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, diarrhea, and nausea and vomiting can occur but are less common (< 10%).
  • Loss of taste and smell have been reported.
  • 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 death more likely are serious disorders such as

  • Disorders or use of drugs that interfere with the immune system
  • Chronic kidney disease
  • Cancer
  • Liver disease
  • Prior stroke
  • Heart and lung disorders
  • Longstanding cigarette smoking
  • Diabetes
  • High blood pressure
  • Severe obesity


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 or blood pressure).

A preliminary study suggested that people with type A blood may be at significantly higher risk of respiratory failure due to COVID-19 while type O blood may be protective.



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.



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

Current CDC recommendations ( include:

  • Staying at home as much as possible
  • Keeping a 6-foot (2-meter) distance between yourself and others in public places
  • Frequent hand-washing particularly after being in a public place, or sneezing or coughing
  • Coughing into a tissue or your elbow and then immediately washing your hands
  • Avoiding touching your eyes, nose, or mouth with unwashed hands
  • Cleaning and disinfecting frequently touched surfaces (eg, tables, doorknobs and handles, phones, keyboards)
  • Wear a cloth face covering when in public settings and when around people who don't live in your household when social distancing measures are difficult to maintain. Continue to keep about 6 feet (about 2 meters) between yourself and others. The cloth face cover is not a substitute for social distancing.
  • 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

Wash hands with soap and water for at least 20 seconds. If soap and water are not available, use a hand sanitizer containing at least 60% ethyl alcohol). For cleaning, any household disinfectant is satisfactory.



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

  • Use a face mask and gloves when caring for someone who is sick with COVID-19
  • Wear a cloth face covering when in public settings and when around people who don't live in your household, especially when social distancing measures are difficult to maintain.Continue to keep about 6 feet (about 2 meters) between yourself and others. The cloth face cover is not a substitute for social distancing.
  • 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 generally do not get so sick that they need to go to the hospital.

    Reported symptoms in children include

    • Fever
    • Cough
    • Nasal congestion or rhinorrhea
    • Sore throat
    • Shortness of breath or difficulty breathing
    • Diarrhea
    • Nausea and vomiting
    • 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.




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

Other coronaviruses cause disease in animals that can be spread to humans, but this is rare. However, there are isolated reports from outside the US of a few companion animals testing positive for COVID-19 after prolonged contact with humans. There are also reports of 2 cats in separate locations in New York State with COVID-19 infections which they likely caught from their owners. These 2 cats had mild symptoms. Also, tigers in a New York City zoo with symptomatic respiratory illness, including cough, were found to have COVID-19 infection.


Researchers in China have published a study online showing that domestic cats can be experimentally infected with the novel coronavirus that causes COVID-19. Cats given a high dose of the virus were also able to infect cats housed in adjacent cages, suggesting aerosol spread. Whether cats can serve as a reservoir host of the virus or transmit it to people has not been determined, but the risk of cats as a source of infection appears to be low. The researchers also attempted infection in ferrets, dogs, pigs, chickens, and ducks. The virus replicated poorly in dogs, pigs, chickens, and ducks, but efficiently in ferrets and cats.


Importantly, there is no evidence that pets can pass the COVID-19 virus infection to humans. However, until the situation is clearer, 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.

Also, doctors have found that some people who have recovered and have developed antibodies tested positive for the virus later on. However, doctors think it is likely they still had a low level of virus from the first infection rather than being infected a second time.



11. When will a COVID-19 vaccine be available?

For extensive information on vaccines in development for COVID-19:



12. How important is lockdown in slowing COVID-19 transmission?

While efforts to develop treatments and vaccines are progressing rapidly, it will still be some time before testing is complete and they will become available to the general population. Until then, the only intervention available to help people stay healthy in areas of significant COVID-19 transmission is to break the COVID-19 transmission cycle by extreme social distancing (that is, a lockdown). The purpose of a lockdown is to decrease the likelihood that uninfected people will come into contact with infected people or surfaces contaminated by coronavirus. Only by decreasing the opportunity for the virus to spread will the pandemic be slowed and eventually controlled. In countries where this strategy was practiced early, the infection rate and death rate indeed seem to be slowing. Areas with limited disease presence and transmission that have appropriate testing and tracking capability may consider less stringent measures than full lockdown (e.g., physical distancing, mask wearing, limiting size of gatherings).



13. 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.



14. 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.

Doctors still are working out guidelines for who should have antibody testing. In people who test negative for the virus, the presence of antibody may help guide which health care workers can return to patient care safely (for themselves and their patients). See Antibody testing may also be part of the return to work decision for the general public—if antibody-positive and presumably resistant to COVID-19, but many other factors also are involved.



15. Is it safe to exercise outside in public?

Solitary exercise in public (eg, walking, jogging, or hiking) is thought to be safe when done alone or with a person one lives with, but when other people are nearby, people should wear a mask and maintain social distancing (> 6 feet) from the others. Group sports (eg, basketball, soccer) are not recommended as the nature of the activity makes appropriate distancing impossible.

It might also be prudent to wear a mask and eye protection and maintain greater than standard social distancing from heavily exercising people (eg, runners, bikers) who are likely to be exhaling more heavily and thus potentially creating a larger zone of contaminated, exhaled air; however, these are not formal recommendations.


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