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Coronaviruses and Acute Respiratory Syndromes (COVID-19, MERS, and SARS)


Brenda L. Tesini

, MD, University of Rochester School of Medicine and Dentistry

Last full review/revision Mar 2021| Content last modified Mar 2021

Coronaviruses are a large family of viruses that cause respiratory illness ranging in severity from the common cold to fatal pneumonia.

There are many different coronaviruses. Most of them cause illness in animals. However, 7 types of coronaviruses are known to cause illness in humans.

Four of these 7 human coronavirus infections involve mild upper respiratory tract illness that causes symptoms of the common cold.

However, 3 of the 7 human coronavirus infections can be much more severe and have recently caused major outbreaks of deadly pneumonia:

  • SARS-CoV-2 is a novel coronavirus that was first identified in Wuhan, China in late 2019 as the cause of coronavirus disease 2019 (COVID-19) and spread worldwide.
  • MERS-CoV was identified in 2012 as the cause of Middle East respiratory syndrome (MERS).
  • SARS-CoV was identified in 2003 as the cause of an outbreak of severe acute respiratory syndrome (SARS) that began in China near the end of 2002.

These coronaviruses that cause severe respiratory infections are transmitted from animals to humans (zoonotic pathogens).


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.

The newly identified coronavirus that causes COVID-19 is called SARS-CoV-2, because it is related to but slightly different from the coronavirus that causes SARS.

Transmission of COVID-19

Early COVID-19 infections were linked to a live animal market in Wuhan, China, suggesting that the virus was transmitted from animals being sold as exotic food to humans.

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. It is typically spread from having close contact (being within 6 feet for 15 minutes or more over a 24-hour period) with a contagious person, but the virus may spread over longer distances or remain in the air longer under certain conditions. In general, the closer and longer the interaction with an infected person, the higher the risk of virus spread. 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).


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.

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

  • Heart disorders including arrhythmias, heart muscle disorders, and acute heart injury
  • Coagulation disorders including blood clots in small and large blood vessels as well as bleeding
  • Guillain-Barré syndrome (rare)

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 these can be similar to the rare condition 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.


  • 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 testing by their healthcare provider or local or state ​health department

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.


The best way to prevent infection is be fully vaccinated. In general, people are considered fully vaccinated

  • 2 weeks after their second dose in a 2-dose series, such as the Pfizer or Moderna vaccines, or
  • 2 weeks after a single-dose vaccine, such as the Janssen vaccine (also known as the Johnson & Johnson 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 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 activities without wearing a mask or physically distancing, except where required, including by local business and workplace guidance.
  • People who are not vaccinated should wear a cloth face covering 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 cloth face covering is not a substitute for social distancing).
  • Everyone should wear a face mask or cloth face covering when sick and around other people (cloth face coverings can be fashioned from household items or made at home from common materials [see CDC's Use of Cloth Face Coverings to Help Slow the Spread of COVID-19]).
  • Everyone should wear a face 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. It is meant to separate and restrict the movement of "close contacts" to see if they become sick within the 14-day incubation period. Except for people who have, within the past 3 months, either had COVID-19 or been fully vaccinated against the disease, 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

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, no vaccines have received approval from the US Food and Drug Administration (FDA), but as of March 1, 2021, the following 3 have emergency use authorization (EUA):

  • The Pfizer-BioNTech vaccine received EUA on December 11, 2020 and now includes authorization for use in individuals 12 years of age and older and is given as a series of 2 injections given 3 weeks apart (see also FDA Fact Sheet on the Pfizer-BioNTech vaccine ).
  • The Moderna vaccine 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 Janssen vaccine (also known as the Johnson & Johnson 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).

The Pfizer-BioNTech and Moderna COVID-19 vaccines are not interchangeable, that is people must receive the vaccine from the same manufacturer for both doses.

All 3 vaccines completely prevented serious complications of COVID-19 including hospitalizations and deaths in their clinical trials. In a clinical trial, the Pfizer-BioNTech 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 Janssen 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, including those taking immunosuppressant drugs, 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.

People should not get the Pfizer-BioNTech 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 Janssen 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-BioNTech 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

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 Janssen 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 Janssen 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


  • Drugs to relieve fever and muscle aches
  • Sometimes, remdesivir and/or dexamethasone

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.

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).

Some people become so severely ill that they need to be treated with mechanical ventilation to assist their breathing.

More Information

The following English-language resources may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

  • FDA Fact Sheet for Recipients and Caregivers: Pfizer-BioNTech
  • FDA Fact Sheet for Recipients and Caregivers: Moderna
  • FDA Fact Sheet for Recipients and Caregivers: Janssen
  • CDC: COVID-19 Vaccines
  • CDC: How COVID-19 Spreads
  • CDC: COVID-19 and Travel
  • CDC: Use of Cloth Face Coverings to Help Slow the Spread of COVID-19
  • CDC: Contact Tracing for COVID-19
  • CDC: When to Quarantine
  • CDC: Isolate If You Are Sick

Middle East Respiratory Syndrome (MERS)

Middle East respiratory syndrome is a coronavirus infection that causes severe flu-like symptoms.

The virus that causes Middle East respiratory syndrome (MERS) is a coronavirus similar to the virus that causes severe acute respiratory syndrome (SARS).

