Skip to Main Content


  • The disease caused by the SARS-CoV-2, the 2019 novel coronavirus
  • Symptoms include fever or chills, cough, shortness of breath, sore throat, new loss of taste or smell, and others
  • Prevention and treatment strategies include vaccinations, medical therapies, and mitigation efforts such as social distancing and wearing masks


Coronavirus disease 2019 (COVID-19) is the official name given by the World Health Organization (WHO) to the disease caused by SARS-CoV-2, the new coronavirus that surfaced in Wuhan, China in 2019 and spread around the globe. By March 2020, COVID-19 was so widespread that the WHO characterized it as a global pandemic, a disease outbreak that covers a wide geographic area and affects an exceptionally high proportion of people.

People who have been infected with COVID-19 respond in different ways—some report mild symptoms or no symptoms at all. Others experience severe symptoms, are hospitalized, and even die from the disease.

Efforts to mitigate the disease have included “social distancing,” masks, and stay-at-home mandates. By early 2021, scientists had developed effective vaccines, and countries around the globe were focused on quickly vaccinating as many people as possible. But there was more to learn, especially with the identification of new SARS-CoV-2 variants (new strains based on mutations in the sequence of the genetic code of the original virus). Meanwhile approaches to treating the disease are still evolving.

What are the origins of the novel coronavirus?

Severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2, was never seen before it surfaced in December 2019—when it was believed to have passed somehow from an animal to a human at a large seafood and live animal market in Wuhan. (Its origins are still under investigation.) It is one of seven known coronaviruses that cause illnesses that range from the common cold to severe acute respiratory syndrome (SARS), an epidemic that killed almost 800 people in 2002 and 2003.

COVID-19 is the first pandemic known to be caused by the emergence of a new coronavirus. Novel influenza viruses caused four pandemics in the last century, which is why some of the response to the new disease has been adapted from existing guidance developed in anticipation of an influenza pandemic.

How does COVID-19 spread?

The disease is believed to spread among people in the following ways, according to the Centers for Disease Control and Prevention (CDC):

  • Through respiratory droplets transmitted in small particles when a person who is infected breathes, coughs, talks, or sneezes. Bystanders may become infected by inhaling respiratory droplets or aerosol particles that contain the virus.
  • Touching the eyes, nose, or mouth with hands or fingers that have infectious virus on them. For instance, it is possible to get COVID-19 by touching a contaminated surface or object, and then touching the eyes, nose, or mouth. But according to the CDC, the risk of contracting COVID-19 in this way is low.

People are more likely to get infected through droplets and particles that circulate in the air if they are within 6 feet (2 arm lengths) of an infected individual.

What are the symptoms of COVID-19?

COVID-19 is most contagious during the early phases of illness, before symptoms begin and when symptoms first develop. Symptoms of COVID-19 can appear anytime between two and 14 days after exposure. Many people report one or more of the following symptoms over the course of their disease:

  • Fever or chills
  • Cough
  • Shortness of breath and difficulty breathing
  • Fatigue
  • Muscle or body aches
  • Headache
  • New loss of taste or smell
  • Sore throat
  • Congestion or runny nose
  • Nausea or vomiting
  • Diarrhea

You should call your medical provider for advice if you have been in close contact with a person known to have COVID-19 or if you live in an area with ongoing spread of the disease and notice these symptoms.

Seek medical attention immediately if you experience emergency warning signs, including difficulty breathing or shortness of breath, persistent pain or pressure in the chest, new confusion or ability to arouse, or bluish lips or face. This list is not inclusive, so consult your medical provider if you notice other concerning symptoms.

Who is at risk for COVID-19 and complications from the disease?

Anyone who may have been exposed to the SARS-CoV-2 is at risk for COVID-19. Experts are still learning about how to predict who will develop severe symptoms that could lead to hospitalization, time in an intensive care unit, or use of a ventilator to help with breathing.

But the risk for complications increases with age. For example, people in their 60s have more risk than those in their 50s. People who are 85 and older are at the highest risk, according to the CDC, which reports that 8 out of 10 deaths from COVID-19 have been reported in people aged 65 and older.

