Merck Manual

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(Coronavirus Disease 2019; COVID)


Brenda L. Tesini

, MD, University of Rochester School of Medicine and Dentistry

Reviewed/Revised Aug 2022 | Modified Dec 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.

Most viruses, including the SARS-CoV-2 virus, evolve and new variants of the virus emerge. Variants with the potential to be more transmissible, more likely to cause severe disease, or that are harder to treat are tracked as Variants of Concern and are commonly referred to by their Greek alphabet label. The dominant variants in the United States and much of the world are Alpha, Beta, Delta, and Omicron. The Omicron variant has been the most common variant worldwide since March 2022, with newer and more transmissible Omicron subvariants (such as BA.4 and BA.5) replacing the original Omicron (B.1.1.529). See also CDC: Omicron Variant: What You Need to Know.

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, elder care or other 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 elder care facilities are also at high risk of severe disease because of age and underlying medical disorders.

Large indoor events, such as meetings or weddings, have also been associated with high transmission rates. These have been called super-spreader events and may lead high rates of infection due to large numbers of people being physically close to each other without adequate airflow or ventilation.

Viral tests Diagnosis in people with COVID-19 may remain positive for at least 3 months regardless of symptoms. However, even people with lingering symptoms are generally not considered infectious after 10 days of illness.

Symptoms of COVID-19

Symptoms vary in people with COVID-19 and may 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 (incubation period) after being infected. For the Omicron variant, symptoms usually appear in only 2 to 4 days. Many infected people have no symptoms or have only mild disease.

The risk of serious disease and death in people with COVID-19 increases

  • With age

  • In people who smoke or previously smoked

  • In people with other serious medical disorders, such as cancer or chronic heart, lung, kidney, or liver disease, sickle cell disease, diabetes, obesity, or immunocompromising disorders


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 some people, symptoms last longer, sometimes with shortness of breath, cough, and extreme fatigue, and persisting for weeks. Prolonged illness appears to be more common in those who originally had severe disease, but even people with mild illness may have persistent symptoms.

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.

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 or other emerging variants (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, severe illness, and death from COVID-19 is to be up to date with COVID-19 vaccinations, including booster shots. In the United States in the fall of 2021, unvaccinated people were 78 times more likely to die from COVID-19 than vaccinated people who had received a booster dose.

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), one adenovirus vector vaccine (Ad26.COV2.S), and one protein subunit adjuvanted vaccine (NVX-CoV2373) 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, additional doses are recommended. People who have received all recommended primary series vaccinations and booster doses are considered "up-to-date" on their vaccine series. (See also CDC: COVID-19 Vaccine Booster Shots.)

In the US, the “updated” booster dose is an mRNA vaccine that is bivalent, which means it stimulates an immune response against 2 viruses or viral strains. The bivalent vaccines are designed to protect against the original COVID-19 strain and also common Omicron variants. The bivalent vaccines may be given as a booster dose or as part of the primary series.

In most situations, the mRNA vaccines and the protein subunit adjuvanted vaccine are preferred over the vector vaccine 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.)

Masking and other routine measures

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.

For people age 2 years or older, the 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:

How To Wash Your Hands

Quarantine and isolation

Quarantine and isolation measures have been recommended in an attempt to limit the local, regional, and global spread of this outbreak.

Quarantine is meant to separate people who had close contact with a contagious person so they do not infect other people. Currently, the CDC does not advise quarantine for people who have been exposed to Covid-19 but are not infected. See CDC: What to Do If You Were Exposed to COVID-19 for information regarding masking and testing after exposure.

Isolation separates people with confirmed or suspected COVID-19 from those without COVID-19. Regardless of vaccination status, people with COVID-19 symptoms and/or who have a positive COVID-19 test should isolate for at least 5 full days. 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. For an additional 5 days, they should wear a mask when around others both in public and at home. (See also CDC: Isolation and Precautions for People with COVID-19.)

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 not authorized for use for longer than 5 consecutive days. It is not recommended for use during pregnancy.

Bamlanivimab plus etesevimab, casirivimab plus imdevimab, and sotrovimab are monoclonal antibody therapies. They are not effective against the Omicron variant. Therefore, the FDA recommended (April 22, 2022) against their use in treatment of COVID-19, because Omicron has become the dominant variant in the United States.

Bebtelovimab is also a neutralizing anti-SARS-CoV-2 monoclonal antibody. It was effective against early Omicron variants but is not expected to neutralize the later and now predominant Omicron subvariants BQ.1 and BQ.1.1. Therefore, effective November 30, 2022, bebtelovimab is not authorized for use in the US (see FDA Announces Bebtelovimab is Not Currently Authorized in Any US Region).

The following therapies are also 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
AK-Dex, Baycadron, Dalalone, Dalalone D.P, Dalalone L.A, Decadron, Decadron-LA, Dexabliss, Dexacort PH Turbinaire, Dexacort Respihaler, DexPak Jr TaperPak, DexPak TaperPak, Dextenza, DEXYCU, DoubleDex, Dxevo, Hemady, HiDex, Maxidex, Ocu-Dex , Ozurdex, ReadySharp Dexamethasone, Simplist Dexamethasone, Solurex, TaperDex, ZCORT, Zema-Pak, ZoDex, ZonaCort 11 Day, ZonaCort 7 Day
7T Gummy ES, Acephen, Aceta, Actamin, Adult Pain Relief, Anacin Aspirin Free, Apra, Children's Acetaminophen, Children's Pain & Fever , Comtrex Sore Throat Relief, ED-APAP, ElixSure Fever/Pain, Feverall, Genapap, Genebs, Goody's Back & Body Pain, Infantaire, Infants' Acetaminophen, LIQUID PAIN RELIEF, Little Fevers, Little Remedies Infant Fever + Pain Reliever, Mapap, Mapap Arthritis Pain, Mapap Infants, Mapap Junior, M-PAP, Nortemp, Ofirmev, Pain & Fever , Pain and Fever , PAIN RELIEF , PAIN RELIEF Extra Strength, Panadol, PediaCare Children's Fever Reducer/Pain Reliever, PediaCare Children's Smooth Metls Fever Reducer/Pain Reliever, PediaCare Infant's Fever Reducer/Pain Reliever, Pediaphen, PHARBETOL, Plus PHARMA, Q-Pap, Q-Pap Extra Strength, Silapap, Triaminic Fever Reducer and Pain Reliever, Triaminic Infant Fever Reducer and Pain Reliever, Tylenol, Tylenol 8 Hour, Tylenol 8 Hour Arthritis Pain, Tylenol 8 Hour Muscle Aches & Pain, Tylenol Arthritis Pain, Tylenol Children's, Tylenol Children's Pain+Fever, Tylenol CrushableTablet, Tylenol Extra Strength, Tylenol Infants', Tylenol Infants Pain + Fever, Tylenol Junior Strength, Tylenol Pain + Fever, Tylenol Regular Strength, Tylenol Sore Throat, XS No Aspirin, XS Pain Reliever
Advil, Advil Children's, Advil Children's Fever, Advil Infants', Advil Junior Strength, Advil Migraine, Caldolor, Children's Ibuprofen, ElixSure IB, Genpril , Ibren , IBU, Midol, Midol Cramps and Body Aches, Motrin, Motrin Children's, Motrin IB, Motrin Infants', Motrin Junior Strength, Motrin Migraine Pain, PediaCare Children's Pain Reliever/Fever Reducer IB, PediaCare Infants' Pain Reliever/Fever Reducer IB, Samson-8
No brand name available
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