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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—people in their 50s are at higher risk for complications than those in their 40s, people in their 60s have more risk than those in their 50s, and 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, paxlovid 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 sever 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 three vaccines being used in the U.S.:

  • Pfizer-BioNTech vaccine. Requires two doses given 21 days apart. In August 2021, the FDA approved the vaccine 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.
  • Moderna vaccine. Requires two doses given 28 days apart. The Moderna vaccine is available to anyone 18 years of age or older under an FDA EUA.
  • Janssen/Johnson & Johnson vaccine. Requires only a single dose. It is available for use in people 18 years of age or older under an FDA EUA.

In December 2021, the FDA announced that current evidence suggests there is 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 now 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.

Because of the increased risk of TTS, the CDC recommends that the Pfizer-BioNTech and Moderna vaccines should be preferred to the Johnson & Johnson vaccine.

However, people who have had a severe allergic reaction to an mRNA vaccine or who have a preference for the Johnson & Johnson vaccine—despite warnings that it increases risk of TTS—can still receive it.

What is the guidance on COVID-19 booster shots?

The CDC recommends that everyone age 18 or over get a booster shot. People in this age group can get a booster shot of any of the three vaccines, though the CDC-recommended timing of the booster shot varies based on which vaccine a person received for their initial vaccination series.

Those who received the Pfizer-BioNTech vaccine should get a booster shot starting five months or longer after their initial vaccination series. People who got Moderna for their primary vaccination series should get a booster shot six months or longer after their second shot. For those who received the Johnson & Johnson vaccine, the CDC recommends a booster shot to anyone age 18 or older provided they received their first dose at least two months earlier. 

The FDA has also authorized a Pfizer-BioNTech booster shot for children and adolescents between the ages of 12 and 17. The CDC recommends that people in this age group get a booster starting five months after completing their initial vaccination series.

People who get a booster shot are free to “mix-and-match” vaccines. This means that the vaccine used for the booster shot does not need to match the vaccine used for the initial vaccination. For example, a person initially vaccinated with the Pfizer-BioNTech vaccine can get a booster dose of the Pfizer-BioNTech, Moderna, or Johnson & Johnson vaccine. 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.

Please visit this CDC webpage to learn more about booster shots and eligibility criteria.

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% against mild to severe COVID-19, respectively. 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.

Though vaccine protection against the coronavirus may lessen over time, scientists and doctors emphasize that the vaccines continue to be highly effective in preventing severe disease, hospitalization, and death, including against the Delta variant.

Researchers are studying the effectiveness of the vaccines against the Omicron variant.

Still, in response to waning effectiveness, the FDA has authorized booster shots of the Pfizer-BioNTech, Moderna, and Johnson & Johnson vaccines for previously vaccinated people  age 18 or older.

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 two variants of concern—Delta and Omicron—each named based on a letter of the Greek alphabet. In late December 2021, Omicron overtook Delta as the dominant variant in the U.S. As of mid-January 2022, the Omicron variant made up an estimated 99.5% of all cases in the U.S. and the Delta variant accounted for only about 0.5% of cases.

Fortunately, research suggests that the two authorized (and one 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?

For people who have not yet been vaccinated against COVID-19, the CDC recommends the following preventive measures:

  • 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.
  • 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.
  • Use a household wipe or spray to clean and disinfect doorknobs, light switches, desks, keyboards, sinks, and other objects and surfaces that are frequently touched.

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.

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).