People of all ages with medical conditions are at higher risk for complications, as are people who smoke. The list of conditions includes, but is not limited to, the following:

  • Cancer
  • Chronic kidney disease
  • Chronic lung diseases including COPD, moderate and severe asthma, and cystic fibrosis
  • Diabetes
  • Heart conditions
  • HIV infection
  • Liver disease
  • Obesity and being overweight
  • Pregnancy
  • Sickle cell disease
  • Solid organ transplant
  • Substance use disorders

Though fewer children than adults have gotten sick with COVID-19, there have been cases where children have gotten severely ill and even died. Doctors are concerned about a rare condition called multisystem inflammatory syndrome (MIS-C) in children, a potentially serious inflammatory condition that may develop after SARS-CoV-2 infection.

How is COVID-19 treated?

Researchers continue to study the safety and effectiveness of a number of treatments for COVID-19. Many treatments are still in the early stages of research and some have shown promise, but scientists need more data on their safety and effectiveness.

Several treatments for COVID-19 are currently available. Monoclonal antibodies that specifically target SARS-CoV-2 may be used to treat patients, though they require intravenous administration and must be used early in the course of COVID-19. In December 2021, two oral antiviral drugs, Paxolvid and molnupiravir, received FDA EUAs. When taken within five days of the onset of symptoms, these drugs can reduce the risk of hospitalization and death due to COVID-19. The two drugs are authorized for use in people who are at high risk for severe disease. Paxlovid may be used for those age 12 or over, while molnupiravir is only authorized for adult use.

What vaccines are available in the U.S.?

There are four vaccines being used in the U.S.:

  • Pfizer-BioNTech vaccine. Requires two doses given three to eight weeks apart. In August 2021, the FDA approved the vaccine, which is known as Comirnaty, for use in people aged 16 and over. It also has received an EUA for use in children and adolescents between the ages of 5 and 15.

    In June 2022, a three-dose series of the vaccine also received an FDA EUA for use in children between 6 months and 4 years of age. Children in this age group should receive two doses three to eight weeks apart followed by a third dose given at least eight weeks after the second dose.

    People ages 5 and over who are immunocompromised with certain conditions may get a third dose of the vaccine as part of their primary series.
  • Moderna vaccine. Requires two doses given four to eight weeks apart. The Moderna vaccine, known as Spikevax, is available to anyone 6 months of age or older under an FDA EUA.

    Individuals 6 years or age or older who are immunocompromised with certain types of conditions can also receive a third dose as part of their primary vaccination series.
  • Novavax vaccine. Requires two doses given three to eight weeks apart. In July 2022, the vaccine received an FDA EUA for people 18 years of age and over and in August 22, the FDA issued an EUA for the vaccine for individuals 12 years of age and older. 
  • Janssen/Johnson & Johnson vaccine. Requires only a single dose. It was available for use in people 18 years of age or older under an FDA EUA. However, in December 2021, the FDA announced evidence suggesting a causal relationship between the Janssen/Johnson & Johnson vaccine and a rare blood clotting disorder called thrombosis with thrombocytopenia (TTS). Because of this, the FDA EUA was changed to says that people who had blood clotting with low platelet counts after receiving the Johnson & Johnson vaccine should not get the Johnson & Johnson vaccine as a booster dose.

    Then, in May 2022, the FDA further revised the EUA, limiting the use of the vaccine to certain groups of people due to risk of the rare blood clotting disorder. The FDA now limits this vaccine to people who are not clinically appropriate for the Moderna or Pfizer vaccines (e.g., they previously had an adverse reaction to them) and those who would otherwise remain unvaccinated either because they have personal concerns about mRNA vaccines and instead opt for the Johnson & Johnson vaccine or do not have access to Moderna or Pfizer vaccines.

What is the guidance on COVID-19 booster shots?

The CDC recommends that all people who are eligible get vaccinated against COVID-19 and stay up to date with booster shots. The agency’s guidelines for booster shots vary based on age, the amount of time since the initial vaccination or previous booster shot, underlying health conditions, and the type of vaccine.

CDC guidelines for immunocompromised individuals may be found here.