The MERS virus was first detected in Jordan and Saudi Arabia in 2012. As of early 2018, there were 2,220 confirmed cases of MERS and 790 deaths. Most occurred in Saudi Arabia, where new cases continue to appear. Cases have also occurred in countries outside the Middle East, including France, Germany, Italy, Tunisia, and the United Kingdom in people who had been traveling or working in the Middle East.

An outbreak of MERS coronavirus occurred in South Korea from May to July 2015 after a South Korean man returned from the Middle East. This outbreak involved more than 180 cases and 36 deaths. Most person-to-person spread occurred in health care settings.

In May 2014, two cases were confirmed in the United States. Both were health care workers who had recently returned from the Persian Gulf. There have been no MERS cases in the United States since May 2014.

In several countries (including Egypt, Oman, Qatar, and Saudi Arabia), dromedary camels are suspected of being the primary source of infection for people, but how the virus spreads from camels to people is unknown.

The infection is more common among men and is more severe in older people and in people with an underlying chronic disorder such as diabetes or a heart or kidney disorder. The infection has been fatal in about one third of infected people.

The MERS virus is spread through close contact with people who have MERS or through airborne droplets that were coughed or sneezed out by an infected person. People are not thought to be contagious until symptoms develop. Most cases of person-to-person spread have occurred in health care workers caring for infected people.

Symptoms usually appear about 5 days (but anywhere from 2 to 14 days) after people are infected. Most people have a fever, chills, muscle aches, and cough. About one third have diarrhea, vomiting, and abdominal pain.


  • Testing of fluids from the respiratory tract
  • Blood tests

Doctors suspect MERS in people who have a lower respiratory tract infection and have traveled to or reside in an area where they could have been exposed to the virus or who have had recent close contact with someone who may have had MERS.

To diagnose MERS, doctors take a sample of fluids from several places in the respiratory tract at different times and test it for the virus. They also do blood tests to detect the virus or antibodies to it. Blood tests are done on all people who have had close contact with someone who may have MERS.


  • Drugs to relieve fever and muscle aches
  • Isolation

There is no specific treatment for MERS. Acetaminophen or a nonsteroidal anti-inflammatory drug (NSAID) such as ibuprofen are given to relieve fever and muscle aches.

Precautions are taken to prevent the spread of the virus. For example, the person is isolated in a room with a ventilation system that limits the spread of microorganisms in the air. People who go into the room must wear a special mask, eye protection, and a gown, cap, and gloves. Doors to the room should be kept closed except when people enter or leave the room, and people should enter and leave as few times as possible.

People traveling to the Middle East should check the World Health Organization (WHO) web site for travel advice (see WHO World-travel advice on MERS-CoV for pilgrimages).

Severe Acute Respiratory Syndrome (SARS)

Severe acute respiratory syndrome is a coronavirus infection that causes flu-like symptoms.

  • No cases had been reported worldwide since 2004.
  • Symptoms of SARS resemble those of other more common respiratory viral infections (such as fever, headache, chills, and muscle aches) but are more severe.
  • Doctors suspect SARS only if people may have been exposed to an infected person.
  • If doctors think a person may have SARS, the person is isolated in a room with a ventilation system that limits the spread of microorganisms in the air.

(See also Overview of Viral Infections.)

Severe acute respiratory syndrome (SARS) was first detected in China in late 2002. A worldwide outbreak occurred, resulting in more than 8,000 cases worldwide, including Canada and the United States, and more than 800 deaths by mid 2003. No cases had been reported worldwide since 2004, and SARS (the disease, but not the virus) is considered to have been eradicated.

The immediate source was presumed to be civets, cat-like mammals, that were being sold in live animal markets as exotic food. How civets became infected is unclear, though bats are thought to be the reservoir host of the SARS virus in nature.

SARS is caused by a coronavirus. SARS is much more severe than most other coronavirus infections, which usually cause only coldlike symptoms. However, the Middle East respiratory syndrome (MERS) is another severe illness caused by a coronavirus.

SARS is spread from person to person through close contact with an infected person or through airborne droplets that were coughed or sneezed out by an infected person.


Symptoms of SARS resemble those of other more common respiratory viral infections but are more severe. They include fever, headache, chills, and muscle aches, followed by a dry cough and sometimes difficulty breathing.

Most people recovered within 1 to 2 weeks. However, some developed severe difficulty breathing, and about 10% died.


  • A doctor's evaluation
  • Tests to identify the virus

SARS is suspected only if people who may have been exposed to an infected person have a fever plus a cough or difficulty breathing.

Tests can be done to identify the virus.


  • Isolation
  • If needed, oxygen
  • Sometimes a ventilator to help with breathing

If doctors think a person may have SARS, the person is isolated in a room with a ventilation system that limits the spread of microorganisms in the air. In the first and only outbreak of SARS, such isolation kept the virus from being transmitted and eventually eliminated it.

People with mild symptoms need no specific treatment. Those with moderate difficulty breathing may need to be given oxygen. Those with severe difficulty breathing may need mechanical ventilation to aid breathing.

Drugs Mentioned In This Article

Generic Name Select Brand Names
lopinavir/ritonavir KALETRA
hydroxychloroquine PLAQUENIL
Acetaminophen TYLENOL
dexamethasone OZURDEX
chloroquine ARALEN
ivermectin STROMECTOL
ibuprofen ADVIL, MOTRIN IB

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