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Coronavirus Disease 2019; COVID


The Manual's Editorial Staff

Last full review/revision Dec 2021| Content last modified Dec 2021
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What is COVID-19?

  • It's caused by a new type of coronavirus

  • The virus spreads very easily to other people

  • COVID-19 is now everywhere in the world

  • Some infected people have no symptoms, but others get very sick, and some people die

  • Doctors test swabs from your nose or throat to diagnose COVID-19

  • No medicines cure COVID-19, but some might help people with severe infection, and doctors are always testing additional ones

  • Vaccines for COVID-19 are being given to people in the United States and around the world

What is a pandemic?

COVID-19 is a pandemic. That's an infection that's spreading rapidly in many parts of the world all at once.

What causes COVID-19?

COVID-19 is caused by a new type of coronavirus called SARS-CoV-2. Doctors call it new because it hasn't been around before. It recently changed (mutated) from a type of coronavirus that infects bats.

The virus spreads:

  • Through the air, in droplets that an infected person spreads through a cough, sneeze, singing, or talking

  • Through touching things that infected people have touched and then touching your face

Unlike with many respiratory viruses, you can spread the virus that causes COVID-19 before you have any symptoms. If you don't know you have the virus, you can pass it on and not know it.

What is a coronavirus?

There are hundreds of different coronaviruses that are all related. Most of them infect only animals.

  • Only a few coronaviruses can infect people

  • Some of them cause only colds

  • Three coronaviruses cause very serious illness and can be fatal

What are the symptoms of COVID-19?

Many people with COVID-19, particularly children, have almost no symptoms.

If you do get symptoms, you'll likely have:

  • Fever

  • Dry cough

  • Trouble breathing

  • Feeling really weak and tired

You may also have:

  • Runny nose and sore throat

  • Loss of your sense of smell and taste

  • Headaches

  • Stomach problems such as nausea, vomiting, and diarrhea

A bad infection gets deep in your lungs, causing viral pneumonia. That will make you very short of breath. A bad infection can also affect a lot of organs, including your brain, heart, and kidneys.

Is COVID-19 fatal?

You can die from COVID-19. The older you are, the higher your risk of death. Most deaths occur in people over 50 years old. People over 80 are at very high risk. But even children and young adults can die.

You're also at higher risk if you have other medical problems such as heart disease, obesity, or diabetes.

How can doctors tell if I have COVID-19?

The most common test uses:

  • A swab taken from your nose or throat

The swab is sent to a lab to look for the virus or other evidence of an infection.

A blood test can look for antibodies What are the main parts of the immune system? The immune system is your body's defense system. It helps protect you from illness and infection. The immune system's job is to attack things that don’t belong in your body, including: Germs... read more your immune system has made to defend against the virus. It takes your body a few weeks to make these antibodies, so this test is not useful when you first get sick. But it can help tell whether you were ever infected.

If you're short of breath, doctors will usually:

  • Do a chest x-ray to look for pneumonia

  • Measure the oxygen level in your blood with a fingertip sensor

How is COVID-19 treated?

If you have a mild or moderate case, you'll:

  • Stay home and not go out or go near other people (isolation)

  • Wear a mask so you don't spread the virus

  • Take acetaminophen or ibuprofen for fever and aches

If you have a severe case, with breathing problems, you'll need to go to the hospital. Doctors will:

  • Give you oxygen

  • Sometimes, give you drugs

  • Sometimes, put you on a ventilator (machine to help you breath)

You may need to be on a ventilator for up to a few weeks.

How can I prevent COVID-19?

You should avoid being exposed to the virus, which can be difficult. People who can spread the virus may not have any symptoms, so you can't tell who has it and who doesn't.

If you’re not fully vaccinated, it's important to:

  • Keep 6 feet away from people who don't live in your household ("social distancing") and

  • Wear a mask over your mouth and nose when around people who don't live with you

If you’re fully vaccinated, you can go about your activities without wearing a mask or physically distancing, except where required, including by local business and workplace guidance.

Whether or not you’re vaccinated, you should also:

  • Wash your hands often with soap and water for at least 20 seconds

  • Use an alcohol-based hand sanitizer

  • Avoid touching your eyes, nose, and mouth with unwashed hands

  • Cover a cough or sneeze with a tissue, then throw the tissue in the trash

  • Use a household disinfectant spray or wipe to clean things people touch a lot, like door handles, phones, remote controls, and countertops

What does it mean to quarantine or isolate?

To help prevent the virus from spreading, certain people should stay away from other people by quarantining or isolating. If you have symptoms of COVID-19, tested positive for COVID-19, or are a "close contact" of someone with COVID-19, you need to isolate or quarantine.

