Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link


(Coronavirus Disease 2019; COVID)


Brenda L. Tesini

, MD, University of Rochester School of Medicine and Dentistry

Last full review/revision Feb 2022| Content last modified Jun 2022
Topic Resources

COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2.

  • Symptoms of COVID-19 vary significantly.

  • Two types of tests can be used to diagnose a COVID-19 infection.

  • Measures should be taken to prevent infection, particularly vaccination, social distancing, and masking.

  • Treatment of COVID-19 depends on how severe the illness is and the likelihood that the person will develop severe disease.

COVID-19 was first reported in late 2019 in Wuhan, China and has since spread extensively worldwide. For current information on the number of cases and deaths, see the Centers for Disease Control and Prevention: 2019 Novel Coronavirus and the World Health Organization Coronavirus (COVID-19) Dashboard.

Social determinants of health (conditions in the places where people are born, live, learn, work, and play) impact a wide range of health risks and outcomes, such as exposure to SARS-CoV-2 infection, severe COVID-19, and death, as well as access to testing, vaccination, and treatment (see CDC: Risk for COVID-19 Infection, Hospitalization, and Death By Race/Ethnicity). In the United States, COVID-19 case, hospitalization, and death rates are higher in some racial and ethnic minority groups, including among people who are Black, Hispanic or Latino, American Indian, and Alaska Native.

Transmission of COVID-19

COVID-19 is mainly spread from person to person through respiratory droplets that are produced when an infected person coughs, sneezes, sings, exercises, or talks. The virus spreads through both large respiratory droplets that can travel short distances and through small respiratory particle aerosols that can linger in the air for several hours and travel longer distances (> 6 feet) before being inhaled.

In general, the closer and longer the interaction with an infected person, the higher the risk of the virus spreading. Factors such as distance from an infected person, duration of time in the presence of an infected person, the size of the air space, and the direction and speed of airflow can contribute to this risk. People may also get a COVID-19 infection by touching something that has the virus on it and then touching their own mouth, nose, or eyes.

The virus is usually transmitted by a person with symptoms of the infection. However, the virus can be transmitted by people before they exhibit symptoms (presymptomatic) and even by people who are infected but never develop symptoms (asymptomatic).

Situations with high risk of transmission include congregate living facilities (for example, nursing homes, long-term care facilities, residential schools, prisons, ships) and crowded, poorly ventilated environments (such as indoor religious services, gyms, bars, night clubs, indoor restaurants, and meat-packing facilities). The residents of nursing homes are also at high risk of severe disease because of age and underlying medical disorders.

The recent Delta and Omicron variants of the SARS-CoV-2 virus are more readily transmitted than the earlier variants (see CDC: Delta Variant: What We Know About the Science). On November 30, 2021, the United States designated Omicron as a "variant of concern." The Centers for Disease Control and Prevention (CDC) has been collaborating with global public health partners to learn about Omicron. (See also CDC: Omicron Variant: What You Need to Know.)

Symptoms of COVID-19

Symptoms vary in people with COVID-19. Most people infected with COVID-19 have mild symptoms or no symptoms at all, but some become severely ill and die. Symptoms can include the following:

  • Fever

  • Cough

  • Sore throat

  • Congestion or runny nose

  • Shortness of breath or difficulty breathing

  • Chills or repeated shaking with chills

  • New loss of smell or taste

  • Fatigue

  • Muscle pain

  • Headache

  • Nausea or vomiting

  • Diarrhea

If people develop symptoms, they usually appear about 2 to 14 days after being infected. For the Omicron variant, symptoms usually appear in only 2 to 4 days.

The risk of serious disease and death in people with COVID-19 increases with age, in people who smoke, and in people with other serious medical disorders, such as cancer, heart, lung, kidney, or liver disease, sickle cell disease, diabetes, obesity, or immunocompromising disorders. For all age groups, risk of serious disease and death decreases in people who are vaccinated against COVID-19 Vaccination COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more .

