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Coronavirus Disease 2019; COVID


Brenda L. Tesini

, MD, University of Rochester School of Medicine and Dentistry

Last full review/revision Oct 2021| Content last modified Nov 2021
Topic Resources

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

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's Novel Coronavirus (COVID-19) situation reports.

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, but it can be transmitted by people before they exhibit symptoms (presymptomatic) and even by people who are infected but never develop symptoms (asymptomatic).

The recent Delta variant of the SARS-CoV-2 virus is more readily transmitted than the earlier variants.

Symptoms of 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

  • Shortness of breath or difficulty breathing

  • Chills or repeated shaking with chills

  • Fatigue

  • Muscle pain

  • Headache

  • Sore throat

  • New loss of smell or taste

  • Congestion or runny nose

  • Nausea or vomiting

  • Diarrhea

Symptoms usually appear about 2 to 14 days after people are infected, most commonly within 4 to 5 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. Risk of serious disease and death decreases in people who are vaccinated against COVID-19.

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. 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 to months. Viral detection tests in these people usually do not show any active virus, and they are generally not considered contagious.

It is known that immunity after infection with other coronaviruses is temporary. The COVID-19 pandemic has not been going on long enough for researchers to be sure how long people might remain immune following COVID-19 illness. However, recently, a very small number of cases have been reported in which people who had recovered from COVID-19 became sick again with a genetically different strain of SARS-CoV-2. This reinfection so far appears extremely rare considering the tens of millions of people who have had COVID-19, but researchers do not know what will happen over time.

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.

Viral tests, such as a polymerase chain reaction (PCR) test, can be done on upper and lower respiratory secretions (samples from nasal or oral swabs or saliva) to identify the virus. Tests that detect specific SARS-CoV-2 antigens can also be done, but they are generally less accurate than PCR tests. (See also CDC: Overview of Testing for SARS-CoV-2 [COVID-19].)

People should be tested for COVID-19 if they

  • Have symptoms of COVID-19

  • Have had close contact (within 6 feet of an infected person for at least 15 minutes over a 24-hour period) with someone with confirmed COVID-19

  • Have been asked or referred to get tested by their healthcare provider or local or state ​health department or as part of routine screening at workplaces or other settings, especially when community transmission levels are high

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

Prevention of COVID-19

  • 2 weeks after their second dose in a 2-dose series, such as the Pfizer-BioNTech COVID-19 vaccine (mRNA) or the Moderna COVID-19 vaccine (mRNA), or

  • 2 weeks after a single-dose vaccine, such as the Johnson & Johnson COVID-19 vaccine (adenovirus vector), which is also known as the Janssen vaccine

People should avoid being exposed to this virus, which can be difficult because some infected people do not know they have the virus. For people who are not fully vaccinated, it is important to practice "social distancing" (keeping 6 feet between people who do not live in the same household) and wear a face mask or cloth face covering over the mouth and nose when around people not living in the same household.

The Centers for Disease Control and Prevention (CDC) recommends the following:

  • Fully vaccinated people can resume many activities without wearing a mask or physically distancing, except where required by local laws, regulations, and rules and by local business and workplace guidance. To maximize protection from the Delta variant and prevent possible transmission of this virus to others, fully vaccinated people should wear a mask indoors in public if in an area of substantial or high transmission. People with a weakened immune system or people at increased risk for severe disease because of age or an underlying medical condition must take extra precautions and mask when with other people. Wearing a mask is also important if you have someone in your household who has a weakened immune system, is at increased risk for severe disease, or is unvaccinated. 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 the level of transmission.

  • People who are not vaccinated should wear a mask if healthy (not exhibiting symptoms) when in public settings and when around people who do not live in the same household, especially when other social distancing measures are difficult to maintain (the mask is not a substitute for social distancing).

  • Everyone should wear a mask when sick and around other people (see CDC: Types of Masks and Respirators).

  • Everyone should wear a mask when caring for someone who is sick.

In addition, the CDC recommends the following routine actions to help prevent the spread of respiratory viruses:

  • Washing hands often with soap and water for at least 20 seconds, especially after going to the bathroom, before eating, and after blowing the nose, coughing, or sneezing

  • Using an alcohol-based hand sanitizer with at least 60% alcohol if soap and water are not readily available

  • Avoiding touching eyes, nose, and mouth with unwashed hands

  • Avoiding close contact with people who are sick

  • Staying home when sick

  • Covering cough or sneeze with a tissue, then throwing the tissue in the trash

  • Cleaning and disinfecting frequently touched objects and surfaces using a regular household cleaning spray or wipe

  • Monitoring health for potential symptoms and taking temperature if symptoms develop

Quarantine and isolation

To help prevent transmission, the Centers for Disease Control and Prevention (CDC) recommends quarantine and isolation measures.