You are a close contact if you

  • Were within 6 feet of an infected person (with or without a mask) for at least 15 minutes, including the 2 days before that person showed symptoms or had a positive test for COVID-19

  • 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

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

Quarantine is for people who are close contacts. Close contacts may not know if they have the virus or not, because the virus can take 14 days to make you sick or show up in a test. The goal of quarantine is to keep close contacts away from other people until those 14 days are over. If you are a close contact, you need to quarantine for 14 days even if:

  • You don't have symptoms

  • You tested negative—even if you tested negative during the 14 days, you still need to quarantine for the rest of the 14 days

You do not need to quarantine after a close contact if you have had COVID-19 in the past 3 months or been fully vaccinated.

Isolation is for people with COVID-19. If you were a close contact but then you got symptoms or tested positive, you need to isolate. Any days you spent in quarantine don't count for any of the 10 days (at least) you now need to isolate. You should isolate if you:

  • Are sick with COVID-19 symptoms but have not been tested

  • Tested positive (whether or not you have symptoms)

Isolation can generally end 10 days after your symptoms started, as long as you don't have a fever and aren't taking drugs to lower a fever and your symptoms are getting better. If you never had symptoms, you can stop isolating 10 days after the date of your first positive test.

Why do people who might not even have the virus need to quarantine for 14 days, but people who actually have COVID-19 may only need to isolate for 10 days?

  • Before you have symptoms or test positive, you can be contagious (able to spread the virus). It can take up to 14 days after you're exposed to the virus for it to make you sick or show up in a test (called the incubation period). That's why you need to isolate for 14 days, even if at the end it turns out that you never had the virus at all. There was no way to know that you weren't contagious until the 14 days were over.

  • Once you're sick with or test positive for COVID-19, the 14-day period doesn't matter anymore. No matter how many days you were already quarantined (0 to 14 days), once you know you have COVID-19, you start the 10-day isolation clock. Scientists have learned that you most likely are not contagious after 10 days from when you first felt sick or tested positive (unless you still have a fever).

What is contact tracing?

Contact tracing is when health departments look for people who came into contact with someone who has certain infections, including COVID-19. Contact tracers tell you that you may have been exposed to someone with COVID-19, help you get tested, and talk to you about isolating or quarantining. Cooperating with contact tracers helps slow the spread of COVID-19 in the community.

Is there a vaccine for COVID-19?

Yes. Vaccines for COVID-19 have been developed and are being given to people around the world. Three of these vaccines are being given to people in the United States. The vaccines are known by the name of the company making the vaccine:

  • The Pfizer-BioNTech COVID-19 vaccine (mRNA) is for people 5 years and older and is given as 2 shots about 3 weeks apart.

  • The Moderna COVID-19 vaccine (mRNA) is for people 18 years and older and is given as 2 shots about 4 weeks apart.

  • The Johnson & Johnson COVID-19 vaccine (adenovirus vector) is for people 18 years and older and requires only a single shot.

An additional primary series dose of the Pfizer and Moderna vaccines may be given to people who are at a very high risk of severe disease. This additional dose should be administered at least 28 days after an initial 2-dose series. People should talk to their healthcare provider about their medical condition, and whether getting an additional primary dose is appropriate for them. (See CDC: COVID-19 Vaccines for Moderately to Severely Immunocompromised People.)

People 18 years and older who are fully vaccinated against COVID-19 can get a booster shot. (See CDC: COVID-19 Booster Shots.)

  • Booster doses are recommended for people who are 18 years and older and completed their initial 2-dose series (or their third primary dose, if immunocompromised) of the Pfizer or Moderna vaccine at least 6 months ago.

  • Booster doses are recommended for people who are 18 years and older and who received their first dose of the Johnson & Johnson vaccine at least 2 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.

People 16 and 17 years old who had their initial 2 shots (or their third primary shot, if immunocompromised) of the Pfizer vaccine at least 6 months ago can get a booster shot of the Pfizer vaccine only. (See CDC Expands COVID-19 Booster Recommendations to 16-and-17-year-olds.)

  • People 16 and 17 years old who received the Moderna or Johnson & Johnson vaccine cannot get a booster shot.

  • Unlike people 18 years and older who can choose any available COVID-19 vaccine as a booster shot, only a Pfizer booster shot is available to people 16 and 17 years old.

All 3 vaccines were highly effective at preventing serious complications of COVID-19 including hospitalizations and deaths in their clinical trials.

Some people may have mild side effects to the vaccines including:

  • Pain, swelling, and redness where you got the shot

  • Feeling tired

  • Headache

  • Muscle pain

  • Chills

  • Joint pain

  • Fever

  • Nausea

  • Feeling unwell

  • Swollen lymph nodes

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

There is a small chance of having a severe allergic reaction. This usually happens 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). Signs of a severe allergic reaction include:

  • Difficulty breathing

  • Swelling of your face and throat

  • A fast heartbeat

  • A bad rash all over your body

  • Feeling dizzy and weak

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