Merck Manual

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Kenneth M. Kaye

, MD, Harvard Medical School

Reviewed/Revised Sep 2021 | Modified Sep 2022
Topic Resources
  • Chickenpox most often affects children, but the vaccine has greatly decreased the number of cases.

  • Before the rash appears, children have a mild headache, moderate fever, loss of appetite, and a general feeling of illness.

  • The diagnosis is based on symptoms, particularly the rash.

  • Most children recover completely, although some children get very sick and can even die.

  • Routine vaccination can prevent chickenpox.

  • Usually, only the symptoms need to be treated.

Before the introduction of a vaccine in 1995, about 90% of children developed chickenpox by age 15. Now, the use of the vaccine has decreased the number of cases of chickenpox per year by approximately 90%. However, in some countries, the vaccine is not available or is not included as a routinely recommended childhood vaccine.

A person with chickenpox is contagious from 2 days before the rash appears and remains contagious until the last blisters have crusted.

Did You Know...

  • A person with chickenpox is contagious from 2 days before the rash appears and remains contagious until the last blisters have crusted.

A person who has had chickenpox develops immunity and cannot contract it again. However, the varicella-zoster virus remains inactive (dormant) in the body after an initial infection with chickenpox, sometimes reactivating in later life, causing shingles Shingles Shingles is a painful skin rash caused by a viral infection that results from reactivation of the varicella-zoster virus, the virus that causes chickenpox. What causes the virus to reactive... read more Shingles . A shingles vaccine Shingles Vaccine The herpes zoster virus that causes shingles is the same virus that causes chickenpox. After chickenpox resolves, the virus remains in the body. It can be reactivated years later and cause shingles... read more is available for older adults. This vaccine may decrease the risk of developing shingles in later life.

Transmission of chickenpox

Chickenpox is spread in the following ways:

  • Through airborne droplets of moisture containing the varicella-zoster virus

  • Through contact with the rash caused by chickenpox or shingles

  • From a pregnant women to the fetus

Symptoms of Chickenpox

Symptoms of chickenpox begin 7 to 21 days after infection occurs. They include

  • Mild headache

  • Moderate fever

  • Loss of appetite

  • A general feeling of illness (malaise)

Younger children often do not have these symptoms, but symptoms are often severe in adults.

About 24 to 36 hours after the first symptoms begin, a rash of small, flat, red spots appears. The spots usually begin on the trunk and face, later appearing on the arms and legs. Some people have only a few spots. Others have them almost everywhere, including on the scalp and inside the mouth.

Within 6 to 8 hours, each spot becomes raised. It forms an itchy, round, fluid-filled blister against a red background and finally crusts. Spots continue to develop and crust for several days. A hallmark of chickenpox is that the rash develops in crops so that the spots are in various forms of development at any affected area. Very rarely, the spots become infected by bacteria, which can cause a severe skin infection (cellulitis Cellulitis Cellulitis is a spreading bacterial infection of the skin and the tissues immediately beneath the skin. This infection is most often caused by streptococci or staphylococci. Redness, pain, and... read more Cellulitis or necrotizing fasciitis Necrotizing Skin Infections Necrotizing skin infections, including necrotizing cellulitis and necrotizing fasciitis, are severe forms of cellulitis. These infections cause infected skin and tissues to die (necrosis). The... read more Necrotizing Skin Infections ).

New spots usually stop appearing by the fifth day, the majority are crusted by the sixth day, and most disappear in fewer than 20 days.

Sometimes children who have been vaccinated develop chickenpox. In these children, the rash is typically milder, fever is less common, and the illness is shorter. However, contact with the sores can spread the infection.

Spots in the mouth quickly rupture and form raw sores (ulcers), which often make swallowing painful. Raw sores may also occur on the eyelids and in the upper airways, rectum, and vagina. The worst part of the illness usually lasts 4 to 7 days.


Risk of complications from chickenpox is increased for newborns, adults, and people who have a weakened immune system or certain disorders.

Lung infection (pneumonia Overview of Pneumonia Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them. Pneumonia is one of the most common causes of death worldwide. Often, pneumonia is the final... read more Overview of Pneumonia ) occurs in about 1 out of 400 adults, resulting in cough and difficulty breathing. Pneumonia rarely develops in young children who have a normal immune system.

Brain infection (encephalitis Encephalitis Encephalitis is inflammation of the brain that occurs when a virus directly infects the brain or when a virus, vaccine, or something else triggers inflammation. The spinal cord may also be involved... read more ) is less common and causes unsteadiness in walking, headache, dizziness, confusion, and seizures. In adults, encephalitis can be life threatening. It occurs in 1 to 2 of 1,000 cases of chickenpox.

Inflammation of the liver and bleeding problems may also occur.

Reye syndrome Reye Syndrome Reye syndrome is a very rare but life-threatening disorder that causes inflammation and swelling of the brain and impairment and loss of function of the liver. The cause of Reye syndrome is... read more is a rare but very severe complication that occurs almost only in those younger than 18 following the use of aspirin. Therefore, aspirin should not be given to children with chickenpox. Reye syndrome may begin 3 to 8 days after the rash begins.