  • Pfizer-BioNTech. People ages 6 months and older are eligible for a booster dose two months or longer after completing a primary vaccination series or receiving a previous booster dose.
    In December 2022, the FDA authorized the Pfizer bivalent booster shot for children as young as 6 months of age. Children between 6 months and 4 years of age who have gotten two doses of the original Pfizer vaccine are eligible for a bivalent Pfizer booster dose beginning two months after completing their primary vaccination series. The FDA has also authorzed the Pfizer bivalent booster for people over 5 years of age starting two months after receiving their most recent vaccine dose. This vaccine is designed to protect against disease caused by the original strain of the SARS-CoV-2 virus as well as the Omicron variant.
  • Moderna. The FDA has issued an EUA for a bivalent Moderna booster designed to provide protection against COVID-19 caused by infection with the original virus strain as well as the Omicron variant. It is authorized for use in individuals 6 months of age or older. Children between the ages of 6 months and 5 years who received two doses of the original Modern vaccine are eligible for a Moderna booster dose starting two months after completing their initial two-dose vaccination series. For those age 5 and above, the bivalent booster can be received starting two months after completing the two-dose primary series or after getting a previous booster shot.
  • Novavax. In October 2022, the FDA authorized the Novavax vaccine as a first booster dose for people aged 18 and over. The Novavax vaccine is monovalent, meaning it targets only one strain (the original strain) of the coronavirus. Adults who have completed their primary COVID-19 vaccination can get a Novavax booster provided they have not yet received another booster shot. The CDC notes that the Novavax booster is an option for individuals who “cannot or will not receive mRNA vaccines.”
  • Johnson & Johnson. People who were vaccinated with the Johnson & Johnson vaccine are eligible for a booster shot two months or longer after receiving their initial vaccination. The CDC further notes that those who received the Johnson & Johnson vaccine and booster shot are eligible for a second booster dose of either the Moderna or Pfizer vaccine starting four months after they got their initial booster shot.
    The CDC says that booster doses of the Pfizer-BioNTech and Moderna vaccines are preferred to the Johnson & Johnson vaccine, though in some cases, the Johnson & Johnson may be considered.

How effective are vaccines against COVID-19?

The Pfizer-BioNTech and Moderna vaccines were shown in clinical trials to have an overall efficacy of 95% and 94.1%, respectively, against mild to severe COVID-19 caused by the original SARS-CoV-2 strain. The Johnson & Johnson vaccine was shown to have an overall efficacy of 66% efficacy against moderate to severe COVID-19. People are considered fully vaccinated two weeks after receiving their second shot of the Pfizer-BioNTech and Moderna vaccines and two weeks after receiving the Johnson & Johnson vaccination.

While each of these vaccines protects against COVID-19, some people who have been fully vaccinated will still get COVID-19. This is because none of the vaccines is 100% effective against infection or illness. The CDC tracks “breakthrough” cases that result in hospitalization and death to learn about the virus variants and types of vaccines involved, as well as the general health of affected individuals

Some studies have shown that vaccine effectiveness against COVID-19 may wane over time and that the vaccines may not work as well against the Delta and Omicron variants. For instance, a study published by the CDC in September 2021 found that as the Delta variant became the dominant variant in New York state, vaccine protection against breakthrough infections dropped from nearly 92% to 75%. 

Other studies have found that vaccine effectiveness against mild-to-moderate disease may also decline over time, meaning that fully vaccinated people who get infected by the coronavirus develop some symptoms.

Researchers are studying the effectiveness of the vaccines against the Omicron variant. The CDC expects the vaccines currently in use in the U.S. to protect against severe disease, hospitalization, and death in those infected with the Omicron variant. People who are vaccinated against COVID-19, however, can still get infected and the CDC anticipates that they can also spread the virus to others. 

What are virus variants?

Scientists have identified several variants of the original coronavirus strain that causes COVID-19. Some of these variants are known as “variants of concern” because they may be more contagious, cause more severe illness, and/or reduce the relative effectiveness of treatments or vaccines compared to the original COVID-19 strain. Scientists are also concerned that other variants that might emerge in the future.