In addition to respiratory disease that can be severe and lead to death, other serious complications include

A rare complication called multisystem inflammatory syndrome in children (MIS-C) that may be linked to COVID-19 has been reported in children. Symptoms of this can be similar to the rare condition Kawasaki disease Kawasaki Disease Kawasaki disease causes inflammation of blood vessels throughout the body. The cause of Kawasaki disease is unknown but may be associated with an infection. Children typically have fever, rash... read more Kawasaki Disease and include fever, abdominal pain, and rash. Vaccination helps protect against developing MIS-C. A similar complication has been reported in young and middle-aged adults (multisystem inflammatory syndrome in adults [MIS-A]).

In most people, symptoms resolve over about a week. However, in a few people, symptoms last longer, most often with shortness of breath, cough, and extreme fatigue, sometimes persisting for weeks. In 25 to 50% of people with COVID-19, symptoms last for months. This has been referred to by many names, including long COVID, long-haul COVID, and post-acute COVID-19 syndrome or condition.

Although infection with coronaviruses is generally believed to provide some degree of immunity to reinfection, the duration and effectiveness of immunity following COVID-19 remain unknown. Symptoms associated with reinfection tend to be similar to or milder than the initial infection.

Diagnosis of COVID-19

  • Tests to identify the virus

Doctors suspect COVID-19 in people who have symptoms of the infection. Recent close contact with someone who has COVID-19 increases the likelihood of infection. People who suspect they may have COVID-19 should call their doctor before being tested and before arriving at a clinic so appropriate precautions can be taken.

The following people should be tested for COVID-19 (also see COVID-19 Testing):

People who took part in activities that put them at higher risk for COVID-19, such as attending large social gatherings or being in crowded indoor settings without correct and consistent masking, may also want to be tested.

There are two types of tests to diagnose COVID-19 infection:

  • NAAT (nucleic acid amplification test)

  • Antigen tests

There are multiple types of NAATs. RT-PCR (real-time reverse transcriptase–polymerase chain reaction) tests, often simply called PCR tests, are the type of NAAT that have the highest sensitivity and specificity, meaning they are more accurate and thus the preferred initial test for COVID-19. RT-PCR tests can be done on upper and lower respiratory secretions (samples from nasal or oral swabs or saliva) to identify the virus.

Antigen tests can be done at home or in a health care setting, but they are generally less accurate than NAATs, including RT-PCR tests. Therefore, it may be necessary to confirm some antigen test results (for example, a negative test in a person with symptoms) with a RT-PCR or other NAAT. Many antigen test kits also recommend repeating the test over several days to increase the likelihood of detecting infection. Also, some tests may not detect the Omicron variant (see FDA: SARS-CoV-2 Viral Mutations: Impact on COVID-19 Tests).

Another type of test is called an antibody test. Antibody tests (also called serologic tests) are not used for diagnosing current infections. Antibody tests help determine whether the person being tested was previously infected, which is important for tracking cases and studying the virus.

Prevention of COVID-19

The best way to prevent infection is to be up to date with vaccinations Vaccination COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more . In addition to being vaccinated, people should avoid being exposed to the virus by taking steps recommended by the Centers for Disease Control and Prevention (CDC). The CDC varies its recommendations regarding prevention measures based on COVID-19 Community Levels. Levels can be low, medium, or high and are determined by looking at hospital beds being used, hospital admissions, and the total number of new COVID-19 cases in an area.

Masking and other routine measures

For people age 2 years or older, the Centers for Disease Control and Prevention (CDC) provides the following guidance regarding wearing a well-fitting face mask, covering both mouth and nose:

  • Everyone, regardless of vaccination status, should wear a mask when in indoor public places in areas where the COVID-19 Community Level is high.

  • People who are at increased risk for severe illness, or who live with or spend time with someone at higher risk, should wear a mask in areas where the COVID-19 Community Level is medium, if advised by a health care practitioner to wear one.

  • Everyone should wear a mask when sick and around other people.

  • Everyone should wear a mask when caring for someone who has COVID-19.

  • Everyone should wear a mask when on public transportation (for example, planes, buses, trains) and while indoors at transportation hubs (for example, airports, train stations) regardless of COVID-19 Community Level.

