Merck Manual

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Kenneth M. Kaye

, MD, Harvard Medical School

Reviewed/Revised Sep 2021 | Modified Sep 2022
Topic Resources

Chickenpox is an acute, systemic, usually childhood infection caused by the varicella-zoster virus (human herpesvirus type 3). It usually begins with mild constitutional symptoms that are followed shortly by skin lesions appearing in crops and characterized by macules, papules, vesicles, and crusting. Patients at risk of severe neurologic or other systemic complications (eg, pneumonia) include adults, neonates, and patients who are immunocompromised or have certain underlying medical conditions. Diagnosis is clinical. Those at risk of severe complications receive postexposure prophylaxis with immune globulin and, if disease develops, are treated with antiviral drugs (eg, valacyclovir, famciclovir, acyclovir). Vaccination provides effective prevention in immunocompetent patients.

Chickenpox is caused by the varicella-zoster virus (human herpesvirus type 3); chickenpox is the acute invasive phase of the infection, and herpes zoster Herpes Zoster Herpes zoster is infection that results when varicella-zoster virus reactivates from its latent state in a posterior dorsal root ganglion. Symptoms usually begin with pain along the affected... read more Herpes Zoster (shingles) represents reactivation of the latent phase.

Chickenpox, which is extremely contagious, is spread by

  • Mucosal (usually nasopharyngeal) inoculation via infected airborne droplets or aerosolized particles

  • Direct contact with the virus (eg, via skin lesions)

Chickenpox is most communicable during the prodrome and early stages of the eruption. It is communicable from 48 hours before the first skin lesions appear until the final lesions have crusted. Indirect transmission (by carriers who are immune) does not occur.

Symptoms and Signs of Chickenpox

In immunocompetent children, chickenpox is rarely severe. In adults and immunocompromised children, infection can often be serious.

Mild headache, moderate fever, and malaise may occur 7 to 21 days after exposure, about 24 to 36 hours before lesions appear. This prodrome is more likely in patients > 10 years and is usually more severe in adults.

Initial rash

The initial rash, a macular eruption, may be accompanied by an evanescent flush. Within a few hours, lesions progress to papules and then characteristic, sometimes pathognomonic teardrop vesicles, often intensely itchy, on red bases. The lesions become pustular and then crust.

Lesions evolve from macules to papules and vesicles, which then crust. A hallmark of chickenpox is that lesions develop in crops so that they are in various stages of development in any affected region. The eruption may be generalized (in severe cases) involving the trunk, extremities, and face, or more limited but almost always involves the upper trunk.

Ulcerated lesions may develop on the mucous membranes, including the oropharynx and upper respiratory tract, palpebral conjunctiva, and rectal and vaginal mucosa.

In the mouth, vesicles rupture immediately, are indistinguishable from those of herpetic gingivostomatitis, and often cause pain during swallowing.

Scalp lesions may result in tender, enlarged suboccipital and posterior cervical lymph nodes.

New lesions usually cease to appear by the 5th day, and the majority are crusted by the 6th day; most crusts disappear < 20 days after onset.

Breakthrough varicella

Sometimes vaccinated children develop varicella (called breakthrough varicella); in these cases, the rash is typically milder, fever is less common, and the illness is shorter; the lesions are contagious.


Secondary bacterial infection (typically streptococcal or staphylococcal) of the vesicles may occur, causing cellulitis or rarely necrotizing fasciitis or streptococcal toxic shock.

Pneumonia may complicate severe chickenpox in adults, neonates, and immunocompromised patients of all ages but usually not in immunocompetent young children.

Myocarditis, hepatitis, and hemorrhagic complications may also occur.

Acute postinfectious cerebellar ataxia is one of the most common neurologic complications; it occurs in 1/4000 cases in children.

Diagnosis of Chickenpox

  • Clinical evaluation

Chickenpox is suspected in patients with the characteristic rash, which is usually the basis for diagnosis. The rash may be confused with that of other viral skin infections.

If the diagnosis is in doubt, laboratory confirmation can be done; it requires one of the following:

  • Polymerase chain reaction (PCR) for viral DNA

  • Immunofluorescent detection of viral antigen in lesions

  • Serologic tests

  • Viral culture

  • Tzanck smear

In serologic tests, detection of IgM antibodies to varicella-zoster virus (VZV) or seroconversion from negative to positive for antibodies to VZV indicate acute infection.

Samples are generally obtained by scraping the base of lesions and are transported to the laboratory in viral media. A Tzanck smear of a superficial scraping from the base of a freshly ruptured vesicle stained with Wright-Giemsa or toluidine blue stain demonstrates multinucleated giant cells and epithelial cells with eosinophilic intranuclear inclusion bodies in herpes simplex and herpes zoster infection. Culture can be used but has lower sensitivity than PCR and a long turnaround time (1 to 2 weeks).

Prognosis for Chickenpox

Chickenpox in children is rarely severe. Severe or fatal disease is more likely in the following:

  • Adults

  • Patients with depressed T-cell immunity (eg, lymphoreticular cancer)

  • Those receiving corticosteroids or chemotherapy or who are otherwise immunosuppressed

  • Patients being treated with tumor necrosis factor (TNF) antagonists

Treatment of Chickenpox

  • Symptomatic treatment

  • Sometimes oral antiviral agents

  • IV acyclovir for immunocompromised patients and others at risk of severe disease

Mild cases of chickenpox in children require only symptomatic treatment. Relief of itching and prevention of scratching, which predisposes to secondary bacterial infection, may be difficult. Wet compresses or, for severe itching, systemic antihistamines and colloidal oatmeal baths may help.

