COVID-19 is a respiratory illness caused by the SARS-CoV-2 virus, which emerged in late 2019. Understanding how this virus spreads is important for public health. The virus can be highly contagious and spread rapidly among individuals. This knowledge allows for the implementation of measures that can help reduce transmission and protect communities.
Primary Modes of Spread
The SARS-CoV-2 virus primarily spreads between people through various respiratory pathways. Infected individuals release virus-laden fluid particles, or droplets, from their mouth and nose when they breathe, talk, cough, sneeze, or sing. These particles vary in size, influencing how they travel and cause infection.
Airborne Transmission
Airborne transmission occurs when tiny respiratory droplets, called aerosols, remain suspended in the air. These particles are less than 5 micrometers in diameter and can travel over distances greater than 1 meter, lingering in the air for extended periods, sometimes hours, especially in indoor spaces with poor ventilation. Inhaling these aerosols directly into the respiratory tract can lead to infection.
Droplet Transmission
Droplet transmission involves larger respiratory droplets, greater than 5-10 micrometers in diameter, that are expelled by an infected person. These larger droplets do not remain airborne for long distances and tend to fall out of the air quickly, usually within about 6 feet or 2 meters of the source. Infection occurs when these droplets land directly on the mucous membranes of another person’s mouth, nose, or eyes.
Contact Transmission
Contact transmission can also occur, though it is a less common route than airborne or droplet spread. Direct contact involves physical touch with an infected individual. Indirect contact, or fomite transmission, happens when a person touches a surface or object contaminated with the virus (a fomite) and then touches their own eyes, nose, or mouth before cleaning their hands. The risk of infection from contaminated surfaces is low.
Factors Influencing Spread
Several environmental and situational factors influence the likelihood of COVID-19 transmission, even when the primary modes of spread are present. These conditions can either concentrate or disperse viral particles, affecting the risk of exposure.
Poor Indoor Ventilation
Poor indoor ventilation increases the risk of transmission because it allows virus-laden aerosols to accumulate in the air. Conversely, good air circulation, such as that provided by open windows, doors, or mechanical ventilation systems like HVAC, helps to dilute and disperse these viral particles, reducing their concentration.
Crowding and High Population Density
Crowding and high population density in a space increase the chances of exposure to infected individuals and make physical distancing difficult. Outbreaks have been reported in various crowded indoor settings, including restaurants, fitness classes, and places of worship, where people may be talking loudly, shouting, or singing, which increases the emission of respiratory fluids.
The Duration of Exposure
The duration of exposure to an infected person also plays a role in transmission risk. Longer periods spent in close proximity to someone with the virus, particularly in indoor settings, increase the cumulative exposure to viral particles and, consequently, the likelihood of infection.
Indoor Settings
Indoor settings pose a higher risk of transmission compared to outdoor environments. Outdoors, natural ventilation and air dilution help to quickly disperse viral particles, making it much less likely for them to reach infectious concentrations.
Reducing Transmission Risk
Implementing a combination of strategies is effective in minimizing the spread of COVID-19. These measures help to interrupt the transmission pathways and protect individuals and communities.
Vaccination
Vaccination plays a primary role in reducing both severe illness and the transmission of SARS-CoV-2. Studies indicate that fully vaccinated individuals are less likely to transmit COVID-19 to their contacts compared to unvaccinated individuals. Vaccination has shown effectiveness against transmission, even with newer variants.
Wearing Well-Fitting Masks
Wearing well-fitting masks is another effective strategy to reduce the spread of respiratory droplets and aerosols. Masks act as a source control by blocking the exhalation of virus-containing particles from an infected person, which is important since people can spread the virus even without symptoms. N95 and KN95 respirators, along with surgical masks, offer protection.
Maintaining Physical Distancing
Maintaining physical distancing helps reduce exposure to respiratory droplets. The recommended distance is 6 feet or 2 meters between individuals who are not from the same household. This practice lessens the opportunity for direct person-to-person transmission, especially since infected individuals can be asymptomatic or pre-symptomatic.
Frequent Hand Hygiene
Frequent hand hygiene, using soap and water for at least 20 seconds or an alcohol-based hand sanitizer with at least 60% alcohol, is an effective defense against the virus. This practice helps to remove the virus from hands, reducing spread, especially if contaminated surfaces are touched.
Improving Ventilation
Improving ventilation in indoor spaces helps to disperse viral particles and reduce their concentration. This can be achieved by opening windows and doors to increase natural airflow, using fans to enhance air circulation, and operating exhaust fans in kitchens and bathrooms. Running HVAC systems continuously and using exhaust fans in bathrooms and kitchens can further aid in removing contaminated air. Portable air purifiers with HEPA filters can also remove airborne particles.
Testing for COVID-19
Testing for COVID-19 when experiencing symptoms or after exposure is important for early detection. If a test result is positive, staying home and isolating from others helps prevent further spread of the virus. Individuals remain infectious for up to 10 days, with the highest infectiousness occurring in the two days before symptoms appear and while acute symptoms are present.