In addition to following the CDC recommendations, people may be required to wear a mask by local laws, regulations, or rules or business or workplace guidance, and this may vary by vaccination status. People who are at increased risk for severe disease or who have someone in their household at increased risk might choose to wear a mask regardless of any requirements or the COVID-19 Community Level. People who are at increased risk for severe disease include those who are unvaccinated, have a weakened immune system, have an underlying medical condition, are pregnant or recently pregnant, and people older than 65 (Different Groups of People at Increased Risk for Severe Illness). Different types of masks provide different levels of protection, including (in increasing order of protection): multi-layer cloth masks; multi-layer surgical masks and K95 masks; and N95 masks (see CDC: Types of Masks and Respirators).

In addition to being up to date with vaccinations and wearing a mask, the CDC recommends the following steps to help prevent the spread of COVID-19, regardless of COVID-19 Community Level:

Quarantine and isolation

To help prevent the spread of COVID-19, the Centers for Disease Control and Prevention (CDC) recommends quarantine and isolation measures. (See also CDC: Quarantine and Isolation.)

Quarantine is meant to separate people who had "close contact" with a contagious person so they do not infect other people.

  • Close contact is having been within 6 feet of a person with COVID-19 for a total of 15 minutes or more during a 24-hour period. An infected person can start spreading the infection 2 days before symptoms start or before testing positive for COVID-19.

  • For students in kindergarten through 12th grade in an indoor or outdoor class setting where masks were worn correctly and consistently, close contact is defined as having been within 3 feet of an infected person (see CDC: Close Contact).

Quarantine begins on the day of the close contact, which is considered Day 0 (counting days of quarantine starts on Day 1). People who are not up-to-date on COVID-19 vaccinations should quarantine through Day 5 and wear a well-fitting mask through Day 10. If quarantine is not possible, the person should wear a well-fitting mask at all times when around others through Day 10.

The following people who had close contact with an infected person do not need to quarantine but should wear a well-fitting mask through Day 10:

  • People who are up-to-date on COVID-19 vaccinations

  • People who had COVID-19 infection (confirmed by a positive test) within 90 days prior to exposure

Exposed people should have a COVID-19 test 5 to 7 days after having a close contact, regardless of whether they have symptoms or are vaccinated. If the exposed person has symptoms that could be from COVID-19, that person should immediately isolate until getting a negative COVID-19 test.

Isolation separates people with confirmed or suspected COVID-19 from those without COVID-19. The CDC recommends isolation for people with COVID-19 symptoms and/or who have a positive COVID-19 test. People who are in isolation should stay home and separated from others, or wear a well-fitting mask when they need to be around others in the home.

Isolation should begin the day symptoms start or the day a person tests positive for COVID-19, which is considered Day 0 (counting days of isolation starts on Day 1) and lasts at least through Day 5.

  • People may stop isolating on Day 6 as long as the person has been fever-free for at least 24 hours, without the use of fever-reducing drugs, and the symptoms are getting better. They should wear a well-fitting mask through Day 10 when around others.

  • If a person has access to a COVID-19 test, the test may be done on or after Day 5 of the isolation period. If the test result is positive, isolation should continue through Day 10. If the test result is negative, isolation may end, but a well-fitting mask should be worn around others at home and all other places through Day 10.

  • People who were severely ill should isolate through Day 10 at least.


Vaccination is the most effective way to prevent severe illness and death from COVID-19, including from the Delta and Omicron variants. For a detailed discussion of the vaccines approved in the United States, see COVID-19 Vaccine COVID-19 Vaccine Coronavirus disease 2019 (COVID-19) vaccines provide protection against COVID-19. COVID-19 is the disease caused by infection with the SARS-CoV-2 virus. There are multiple COVID-19 vaccines... read more .

Following the recommendations for vaccination is very important for people's own health and for the health of their family and the people in their community. Multiple COVID-19 vaccines are currently in use worldwide (see COVID-19 Vaccine Tracker). Two mRNA vaccines (BNT162b2 and mRNA-1273) and one adenovirus vector vaccine (Ad26.COV2.S) are used in the United States. Vaccines available in the United States are administered as a primary series of 1, 2, or 3 injections (see CDC: Use of COVID-19 Vaccines in the United States).

