COVID-19, caused by the SARS-CoV-2 virus, became a global pandemic because it is a highly contagious respiratory illness. The spread of the virus from person to person is a complex process involving how the virus travels, the conditions that aid its spread, and how it has changed over time. The ability of SARS-CoV-2 to transmit effectively is a result of multiple factors, from its biological properties to the environments where people gather.
Primary Modes of Transmission
The primary way SARS-CoV-2 spreads is through the air. When an infected person breathes, talks, coughs, or sneezes, they release fluid particles from their respiratory tract containing the virus. These particles are categorized into two types: larger respiratory droplets and smaller aerosols, with the distinction based on particle size.
Respiratory droplets are larger and heavier, causing them to fall out of the air quickly and within a short distance of the infected person. Transmission occurs when these droplets land on the mucous membranes of a person nearby, such as the mouth, nose, or eyes. This is why close contact with an infected individual increases the risk of exposure.
Aerosols are much smaller particles that can remain suspended in the air for extended periods. These particles can travel much farther than larger droplets, posing a risk of airborne transmission. This mode of transmission is most efficient in enclosed, poorly ventilated indoor spaces, where the concentration of viral particles can build up.
Another mode involves contact with contaminated surfaces, known as fomites. This happens when a person touches a surface with the virus on it and then touches their own mouth, nose, or eyes, though this is not the main way the virus spreads. Distinguishing between transmission from respiratory droplets and fomites can be difficult, as close contact with an infected person often involves both potential exposures.
Key Factors Affecting Transmission Risk
The environment significantly influences the risk of transmission. Crowded, poorly ventilated indoor spaces are the highest-risk settings because virus-containing aerosols can accumulate in the air. Outdoor settings are safer because air circulation disperses viral particles more effectively. Improving ventilation, for example by opening windows and doors or using air filtration systems, can reduce the concentration of viral particles indoors.
Proximity and duration also affect transmission risk. Being physically closer to an infected person for a longer period increases the likelihood of inhaling infectious particles. This is why “close contact” is a recurring theme in public health guidance. Activities like singing or shouting, which involve heavy breathing, can further elevate risk in close-proximity settings.
The amount of virus an infected person carries and sheds, known as their viral load, is another factor. An individual with a higher concentration of the virus in their respiratory system is likely to release more viral particles when they breathe or talk, making them more contagious. Viral load is highest around the time symptoms first appear and for the first few days of illness, corresponding to the period of highest infectiousness.
Viral Evolution and Increased Contagiousness
Viruses mutate as they replicate, and SARS-CoV-2 is no exception. These changes can result in new variants of the virus that are more contagious than previous strains. Over the course of the pandemic, several variants of concern have emerged, including Alpha, Delta, and Omicron, each demonstrating an increased ability to spread.
The enhanced transmissibility of these variants is linked to mutations in the virus’s spike protein, which it uses to attach to and enter human cells. For example, the Alpha variant was estimated to be 50-100% more transmissible than the original virus. The subsequent Delta variant was about 60% more transmissible than Alpha, partly due to mutations that improved the spike protein’s ability to bind to human cells.
The Omicron variant displayed an even greater leap in transmissibility, spreading more rapidly than Delta. It possessed a large number of mutations on its spike protein, which are believed to have increased its ability to infect human cells. This continuous evolution, leading to variants that are better at spreading, is a reason the virus has remained a persistent public health challenge.
Asymptomatic and Presymptomatic Transmission
A challenging aspect of controlling COVID-19 is its ability to spread from people who do not appear sick. This occurs through presymptomatic and asymptomatic transmission. Presymptomatic transmission is the spread of the virus by an infected individual in the days before they develop symptoms.
Asymptomatic transmission occurs when an individual with a confirmed infection never develops symptoms but still passes the virus to others. While people without symptoms may be less infectious than those with symptoms, they can still contribute to community spread, often because they are unaware they are infected and continue their normal activities. Studies have found that individuals with asymptomatic infections can have similar viral loads to those with symptoms.
The existence of both presymptomatic and asymptomatic spread has been a major factor in the rapid transmission of the virus. This means that simply identifying and isolating people who are visibly ill is not enough to stop the chain of transmission, as individuals can be contagious without any outward signs of illness.