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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 coughs, talks, or sneezes. Bystanders may become infected by inhaling respiratory droplets or aerosol particles that contain the virus.
  • Between people who are within 6 feet (2 arm lengths) of each other.
  • There is evidence that transmission can occur when people unknowingly inhale virus that remains suspended in the air at distances greater than 6 feet in indoor settings. In these cases, a person with COVID-19 spent over 15 minutes in the indoor setting. For this reason, indoor environments without good ventilation (including gyms, restaurants, and churches) are considered risky.
  • By touching a surface or object where droplets have landed, and then touching the eyes, nose, or mouth. According to the CDC, the risk of contracting COVID-19 from contaminated surfaces or objects is low.

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.

What vaccines are available in the U.S.?

As of late-2021, there are three vaccines being used in the U.S.:

  • Pfizer-BioNTech vaccine. Requires two doses given 21 days apart. In August 2021, the Food and Drug Administration (FDA) approved the vaccine for use in people aged 16 and over. It also has received emergency use authorization (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 September and October 2021, the CDC began to recommend a vaccine booster shot for select populations. The eligibility criteria for a booster shot varies based on the vaccine with which a person was initially vaccinated.

For those who received the Pfizer-BioNTech and Moderna vaccines, the agency recommends a booster shot to those ages 65 and older and anyone over 18 with certain underlying medical conditions, whose job puts them at high-risk of exposure to COVID-19, or who live in long-term care settings. Anyone in these groups is eligible for a booster dose starting six months or longer after their initial vaccination.

For those who received the Johnson & Johnson vaccine, the CDC recommends a booster shot to anyone age 18 and older provided they received their first dose at least two months earlier.

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.

Please visit this CDC webpage to learn more about 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, a small percentage of 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 is tracking these so-called “breakthrough” cases.

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 variant. 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. 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 who meet certain eligibility requirements.

What are the 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.

To date, the CDC has identified four variants of concern—Alpha, Beta, Delta, and Gamma—each named based on a letter of the Greek alphabet. By late August 2021, the Delta variant accounted for an estimated 86% of cases in the U.S.

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:

  • Wear a mask that covers both the mouth and nose around people who don’t live in the household. The CDC recommends a cloth mask made of at least two layers of breathable fabric or a disposable mask of multiple layers of material. The mask should provide a close fit that does not leave gaps through which air can enter or escape, and have a wire that allows it to be fitted over the bridge of the nose.
  • 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 (about 2 arms’ lengths) away from other people.
  • Limit unnecessary face-to-face interactions with others outside the household and keep interactions as brief as possible.
  • 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.

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).