Skip to Main Content



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.

Some people may continue to have symptoms or develop new or recurrent ones four or more weeks after an initial SARS-CoV-2 infection. This is known as Long COVID, or Post-COVID Conditions (PCC).

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. Paxlovid was granted FDA approval in May 2023. 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 three vaccines being used in the U.S.: the Pfizer-BioNTech messenger-RNA (mRNA) vaccine known as Comirnaty, the mRNA Moderna vaccine known as SpikeVax, and a vaccine made by Novavax. The FDA has approved or authorized each of these vaccines for use in the U.S.

For more details about COVID-19 vaccines and recommended vaccine schedules, see this CDC webpage. For details on recommendations for immunocompromised people, see this CDC webpage.

Starting in the fall of 2023, new COVID-19 vaccine formulations are being used in the U.S. The new, 2023-2024 COVID-19 vaccines manufactured by Pfizer-BioNTech, Moderna, and Novavax are based on XBB.1.5, an Omicron variant of the SARS-CoV-2 virus first detected in New York City in October 2022. An FDA advisory committee recommended that the updated vaccines target the XBB sublineage of the coronavirus based on expectations that XBB variants would be in wide circulation by fall 2023. The CDC anticipates that the new vaccine formulations will offer better protection against currently circulating coronavirus variants than previous vaccine formulations.

The CDC recommendations for the updated COVID-19 vaccines include:

Everyone 5 years of age and older should receive an updated Pfizer-BioNTech, Moderna, or Novavax vaccine. The CDC does not recommend one vaccine over another.

As of February 28, 2024, the CDC recommends an additional dose of the updated 2023-2024 vaccine for adults ages 65 and older.

Children between 5 and 11 years of age should get one dose of an updated Pfizer or Moderna vaccine. The updated dose should be received two or more months after their most recent COVID-19 vaccination.

People ages 12 and older who have not previously been vaccinated against COVID-19 should get one dose of the Pfizer or Moderna vaccine, or two doses of the Novavax vaccine. People in this age group should get the updated vaccine two or more months after their last COVID-19 vaccine dose.

Children between 6 months and 4 years of age must get multiple vaccine doses to be considered up to date on COVID-19 vaccinations, one of which must be a dose of an updated vaccine. Specific vaccine schedules for children in this age group are based on whether a child has been previously vaccinated and the vaccine manufacturer:

  • Unvaccinated children in this age group should get two or three doses of an updated mRNA COVID-19 vaccine as follows:
    • For Pfizer-BioNTech, children should get three doses. The second dose should be given three to eight weeks after the first dose and the third dose at least 8 weeks after the second dose.
    • For Moderna, children should get two doses. The second dose should be given four to eight weeks after the first dose.
  • Children who have gotten previous COVID-19 vaccine doses should get one or two doses of an updated mRNA COVID-19 vaccine as follows:
    • For Pfizer-BioNTech, children who have gotten one dose of a previous Pfizer COVID-19 vaccine should get two doses of the updated Pfizer-BioNTech vaccine. The first dose should be given three to eight weeks after the first dose, and the second dose should be given at least eight weeks after the second dose. Children who have received two or more previous doses of the Pfizer vaccine should get one dose of the updated vaccine at least eight weeks after the last dose.
    • For Moderna, children who have received one dose of a previous Moderna vaccine should get one dose of the updated Moderna vaccine four to eight weeks after their previous dose. Children who have received two or more doses of previous Moderna vaccine should get one dose of the updated Moderna vaccine at least eight weeks after the last dose.
  • Those who are moderately or severely immunocompromised may receive additional doses. They should discuss getting additional doses with a health care provider.

What happened to the Janssen/Johnson & Johnson vaccine?

The U.S. government stock of the Johnson & Johnson COVID-19 vaccine expired on May 7, 2023 and on June 1, 2023, the FDA revoked its EUA at the request of Janssen Biotech. The Johnson & Johnson COVID-19 vaccine is no longer available for use in the U.S.

The Johnson & Johnson vaccine was available for use in people 18 years of age or older under an FDA EUA. The primary series consisted of a single dose of the vaccine. However, in December 2021, the FDA announced evidence suggesting a causal relationship between the Janssen/Johnson & Johnson vaccine and a rare but serious blood clotting disorder called thrombosis with thrombocytopenia (TTS). Because of this, the FDA EUA was changed to say 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. In June 2023, the FDA withdrew the EUA for the Johnson & Johnson COVID-19 vaccine.

People who received the Johnson & Johnson vaccine are considered up to date after getting one dose of an updated COVID-19 vaccine.

How effective are vaccines against COVID-19?

The original 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 ancestral SARS-CoV-2 strain. The Johnson & Johnson vaccine was shown to have an overall efficacy of 66% efficacy against moderate to severe COVID-19, while the Novavax vaccine was shown to have a 90.4% efficacy against symptomatic COVID-19.

While the vaccines in use in the U.S. protect against severe illness and death, people who have been vaccinated can still get COVID-19. This is because none of the vaccines is 100% effective against infection or illness. People who are vaccinated against COVID-19 can also spread the virus to others and should take precautions to avoid spreading the virus. The CDC conducts studies to monitor and determine vaccine effectiveness "under real-world conditions." Among other factors, these studies are designed to understand how well vaccines work for different age groups, among people with risk factors such as underlying medical conditions or pregnancy, and according to vaccine dose and length of time since the last dose.

Several studies have shown that vaccine effectiveness against SARS-CoV-2 infection and protection against mild, moderate, and severe disease decreases over time. For instance, a study published in the British Medical Journal (BMJ) in October 2022 found that the effectiveness of mRNA COVID-19 vaccines waned over time. The study assessed data from nearly 900,000 adults admitted to hospitals, emergency departments, and urgent care clinics across 10 states.

Evidence also suggests that the vaccines do not work as well against emerging variants, including Omicron variants, because these are able to partially evade the immune response. Because vaccine effectiveness can wane over time and emerging coronavirus variants are capable of partially evading the immune response, Pfizer and Moderna have formulated bivalent mRNA vaccines that target both the original strain of the coronavirus and Omicron BA.4/BA.5 subvariants. Researchers are actively studying the effectiveness of the vaccines against Omicron variants.

While vaccine effectiveness against disease may decline over time in vaccinated people, the COVID-19 vaccines in use in the U.S. still provide good protection against severe disease and death. The CDC recommends that everyone stay “up to date” with COVID-19 vaccinations and boosters to ensure the best protection against COVID-19.

What are virus variants?

Scientists have identified several variants of the original coronavirus strain that causes COVID-19. Some of these variants may be classified 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 might emerge in the future.

In late December 2021, Omicron overtook Delta as the dominant variant in the U.S. Since May 2022, Omicron subvariants have 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 JN.1, may have certain mutations that make them more transmissible and/or 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.

As of late April 2024, the Omicron KP.2 subvariant made up an estimated 24.9% of COVID-19 cases in the U.S., while the JN.1 subvariant accounted for an estimated 22% of cases. Several other Omicron subvariants were circulating as well, including JN.1.7, JN.1.13.1, KP.1.1, JN.1.16, and JN.1.8.1. These variants made up an estimated 13.7%, 8.8%, 7.5%, 6.9%, and 5.3% of U.S. cases, respectively. Several 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 three COVID-19 vaccines in use in the U.S. 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 updated vaccines.
  • 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.
  • 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).