The CDC currently recognizes one variant of concern—Omicron, named based on a letter of the Greek alphabet. In late December 2021, Omicron overtook Delta, another coronavirus variant, as the dominant variant in the U.S. As of late May 2022, the Omicron variant made up an estimated 100% of all cases in the U.S. Scientists have detected several subvariants of Omicron. Some of these subvariants, such as one known as BA.5, have certain mutations that make them more transmissible and better able to evade immunity acquired from vaccination or a previous infection than the original Omicron variant. Scientists are studying recently detected Omicron subvariants to better understand their transmissibility and whether they cause more severe disease. In mid-January 2023, the Omicron XBB.1.5 subvariant made up an estimated 49.1% of COVID-19 cases in the U.S., while the BQ.1.1 subvariant accounted for an estimated 26.9% of cases. Several other Omicron subvariants were circulating as well, including BQ.1, XBB, BA.5, and BN.1. These variants made up an estimated 13.3%, 3.3%, 2.0%, and 1.8% of U.S. cases, respectively. A handful of other Omicron subvariants were also in circulation, though each of them made up only a small percentage of total cases.

Fortunately, research suggests that the one authorized (and two fully approved) COVID-19 vaccines protect against severe illness caused by the known variants, and scientists are monitoring the virus to detect any new variants that appear.

What precautions can I take to avoid COVID-19?

The CDC recommends several preventive measures to protect against getting infected with the coronavirus and reduce the chances of spreading it to others, including:

  • Get vaccinated against COVID-19 and stay up to date with booster shots.
  • The CDC recommends that people wear the most protective mask that they will wear consistently. To be effective, a mask should be tight fitting, without gaps that allow respiratory droplets to escape or enter the mask. Various types of masks are available including cloth masks, disposable “surgical” masks, and the most protective option, respirators such as N95s or KN95s. This CDC webpage provides details on different types of masks, how to check and improve mask fit, and mask recommendations for children and others.
  • Wash hands often with soap and water for at least 20 seconds, especially if you have been in a public place. If soap isn’t available, use a hand sanitizer with at least 60% alcohol.
  • Cover coughs and sneezes
  • Practice social distancing by staying at least 6 feet away from other people.
  • Avoid touching eyes, mouth, or nose unless hands are clean.
  • Avoid crowded indoor places and indoor spaces with poor ventilation.
  • Avoid close contact with people who have COVID-19. Those who have been in the presence of someone who has a coronavirus infection for 15 minutes or more within a 24-hour period should follow the CDC’s quarantine recommendations. 
  • Regularly clean and disinfect frequently touched surfaces such as doorknobs, light switches, desks, keyboards, sinks, and other objects and surfaces, especially if someone in the household has tested positive for COVID-19 or is sick.

Testing can help prevent the spread of COVID-19. If someone tests positive for a coronavirus infection, they can take steps to avoid spreading the virus to others such as isolating. Two types of tests are available that can check for a current coronavirus infection.

In February 2022, as the number of Omicron infections declined substantially, the CDC relaxed its guidance around masks, physical distancing, and other preventive measures. The health agency now recommends that people wear a mask when “Community Levels” of COVID-19 are medium or high. The COVID-19 Community Level is determined based on county-level data on the number of new cases, hospital admissions, and the percentage of hospital beds used by COVID-19 patients. The CDC provides a tool to check COVID-19 Community Level in every county in the U.S.

How is Yale Medicine prepared to handle patients with COVID-19?

Yale Medicine Infectious Diseases has an entire team with experience treating both existing and emerging diseases. This team is at the forefront of the latest testing, diagnostic, and treatment approaches.

The Yale Medicine Winchester Center for Lung Disease’s Post-COVID-19 Recovery Program offers pulmonary-focused, multidisciplinary evaluation and care for patients recovering from COVID-19. The program partners with teams taking care of patients in the hospital, as well as community providers to identify patients who have persistent symptoms or appear at risk of developing post-COVID-19 complications.

Yale New Haven Health offers a call center for patients and people in the community who have questions about COVID-19 at 833-ASK-YNHH (833-275-9644).