Pregnant women who get varicella are at risk of serious complications, such as pneumonia Overview of Pneumonia Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them. Pneumonia is one of the most common causes of death worldwide. Often, pneumonia is the final... read more Overview of Pneumonia , and may die as a result. Chickenpox can also be transmitted to the fetus, especially if chickenpox develops during the 1st or early 2nd trimester. Such an infection can result in scars on the skin, birth defects, and a low birth weight.

Diagnosis of Chickenpox

  • A doctor's evaluation

  • Rarely blood tests and testing of a sample taken from a sore

A doctor is usually certain of the diagnosis of chickenpox because the rash and the other symptoms are so typical.

Blood tests to measure levels of antibodies in the blood and laboratory tests to identify the virus (usually using a sample scraped from the sores) are rarely needed. (Antibodies Antibodies One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more Antibodies are produced by the immune system to help defend the body against a particular attacker, such as the varicella-zoster virus.)

Prognosis of Chickenpox

Healthy children nearly always recover from chickenpox without problems. Before routine immunization, about 4 million people developed chickenpox annually in the United States, and about 100 to 150 of them died each year because of complications of chickenpox.

In adults, chickenpox is more severe, and the risk of dying is higher.

Prevention of Chickenpox

  • Vaccination

  • Sometimes immune globulin


In the United States, children are routinely vaccinated against varicella-zoster. They are given two doses: one at 12 to 15 months of age, with the second given at 4 to 6 years of age ( see Childhood Vaccination Schedules Childhood Vaccination Schedules Vaccination protects children against many infectious diseases. Vaccines contain either noninfectious components of bacteria or viruses or whole forms of these organisms that have been weakened... read more ).

Older children and adults (particularly women of childbearing age and adults with chronic disorders) who have not had chickenpox and have not been vaccinated may also be vaccinated. They are given two doses 4 to 8 weeks apart.

Certain people should not be vaccinated:

  • People with a weakened immune system, including those taking high doses of corticosteroids

  • Children who take aspirin regularly

  • Pregnant women and those who intend to become pregnant within 1 to 3 months of vaccination

  • People who are more than slightly ill at the time of vaccination

Preventing spread of chickenpox

Isolation of an infected person helps prevent the spread of infection to people who have not had chickenpox. Children should not return to school and adults should not return to work until the final blisters have crusted.

After exposure to chickenpox

Susceptible people who are at high risk of complications and who have been exposed to someone with chickenpox may be given antibodies against the varicella virus (varicella-zoster immune globulin). These people include

  • Those with leukemia or a weakened immune system

  • Pregnant women who have not had chickenpox or the vaccine

  • Newborns whose mother developed chickenpox 5 days before or 2 days after delivery

Treatment with varicella-zoster immune globulin may prevent the infection or reduce its severity.

If healthy people have been exposed to chickenpox and have not already been vaccinated, vaccination within 3 to 5 days of exposure can help prevent chickenpox or reduce its severity.

Treatment of Chickenpox

  • For people with mild cases, measures to relieve symptoms

  • For people at risk of moderate to severe symptoms, antiviral drugs

Mild cases of chickenpox in children require only the treatment of symptoms. Wet compresses on the skin help soothe itching, which may be intense, and prevent scratching, which may spread the infection and cause scars. Because of the risk of bacterial infection, the skin is bathed often with soap and water, the hands are kept clean, the nails are clipped to minimize scratching, and clothing is kept clean and dry. If the itching is severe, drugs that relieve itching, such as antihistamines, may be given by mouth. Taking colloidal oatmeal baths may also help.

If a bacterial infection develops, antibiotics may be needed.

For people over 1 year of age who have a weakened immune system, doctors may prescribe acyclovir given by vein (IV).

Antiviral drugs can reduce the severity and duration of symptoms and should be given within 24 hours of the start of disease, if possible.

Because pregnant women are at high risk of severe complications from chickenpox, some experts recommend treating pregnant women who have chickenpox with acyclovir or valacyclovir.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Anacin Adult Low Strength, Aspergum, Aspir-Low, Aspirtab , Aspir-Trin , Bayer Advanced Aspirin, Bayer Aspirin, Bayer Aspirin Extra Strength, Bayer Aspirin Plus, Bayer Aspirin Regimen, Bayer Children's Aspirin, Bayer Extra Strength, Bayer Extra Strength Plus, Bayer Genuine Aspirin, Bayer Low Dose Aspirin Regimen, Bayer Womens Aspirin , BeneHealth Aspirin, Bufferin, Bufferin Extra Strength, Bufferin Low Dose, DURLAZA, Easprin , Ecotrin, Ecotrin Low Strength, Genacote, Halfprin, MiniPrin, St. Joseph Adult Low Strength, St. Joseph Aspirin, VAZALORE, Zero Order Release Aspirin, ZORprin
Sitavig, Zovirax, Zovirax Cream, Zovirax Ointment, Zovirax Powder, Zovirax Suspension
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