To prevent secondary bacterial infection, patients should bathe regularly and keep their underclothing and hands clean and their nails clipped. Antiseptics should not be applied unless lesions become infected; bacterial superinfection is treated with antibiotics.

Patients should not return to school or work until the final lesions have crusted.

Antivirals and chickenpox

Oral antivirals, when given to immunocompetent patients within 24 hours of the rash’s onset, slightly decrease symptom duration and severity. However, because the disease is generally benign in children, antiviral treatment is not routinely recommended.

Oral valacyclovir, famciclovir, or acyclovir should be given to healthy people at risk of moderate to severe disease, including patients

  • 12 years of age or older (18 years of age or older for famciclovir)

  • With skin disorders (particularly eczema)

  • With chronic lung disease

  • Receiving long term salicylate therapy

  • Receiving corticosteroids

The dose is famciclovir 500 mg 3 times a day or valacyclovir 1 g 3 times a day for adults. Acyclovir is a less desirable choice because it has poorer oral bioavailability, but it can be given at 20 mg/kg 4 times a day for 5 days for children ≥ 2 years and ≤ 40 kg. The dose for children > 40 kg and adults is 800 mg 4 times a day for 5 days.

Immunocompromised children > 1 year should be given acyclovir10 mg/kg every 8 hours IV. Immunocompromised adults should be treated with acyclovir 10 to 12 mg/kg IV every 8 hours.

Because pregnant women are at high risk of varicella complications, some experts recommend oral acyclovir or possibly valacyclovir for pregnant women with varicella. Although available safety data are reassuring, the safety of antiviral therapy during pregnancy is not firmly established, and there is longer experience with acyclovir in pregnancy compared to valacyclovir. Acyclovir and valacyclovir are pregnancy category B drugs. IV acyclovir is recommended for serious varicella disease in pregnant women. There are little data regarding the safety of famciclovir in pregnancy so it is not generally recommended for pregnant women.

Prevention of Chickenpox

Infection provides lifelong protection.

Potentially susceptible people should take strict precautions to avoid people capable of transmitting the infection.


All healthy children and susceptible adults should receive 2 doses of live-attenuated varicella vaccine (see Centers for Disease Control and Prevention [CDC]: Child and Adolescent Immunization Schedule by Age). Vaccination is particularly important for women of child-bearing age, those at high risk for exposure, and those who have contact with individuals at higher risk for severe disease. These include healthcare professionals, teachers, child care workers, and residents and staff of nursing homes or other institutional settings. Serologic testing to determine immune status before vaccination in adults is usually not required. Rarely the vaccine may cause chickenpox lesions in immunocompetent patients, but disease is usually mild (< 10 papules or vesicles) and brief and causes few systemic symptoms.

Vaccination of health care workers who do not have evidence of varicella immunity is recommended. Susceptible health care workers who have been exposed to varicella should be vaccinated as soon as possible and kept off duty for 21 days.

Vaccination is contraindicated in

  • Patients with moderate to severe acute concurrent illness (vaccination is postponed until illness resolves)

  • Immunocompromised patients

  • Pregnant women and those who intend to become pregnant within 1 month of vaccination (based on Centers for Disease Control and Prevention recommendations) or within 3 months of vaccination (based on vaccine labeling)

  • Patients taking high doses of systemic corticosteroids

  • Children using salicylates

Postexposure prophylaxis

After exposure, chickenpox can be prevented or attenuated by intramuscular (IM) administration of varicella-zoster immune globulin (VariZIG). Candidates for postexposure prophylaxis include

  • People with leukemia, immunodeficiencies, or other severe debilitating illness

  • Susceptible pregnant women

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

  • Neonates born at < 28 weeks and exposed to a nonmaternal source even if their mother has evidence of immunity (exposed neonates born at ≥ 28 weeks should receive immune globulin if their mother has no evidence of immunity)

The VariZIG immune globulin should be given as soon as possible (and within 10 days of exposure) and may modify or prevent varicella.

Vaccination should be given as soon as possible to exposed, susceptible healthy patients eligible for vaccination (eg, age ≥ 1 year and no contraindications). Vaccination can be effective in preventing or ameliorating disease within 3 days and possibly up to 5 days after exposure.

To prevent nosocomial transmission, the Centers for Disease Control and Prevention recommends postexposure prophylaxis with vaccination or varicella-zoster immunoglobulin, depending on immune status, for exposed health care workers and patients without evidence of immunity (available at Immunization of Health-Care Personnel).

Key Points

  • Chickenpox causes pustular, crusting lesions on the skin (often including scalp) and may cause ulcerated lesions on mucous membranes.

  • Complications include secondary bacterial infection of skin lesions, pneumonia, cerebellar ataxia, and, in adults, encephalitis.

  • Give oral valacyclovir to varicella patients ≥ 12 years, or famciclovir to varicella patients 18 years and to those with skin disorders (particularly eczema) or chronic lung disease.

  • Give IV acyclovir to immunocompromised patients and to other patients at risk of severe disease.

  • Vaccinate all healthy children and susceptible adults.

  • Give postexposure prophylaxis with varicella-zoster immune globulin to immunocompromised patients, susceptible pregnant women, and neonates whose mother developed chickenpox within 5 days before or 2 days after delivery.

  • Give postexposure prophylaxis with varicella vaccine to immunocompetent patients ≥ 1 year who are eligible for vaccination.

More Information

Drugs Mentioned In This Article

Drug Name Select Trade
Sitavig, Zovirax, Zovirax Cream, Zovirax Ointment, Zovirax Powder, Zovirax Suspension
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
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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