Q&A: How is COVID-19 transmitted?

Current evidence suggests that COVID-19 spreads between people through direct, indirect (through contaminated objects or surfaces), or close contact with infected people via mouth and nose secretions. These include saliva, respiratory secretions or secretion droplets. These are released from the mouth or nose when an infected person coughs, sneezes, speaks or sings, for example. People who are in close contact (within 1 metre) with an infected person can catch COVID-19 when those infectious droplets get into their mouth, nose or eyes.

To avoid contact with these droplets, it is important to stay at least 1 metre away from others, clean hands frequently, and cover the mouth with a tissue or bent elbow when sneezing or coughing. When physical distancing (standing one metre or more away) is not possible, wearing a fabric mask is an important measure to protect others. Cleaning hands frequently is also critical.

People with the virus in their noses and throats may leave infected droplets on objects and surfaces (called fomites) when they sneeze, cough on, or touch surfaces, such as tables, doorknobs and handrails. Other people may become infected by touching these objects or surfaces, then touching their eyes, noses or mouths before cleaning their hands. 

This is why it is essential to thoroughly clean hands regularly with soap and water or an alcohol-based hand rub product, and to clean surfaces regularly.

Some medical procedures can produce very small droplets (called aerosolized droplet nuclei or aerosols) that are able to stay suspended in the air for longer periods of time. When such medical procedures are conducted on people infected with COVID-19 in health facilities, these aerosols can contain the COVID-19 virus. These aerosols may potentially be inhaled by others if they are not wearing appropriate personal protective equipment.  Therefore, it is essential that all health workers performing these medical procedures take specific airborne protection measures, including using appropriate personal protective equipment. Visitors should not be permitted in areas where such medical procedures are being performed.

There have been reported outbreaks of COVID-19 in some closed settings, such as restaurants, nightclubs, places of worship or places of work where people may be shouting, talking, or singing.  In these outbreaks, aerosol transmission, particularly in these indoor locations where there are crowded and inadequately ventilated spaces where infected persons spend long periods of time with others, cannot be ruled out.  More studies are urgently needed to investigate such instances and assess their significance for transmission of COVID-19.

Based on what we currently know, transmission of COVID-19 is primarily occurring from people when they have symptoms, and can also occur just before they develop symptoms, when they are in close proximity to others for prolonged periods of time. While someone who never develops symptoms can also pass the virus to others, it is still not clear to what extent this occurs and more research is needed in this area.

Limiting contact with people who are infected with COVID-19, frequent, thorough, hand cleansing and wearing a mask when at least 1 metre of physical distance can’t be guaranteed, help to break chains of transmission.

Yes, infected people can transmit the virus both when they have symptoms and when they don’t have symptoms. This is why it is important that all people who are infected are identified by testing, isolated, and, depending on the severity of their disease, receive medical care.  Even people confirmed to have COVID-19 but who do not have symptoms should be isolated to limit their contact with others.  These measures break chains of transmission.

This is why it is always important to stay at least 1 metre from others, cover your mouth when you cough or sneeze with a bent elbow or tissue, clean hands regularly, and stay home if you become unwell or if asked. In areas where there is widespread transmission, it is also important that people wear a fabric mask where physical distancing and other control measures cannot be implemented.

More information on the use of masks can be found here. 

Yes, both terms refer to people who do not have symptoms. The difference is that asymptomatic refers to people who are infected but never develop symptoms during the period of infection while pre-symptomatic refers to infected people who have not yet developed symptoms but do go on to develop symptoms later. 

This distinction is important for public health strategies to control transmission.  For example, laboratory data suggests that people might be the most infectious at or around the time they develop symptoms.  Therefore, in WHO’s case investigation and contact tracing guidance, it is recommended that people be considered ‘contacts’ if they had contact with an infected person from 2 days before that he/she developed symptoms. 

Yes, COVID-19 is a new disease. While more information becomes available every day, many questions about transmission remain. A vast effort by research teams and networks around the world is underway to answer those questions.

WHO and our partners are working to gain a better understanding about:

• different transmission routes, including through droplets of different sizes, physical contact, fomites, and the role of airborne transmission in the absence of aerosol generating procedures;
• the dose of virus required for transmission to occur;
• the characteristics of people and situations that facilitate superspreading events such as those observed in some closed settings;
• the proportion of infected people who remain asymptomatic throughout the course of their infection;
• the proportion of truly asymptomatic persons who transmit the virus to others;
• the specific factors that drive asymptomatic and presymptomatic transmission;
• and the proportion of all infections transmitted from asymptomatic and presymptomatic individuals.

WHO recommends the following set of measures to prevent person-to-person spread  of COVID-19

• Limit close contact between infectious people and others. Ensure a physical distance of at least 1 metre from others.  In areas where COVID-19 is circulating and this distance cannot be guaranteed, wear a mask.
  
• Identify infected people quickly so that they can be isolated and cared for and all of their close contacts can be quarantined in appropriate facilities.
  
• Clean hands and cover coughs and sneezes with a tissue or bent elbow at all times.
  
• Avoid crowded places, close-contact settings and confined and enclosed spaces with poor ventilation.
  
• Ensure good ventilation in indoor settings, including homes and offices.   
  
• Stay home if feeling unwell and call your medical provider as soon as possible to determine whether medical care is needed.
  
• In countries or areas where COVID-19 is circulating, health workers should use medical masks continuously during all routine activities in clinical areas in health care facilities.
  
• Health workers should also use additional personal protective equipment and precautions when caring for COVID-19 patients. More details for medical professionals are available here and here.
  
• Workplaces should have in place protective measures, details here. 

See more information on how to protect yourself here. 

WHO regularly issues scientific briefs to explain topics in depth for a scientific audience. The brief on COVID-19 transmission summarizes what is known about how the virus spreads between people, who can transmit the virus and when people pass the infection from one to another, and the implications for preventive measures to be adopted. It also outlines some key areas where more research is needed and how these results will help inform advice and guidance.

This information is important for understanding how best to prevent infection and limit the spread of the virus between people.

WHO scientific briefs are living documents, meaning they are updated as more studies become available. COVID-19 is a new disease and we are learning more every day.  

WHO continues to review information provided by published studies, including those that and are available as “pre-prints” (non-peer-reviewed manuscripts that are uploaded to pre-print servers).  WHO also identifies critical questions that need to be answered to understand and improve our response to COVID-19 and guides research efforts on these issues. WHO convenes regular teleconferences with global expert networks of different scientific disciplines to evaluate all available studies and determine how the available evidence, best practices and experience of frontline workers can be translated into guidance and advice.