

[81] Public health authorities recommend that people follow practice good hygiene by washing hands with soap and water before preparing and consuming food.

[82] There is also no evidence that COVID-19 transmission to humans occurs through water in swimming pools, hot tubs, or spas.

[84] COVID-19 is not a sexually transmitted infection; while the virus has been found in the semen of people who have COVID-19, there is no evidence that the virus spreads through semen or vaginal fluid,[53] however transmission during sexual activities is still possible due to proximity during intimate activities which enable transmission through other paths.

[88] Traffic vehicles are also a site for transmission, since the control of the pathogen there is harder due to the weak ventilation system and the high density of people.

In January 2022, William Schaffner, professor of infectious diseases at Vanderbilt University Medical Center, compared the contagiousness of the Omicron variant to that of the measles.

[51] Researchers strongly recommend using the elbow or hands to prevent droplet leakage even when wearing face masks during sneezing and coughing.

[99][100][101] On 22 December 2021, the Imperial College COVID-19 Response Team reported an about 41% (95% CI, 37–45%) lower risk of a hospitalization requiring a stay of at least 1 night compared to the Delta variant, and that the data suggested that recipients of 2 doses of the Pfizer–BioNTech, the Moderna or the Oxford–AstraZeneca vaccine were substantially protected from hospitalization.

Many cases of infection broke through, albeit "a bit less than in the control group", even though trial participants had higher antibody levels after the fourth dose.

Professor Paul Morgan, immunologist at Cardiff University said, "I think a blunting rather than a complete loss [of immunity] is the most likely outcome.

"[112] Professor Francois Balloux of the Genetics Institute at University College London said, "From what we have learned so far, we can be fairly confident that – compared with other variants – Omicron tends to be better able to reinfect people who have been previously infected and received some protection against COVID-19.