Protection against infection from a primary vaccination series has been shown to decrease over time. To maximize protection against infection, severe disease, and death, booster doses are recommended. People who have received a booster dose when eligible are considered "up-to-date" on their vaccine series. Booster doses are recommended beginning 2 to 5 months after the primary series is completed, depending on age group, risk factors for severe COVID-19, and type of vaccine administered in the primary series. (See also CDC: COVID-19 Vaccine Booster Shots.)

In most situations, the mRNA vaccines are preferred over the vector vaccine for primary series and booster doses due to the slight risk of serious side effects with the vector vaccine (see CDC: Johnson & Johnson’s Janssen COVID-19 Vaccine Overview and Safety).

Treatment of COVID-19

  • Drugs to relieve fever and muscle aches

  • Sometimes, remdesivir and/or dexamethasone

  • Sometimes, antiviral drugs for mild to moderate infection

  • Sometimes, monoclonal antibodies

Treatment of COVID-19 depends on how severe the illness is and the likelihood that the person will develop severe disease.

For mild illness, resting at home is often sufficient. Acetaminophen or a nonsteroidal anti-inflammatory drug (NSAID), such as ibuprofen, may be taken to relieve fever and muscle aches. Despite initial anecdotal concerns, there is no scientific evidence that the use of NSAIDs worsens COVID-19. Similarly, there is no scientific evidence that people with COVID-19 who take the blood pressure drugs called angiotensin-converting enzyme inhibitors (ACEI) or angiotensin receptor blockers (ARBs) should stop taking them.

For people with severe illness from COVID-19 or people at high risk for progression to severe disease, some drug and other therapies are recommended. This is a rapidly evolving topic (see National Institutes of Health (NIH) COVID-19 Treatment Guidelines and Infectious Diseases Society of America (IDSA) Guidelines on the Treatment and Management of Patients with COVID-19).

Remdesivir (an antiviral drug) is for the treatment for select people with COVID-19. Remdesivir is given intravenously. The recommended treatment duration is 5 to 10 days. The combination of remdesivir and the corticosteroid dexamethasone is commonly used in hospitalized people who need supplemental oxygen.

The combination drug nirmatrelvir and ritonavir is an antiviral drug taken by mouth. It may be used to treat mild to moderate COVID-19 infection in some adults and adolescents who are at high risk for progression to severe COVID-19, including hospitalization or death. It is not authorized for use for longer than 5 consecutive days.

Molnupiravir is an antiviral drug taken by mouth. It may be used to treat mild to moderate COVID-19 infection in nonhospitalized adults who are at high risk for progression to severe COVID-19, including hospitalization or death, and for whom alternative COVID-19 treatment options are not available or appropriate. Molnupiravir is taken by mouth and is not authorized for use for longer than 5 consecutive days. It is not recommended for use during pregnancy.

Three monoclonal antibodies may be used to treat mild to moderate COVID-19 in adults and pediatric patients (12 years or older and weighing at least 88 pounds) with COVID-19 and who are at high risk for progressing to severe disease (includes those who are 65 years or older or who have certain chronic medical conditions). Only one of the three is effective against the Omicron variant. Monoclonal antibodies are given intravenously or by a single-dose injection.

The following therapies are NOT recommended for the treatment or prevention of COVID-19:

  • Blood plasma from recovered patients

  • Nonspecific immunoglobulin (IVIG) and mesenchymal stem cell therapy

  • Additional immunomodulatory therapies, including interferons, kinase inhibitors, and interleukin inhibitors

  • Azithromycin and antiretrovirals

  • Lopinavir/ritonavir (an HIV retroviral)

  • Chloroquine and hydroxychloroquine (antimalaria drugs)

  • Ivermectin (an antiparasite drug): the FDA and other organizations have issued warnings about toxicity from the inappropriate use of ivermectin preparations intended for large animal use (see FDA: Why You Should Not Use Ivermectin to Treat or Prevent COVID-19).

More Information

Drugs Mentioned In This Article

Generic Name Select Brand Names
quiz link

Test your knowledge

Take a Quiz!