Merck Manual

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COVID-19 Resources Home Page 

1. What are the symptoms of COVID-19?

  • Fever, dry cough, fatigue and loss of appetite are the most common symptoms.
  • Sore throat and dry cough may be the first symptoms.
  • Headache, confusion, runny nose, muscle aches, diarrhea, and nausea and vomiting can occur but are less common.
  • Loss of taste and smell occurs.
  • Shortness of breath is reported by 30-40% of patients. If pneumonia develops, shortness of breath can get much worse requiring hospital treatment with oxygen or even mechanical ventilation.

It should be noted that some people diagnosed with COVID-19 have not yet developed symptoms (pre-symptomatic).

  Also, about 35 % of people infected with the virus that causes COVID-19 don't develop symptoms (are asymptomatic).



2. Can I have COVID-19 and not know it?

It is possible to be infected with COVID-19 and not have any symptoms (asymptomatic) and therefore not know you are infected. Asymptomatic infection seems to be quite common, and reports indicate this may represent between 13% to over 50% of cases. According to the CDC, the best estimate is that about 35% of people infected with the virus that causes COVID-19 are asymptomatic. However, because people without symptoms are generally not tested at the present time it is not known how common asymptomatic infection actually is. Some of these asymptomatic infections progress to symptomatic disease while others remain without symptoms. 




3. How long after I contact COVID-19 will I get sick?

  • An average of about 4 to 5 days, and almost always between 1 and 14 days.
  • Almost 98% of people who develop symptoms will do so in 12 days or less after they become infected.



4. If I get COVID-19, how likely is it I’ll die?

The Case Fatality Rate (%) = the number of deaths / the number of confirmed COVID-19 cases x 100 and therefore will depend on the number of people tested. It is likely that many cases have not been tested and thus identified, whereas the number of deaths attributed to COVID-19 are more accurately identified. Thus, the apparent risk of death varies widely depending on how much testing is done in a given region.

Also, the chances of death vary widely by a person’s age and overall health. Older people are much more likely to die.

Although death is uncommon in younger people, it does occur. We don’t fully understand why some younger people are susceptible.

Other factors that make serious illness or death due to COVID-19 more likely include

  • Disorders or use of drugs that interfere with the immune system
  • HIV infection
  • Stroke or cerebrovascular disease
  • Chronic kidney disease
  • Cancer
  • Liver disease
  • Heart and lung disorders
  • Current or previous cigarette smoking
  • Diabetes
  • Sickle cell disease or thalassemia
  • Organ transplant or stem cell transplant
  • Dementia or other neurologic condition
  • Down syndrome
  • Pregnancy and recently pregnant women (for at least 42 days after end of pregnancy
  • Substance abuse disorders (such as alcohol, opioid, or cocaine use disorders)
  • Overweight (body mass index [BMI] between 25 kg/m2 and < 30 kg/m2 ), obesity (BMI between 30 kg/m2 and < 40 kg/m2), and especially severe obesity (BMI of 40 kg/m2 or above)  


People who have these disorders may be able to decrease their risk by keeping the disorder under control (for example, maintaining the best levels blood sugar).



5. How long does the COVID-19 illness last?

Much about this is still unknown. However, a mild case of COVID-19 appears to last about 2 weeks. Severe disease (usually meaning pneumonia) lasts from about 3 to 6 weeks.

If pneumonia develops, it usually occurs about 1 week after the first symptoms. Symptoms can continue to worsen for another 3 to 7 days or so. People who are admitted to the hospital and survive usually leave the hospital after about 10 to 13 days.

Some people continue to have symptoms such as fatigue, headaches, and shortness of breath for weeks to months after recovery.



6. What can I do to prevent COVID-19?

Getting vaccinated is the most important thing you can do to prevent COVID-19.



7. When should I wear a mask or use gloves?

  According to the CDC, if you've been fully vaccinated,

  • You can resume activities that you did prior to the pandemic.
  • You can resume activities without wearing a mask or staying 6 feet apart, except where required by federal, state, local, tribal, or territorial laws, rules, and regulations, including local business and workplace guidance.
  • You can use a face mask and gloves when caring for someone who is sick with COVID-19.

Cloth face coverings should not be placed on young children under age 2, anyone who has trouble breathing, or anyone who is unconscious, incapacitated, or otherwise unable to remove the mask without assistance.

Cloth face coverings can be fashioned from household items or made at home from common materials. The effectiveness of such cloth face coverings is unknown but is clearly less than medical respirators. However, to preserve the supply of such equipment, people should not use face masks intended for health care workers unless they are caring for someone who has COVID-19.



8. Do babies and children get COVID-19?

  • Babies and children do get COVID-19.

    The symptoms of COVID-19 are similar in children and adults. However, children with COVID-19 generally have milder symptoms. Children are less likely to get so sick that they need to go to the hospital.

    Reported symptoms in children include

    • Fever
    • Cough
    • Nasal congestion or rhinorrhea
    • Sore throat
    • Loss of taste or smell
    • Shortness of breath or difficulty breathing
    • Diarrhea
    • Nausea and vomiting
    • Abdominal pain
    • Fatigue
    • Headache
    • Muscle ache (myalgia)
    • Poor feeding or poor appetite

    It’s not known yet whether children with underlying medical conditions are at higher risk of severe illness. Similar to adults, children with obesity, diabetes, asthma or chronic lung disease, sickle cell disease, or immunosuppression might also be at increased risk for severe illness from COVID-19.

    Information for Pediatric Healthcare Providers (




9. Can pets get COVID-19 or spread it to people?

A small number of pets worldwide, including cats and dogs, have been reported to be infected with the virus that causes COVID-19. The virus that causes COVID-19 can spread from people to animals in some situations. however, the risk of animals spreading COVID-19 to people is considered to be low. Until we learn more about how this virus affects animals, the CDC recommends ( treating pets as you would other human family members to protect them from a possible infection. However, until the situation is clearer, the CDC recommends that pets not interact with people or animals outside the household. People with COVID-19 infection should limit contact with their pets, and they and others in the household should wash their hands before and after contact with their pets.




10. Can I get COVID-19 again?

People with COVID-19, whether or not they have symptoms, start to develop antibodies against the virus within a few days of infection; however, it is as yet too early to tell if these antibodies will make them immune to repeat infection by the virus and, if so, for how long. Studies done of other coronavirus infections in people, including of the strains that cause colds as well as after the original SARS outbreak in the early 2000s, showed that people lost immunity to those coronaviruses after months to years. Doctors’ current best estimate is that most infected people will have some immunity but that it is probably not lifelong.



11. Who should have a COVID-19 test?

The following people should have SARS-CoV-2 testing with viral tests (e.g., nucleic acid or antigen tests) :

Practitioners are also encouraged to test for other causes of respiratory illness such as influenza and bacterial pneumonia.



12. What is the role of the new COVID-19 antibody test?

The presence of antibodies against COVID-19 in a person’s blood indicates that the person was infected and responded by making protective antibodies. People need these antibodies to be immune to another COVID-19 infection. However, although the presence of antibodies is proof that people had been infected, even those without symptoms, having antibodies is not a permanent guarantee against another infection. That is for two reasons

  • The level of antibodies may not be high enough to be protective
  • Even a protective level of antibodies may decrease over time and the body may not build them back up to a protective level quickly enough when re-exposed to the virus

Because it takes the body several days to make antibodies, antibody tests are not particularly useful for diagnosing infection right away.




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