Quarantine is for people who were exposed or may have been exposed to the virus. It is meant to separate and restrict the movement of "close contacts" to see if they become sick within the 14-day incubation period. The following people should quarantine for 14 days after their last exposure to the virus:

  • Close contacts who do not have symptoms and tested negative

  • Close contacts who do not have symptoms and have not been tested

People who have been fully vaccinated against the disease and show no symptoms do not need to quarantine after a close contact with someone who had COVID-19. However, fully vaccinated people should get tested 3 to 5 days after their exposure (even if they do not have symptoms) and wear a mask indoors in public for 14 days following exposure or until their test result is negative.

If a close contact receives a positive test result, that person then isolates for at least 10 days. Strict adherence to these measures have been successful at controlling the spread of infection in select areas.

A close contact is a person who

  • Was within 6 feet of an infected person (with or without a mask) for a cumulative total of 15 minutes or more over a 24-hour period starting from 48 hours before that person developed symptoms or was tested (if they did not have symptoms)

A close contact may also include people who

  • Provided care at home to someone sick with COVID-19

  • Had direct physical contact with the sick person (hugged or kissed them)

  • Shared eating or drinking utensils

  • Was in range of the sick person's respiratory droplets (for example, through a sneeze or cough)

Isolation is meant to separate people who are contagious from those who are susceptible. The following people should isolate:

  • People who are sick with COVID-19 symptoms who have not been tested

  • Anyone who tested positive for SARS-CoV-2 (whether or not they have symptoms)

Isolation can generally end 10 days after symptoms started, 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. In people who never had symptoms, the isolation can stop 10 days after the date of their first positive test.


Multiple COVID-19 vaccines are currently in use worldwide. In the US, 3 vaccines have received either full approval or emergency use authorization (EUA) from the US Food and Drug Administration (FDA):

  • The Pfizer-BioNTech COVID-19 vaccine (mRNA) received full FDA approval on August 24, 2021 for use in individuals 16 years of age and older and is available under EUA for use in individuals 5 to 15 years of age. It is given as a series of 2 injections, 3 weeks apart (see also FDA Fact Sheet on the Pfizer-BioNTech vaccine ).

  • The Moderna COVID-19 vaccine (mRNA) received EUA on December 18, 2020 for use in individuals 18 years of age and older and also requires 2 injections but given 4 weeks apart (see also FDA Fact Sheet on the Moderna vaccine).

  • The Johnson & Johnson COVID-19 vaccine (adenovirus vector), which is also known as the Janssen vaccine, received EUA on February 27, 2021 for use in individuals 18 years of age and older and requires only a single injection (see also FDA Fact Sheet for Recipients and Caregivers: Janssen).

In August 2021, the FDA amended the authorizations for the Pfizer and Moderna vaccines to allow an additional dose to be administered to people with moderately to severely compromised immune systems, such as those who have undergone solid organ transplantation. This additional dose should be administered at least 28 days after an initial 2-dose series.

Third doses (booster doses) are available for the following Pfizer and Moderna vaccine recipients who completed their initial 2-dose series at least 6 months ago:

  • Older adults, age 65 years and older

  • People age 18 years and older with an underlying medical condition

  • People age 18 years and older who live in long-term care settings

  • After considering individual risks and benefits, people age 18 years and older who work or live in high-risk settings such as health care facilities, schools, correctional facilities, and homeless shelters

More detailed explanations of these groups can be found on the CDC web site. Recommendations regarding an additional dose for other groups have not been finalized.

Second doses (booster doses) are recommended for Johnson & Johnson vaccine recipients who

  • Are 18 years or older and who received their first dose of the vaccine 2 or more months ago

Eligible people may choose to receive a booster dose of any available COVID-19 vaccine (Pfizer, Moderna, or Johnson & Johnson), regardless of which vaccine they initially received.

All 3 vaccines prevented serious complications of COVID-19 including hospitalizations and deaths in their clinical trials. In a clinical trial, the Pfizer vaccine prevented COVID-19 disease in 95% of people following 2 doses given 3 weeks apart. In a separate trial, the Moderna vaccine prevented COVID-19 disease in 94.1% of people. The Johnson & Johnson vaccine prevented COVID-19 disease in approximately 67% of people overall and prevented severe/critical COVID-19 disease in 85% after one dose. It is important to note that these trials cannot be compared directly because they were done on different groups of patients at different points in the pandemic. The duration of the protection from the vaccines is currently not known. People with a weakened immune system Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more , including those taking immunosuppressant drugs Secondary immunodeficiency disorders Immunodeficiency disorders involve malfunction of the immune system, resulting in infections that develop and recur more frequently, are more severe, and last longer than usual. Immunodeficiency... read more , may have a diminished response to the vaccine. It is not currently known how well vaccines will prevent the spread of the virus that causes COVID-19, so people who have been vaccinated should still follow general prevention measures such as mask wearing, social distancing, and frequent hand washing.

