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.

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.

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.

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).

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.

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.

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.

In August 2024, the FDA approved and granted EUA for updated mRNA COVID-19 vaccines manufactured by Moderna and Pfizer-BioNTech and an updated vaccine from Novavax. The updated mRNA vaccines, referred to as the 2024–2025 Moderna and Pfizer-BioNTech COVID-19 vaccines, target KP.2, an Omicron variant of SARS-CoV-2 that has been in circulation in the United States through the spring and summer of 2024. The 2024–2025 Novavax vaccine targets the JN.1 variant. Previous formulations of the COVID-19 vaccines targeted virus variants that are no longer in circulation. By targeting KP.2 and JN.1, the 2024–2025 vaccines are designed to provide improved protection against currently circulating variants.

The CDC recommendations for the 2024–2025 COVID-19 vaccines include:

Everyone 6 months of age and older—including those who have previously gotten a COVID-19 vaccine or who have had COVID-19—should receive a 2024–2025 vaccine. The CDC does not recommend one vaccine over another, though the Novavax vaccine is only approved for individuals aged 12 and older. The health agency states that it is particularly important that people in the following groups get a 2024–2025 vaccine:

• Ages 65 and older
• At high risk for severe COVID-19
• Never received a COVID-19 vaccination
• Live in a long-term care facility
• Are pregnant, breastfeeding, or trying to get pregnant or may become pregnant

The CDC recommends that people stay up to date on their COVID-19 vaccinations:

Children ages 6 months–4 years. Children in this age range are considered up to date once they have gotten all recommended vaccine doses. If a child has previously received:

• Zero vaccine doses (never vaccinated), they should get two doses of the 2024–2025 Moderna vaccine or 3 doses of the 2024–2025 Pfizer-BioNTech vaccine
• One or more doses of Moderna vaccine, they should get one dose of the 2024–2025 Moderna vaccine
• One dose of Pfizer-BioNTech vaccine, they should get two doses of the 2024–2025 Pfizer-BioNTech vaccine
• Two or more doses of Pfizer-BioNTech vaccine, they should get one dose of the 2024–2025 Pfizer-BioNTech vaccine

Children ages 5–11 years. People in this age range are considered up to date once they have gotten one dose of the 2024–2025 Moderna vaccine or one dose of the 2024–2025 Pfizer-BioNTech vaccine.

People ages 12 and older. Individuals ages 12 and older are considered up todate when they have received:

• One dose of the 2024–2025 Moderna or one dose of the 2024–2025 Pfizer-BioNTech vaccine, or
• One dose of the 2024–2025 Novavax vaccine (if the individual has not gotten any previous COVID-19 vaccine and they select the Novavax vaccine, they must receive two doses of the 2024–2025 Novavax vaccine to be considered up to date).

The CDC notes that people who have had COVID-19 recently may hold off on getting a COVID-19 vaccine for three months. This is because the risk of getting COVID-19 is lower in the weeks and months after getting infected with SARS-CoV-2. Still, those who are at increased risk for severe COVID-19, have a household member or close contact at risk for COVID-19, or live in an area with a high level of COVID-19 may consider getting a vaccine sooner.

Those 6 months and older who are moderately or severely immunocompromised may receive additional doses. They should discuss getting additional doses with a health care provider.

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.

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, vaccine manufacturers have formulated vaccines that target and provide improved protection against virus variants that are currently circulating. 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.

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-August 2024, the Omicron KP.3.1.1 subvariant made up an estimated 42.2% of COVID-19 cases in the U.S., while the KP.2.3 subvariant accounted for an estimated 14.6% of cases. Several other Omicron subvariants were circulating as well, including KP.3, LB.1, KP.2, and LP.1. These variants made up an estimated 14.2%, 13.5%, 3.1%, and 3.1% 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.

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.