Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

COVID-19: Prevention

COVID-19 Resources Home Page 
By Matthew E. Levison, MD, Adjunct Professor of Medicine, Drexel University College of Medicine




Prevention of COVID-19

The COVID-19 virus spreads by close person-to-person contact, mainly via respiratory droplets produced when an infected person coughs, sneezes, sings, exercises, or talks. Large respiratory droplets spread within 6 feet of a contagious person, but the virus can sometimes be spread more than 20 feet via small respiratory particle aerosols that can linger in air for several hours and infect people separated by distances previously considered safe. Spread of the virus could also occur via contact with a surface contaminated by respiratory droplets. It is known that asymptomatic and presymptomatic people, as well as symptomatic patients, can transmit the virus, making it difficult to control an outbreak.

The most important preventive measure is avoidance of exposure to COVID-19 virus by means of

  • Respiratory and contact precautions
  • Quarantine and isolation

Respiratory precautions involve using face masks. The CDC recommends wearing a face mask or cloth face covering over the mouth and nose when in public settings and when around people who do not live in the same household.

Two types of face masks are available, surgical (whether medical products or other cloth masks) and N-95. Surgical face masks and cloth masks reduce the risk of getting infected. However, surgical masks do not fit tightly enough to definitively protect uninfected people from inhaling infected respiratory emissions. Thus, people in contact with patients infected with COVID-19 (eg, health care providers, household members) should wear N-95 masks, which fit very tightly, and protect the wearer from airborne respiratory emissions. Patients infected with COVID-19 should wear a surgical mask, which helps contain their respiratory secretions, thus protecting others.

Contact precautions include

  • Maintaining good social distance (about 6 feet)
  • Avoiding close contact with people who have COVID-19
  • Avoiding touching one’s eyes, nose, and mouth with unwashed hands
  • Washing hands often with soap and water for at least 20 seconds or using an alcohol-based hand sanitizer that contains at least 60% alcohol if soap and water are not available.
  • Using disposable wipes to routinely clean environmental surfaces that are frequently touched by multiple people (eg, doorknobs, bathroom fixtures, keyboards, elevator buttons)
  • Avoiding indoor restaurants and large parties or social gatherings at people's homes where people will be eating, drinking, and talking to each other in close proximity not wearing face mask, as well as indoor sporting events where people are cheering and shouting. These are high-risk venues for a “superspreader” event in which a single or a few infected people can trigger a large outbreak. Recommendations are that such events where large numbers of people can be exposed in a high-risk setting, such as such as nightclubs and bars, festivals, conferences, and sporting events be postponed until the level of community transmission is low, especially for people with a higher risk of developing severe cases of COVID-19 (1).


Quarantine is essential for well individuals who had close contact with a COVID-19-infected patient. Close contact is defined as within 6 feet of an infected person for a total of 15 minutes or more over a 24-hour period starting from 48 hours before the onset of symptoms or positive test (if asymptomatic). This exposure could be fifteen 1-minute exposures over the 24-hour period. Individuals who have had close contact with a COVID-19 infected person should quarantine at home for the duration of the incubation period (ie, 14 days after the last exposure).

Isolation is essential for those infected with the COVID-19 virus or those who have COVID-19 symptoms who have not been tested. For patients, illness severity helps determine whether they are isolated in a hospital or at home.

  • Those who had mild to moderate symptoms can be discontinue isolation for 10 days after symptom onset provided they have been afebrile for ≥ 24 hours without the use of fever-reducing drugs and whose other symptoms are significantly improving.
  • Asymptomatic patients can discontinue isolation 10 days after the date of their first positive COVID-19 diagnostic test.


  1. Centers for Disease Control and Prevention: Coronavirus Disease 2019 (COVID-19): Considerations for Events and Gatherings. Atlanta, GA, US Department of Health and Human Services, Centers for Disease Control and Prevention. Updated January 8, 2021. Accessed January 16, 2021.

COVID-19 Resources Home Page