The effectiveness of the Pfizer vaccine against the Delta variant was assessed in a large study. After 2 doses, the Pfizer vaccine was 93.7% effective among patients with the alpha variant and 88.0% among those with the Delta variant. A single dose of the vaccine was markedly less effective against either variant and was especially less effective against the Delta variant.

People should not get the Pfizer or Moderna vaccine if they have had a severe allergic reaction to a previous dose of the vaccine or to any component of the vaccine (including polyethylene glycol [PEG]). People should not get the Johnson & Johnson vaccine if they have a history of severe allergic reaction to any of its ingredients.

Side effects to the 3 vaccines are similar and include

  • Pain, swelling, and redness at the injection site

  • Tiredness

  • Headache

  • Muscle pain

  • Chills

  • Joint pain

  • Fever

  • Nausea

  • Feeling unwell

  • Swollen lymph nodes

Side effects typically last several days. For the Pfizer and Moderna COVID-19 vaccines, more people experience side effects after the second dose than after the first dose.

There is a remote chance of a severe allergic reaction. This usually occurs within a few minutes to 1 hour after getting a dose of the vaccine and requires emergency treatment (call for emergency medical care [911 in the United States] or go to the nearest hospital). People who have had severe allergic reactions to other vaccines or injectable drugs should discuss the risk of an allergic reaction with their doctor and be observed after receiving the vaccine. Signs of a severe allergic reaction include

  • Difficulty breathing

  • Swelling of face and throat

  • A fast heartbeat

  • A bad rash all over body

  • Dizziness and weakness

Myocarditis Myocarditis Myocarditis is inflammation of the muscle tissue of the heart (myocardium) that causes tissue death. Myocarditis may be caused by many disorders, including infection, toxins and drugs that affect... read more and pericarditis Acute Pericarditis Acute pericarditis is inflammation of the pericardium (the flexible two-layered sac that envelops the heart) that begins suddenly, is often painful, and causes fluid and blood components such... read more Acute Pericarditis have been reported following the second doses of the Pfizer and Moderna vaccines, suggesting there may be an increased risk of these events following vaccination. Vaccine recipients should seek medical attention right away if they have chest pain, shortness of breath, or feelings of having a fast-beating, fluttering, or pounding heart after vaccination.

There is a remote chance of having blood clots involving blood vessels in the brain, abdomen, and legs along with low levels of platelets (blood cells that help the body stop bleeding) in people who have received the Johnson & Johnson vaccine. In these people symptoms began approximately 1 to 2 weeks following vaccination. Most people who developed these blood clots and low levels of platelets were females ages 18 to 49 years. The chance of having this occur is remote. People who have any of the following symptoms after receiving the Johnson & Johnson vaccine should seek medical attention right away:

  • Shortness of breath

  • Chest pain

  • Leg swelling

  • Persistent abdominal pain

  • Severe or persistent headaches or blurred vision

  • Easy bruising or tiny blood spots under the skin beyond the site of the injection

Treatment of COVID-19

  • Drugs to relieve fever and muscle aches

  • Sometimes, remdesivir and/or dexamethasone

  • Sometimes, monoclonal antibodies

Most people with COVID-19 do not need treatment.

National Institutes of Health (NIH) guidelines recommend remdesivir (an antiviral drug) and dexamethasone (an anti-inflammatory drug) for selected people with severe disease. Remdesivir is the only treatment approved by the US Federal Drug Administration (FDA) for COVID-19, and it is approved to be used only in patients hospitalized with COVID-19. Many drugs are being evaluated in clinical trials, but there is not enough information about how well they work to recommend using them outside of these clinical trials. Multiple clinical trials of the HIV retroviral, lopinavir/ritonavir, and the anti-malaria drugs chloroquine and hydroxychloroquine have shown these drugs to be without benefit. There are also no randomized clinical trials documenting the usefulness of the anti-parasite drug ivermectin for the prevention or treatment of COVID-19.

Three monoclonal antibodies have received emergency use authorization from the FDA for treatment of 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).

People who have recovered from COVID-19 have antibodies to SARS-CoV-2 virus in their blood. A recent study found that blood plasma from recovered patients could aid recovery of selected patients with mild symptoms. Manufactured antibodies (monoclonal antibodies) to SARS-CoV-2 virus have received emergency use authorization from the FDA for treatment of mild to moderate COVID-19 in selected patients. For both of these new treatments, which are only given by intravenous infusion, there are insufficient data from clinical trials to recommend for or against these treatments, and these treatments should not be considered standard of care.

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 should stop taking the blood pressure drugs called angiotensin-converting enzyme inhibitors (ACEI) or angiotensin receptor blockers (ARBs).

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