Clinical Overview of Chickenpox (Varicella)

Key points

  • Varicella (chickenpox) is an acute infectious disease caused by varicella-zoster virus (VZV).
  • The most common chickenpox complications are bacterial infections of the skin and soft tissues in children and pneumonia in adults.
  • The most effective prevention for varicella is chickenpox vaccination.
Pediatrician using stethoscope on girl on mom's lap.


Varicella is highly caused by varicella-zoster virus (VZV), which is a DNA virus that is a member of the herpesvirus group. Primary infection with VZV causes varicella. After the primary infection, VZV stays in the body (in the sensory nerve ganglia) as a latent infection. Reactivation of latent infection causes herpes zoster (shingles).

Who is at risk

People at risk for severe varicella include:

  • Immunocompromised people without evidence of immunity to varicella.
  • Pregnant women without evidence of immunity to varicella.
  • Newborns whose mothers have varicella from 5 days before to 2 days after delivery.
  • Premature babies exposed to varicella or herpes zoster.

Incubation period

The average incubation period for varicella is 14 to 16 days after exposure to a varicella or a herpes zoster rash. It takes from 10 to 21 days after exposure to the virus for someone to develop varicella.

How it spreads

Varicella is highly contagious‎

Varicella is more contagious than mumps and rubella and less contagious than measles.

People with breakthrough varicella are also contagious.

The virus can be spread from person to person by direct contact, inhalation of aerosols from vesicular fluid of skin lesions of acute varicella or zoster; and possibly through infected respiratory secretions that also may be aerosolized.

A person with varicella is considered contagious beginning 1 to 2 days before rash onset until all the chickenpox lesions have crusted. Vaccinated people may develop lesions that do not crust. These people are considered contagious until no new lesions have appeared for 24 hours.

Based on studies of transmission among household members, about 90% of susceptible close contacts will get varicella after exposure to a person with disease.

Although limited data are available to assess the risk of VZV transmission from zoster, one household study found that the risk for VZV transmission from herpes zoster was approximately 20% of the risk for transmission from varicella.

One study of varicella transmission in household settings found that people with mild breakthrough varicella (<50 lesions) who were vaccinated with one dose of varicella vaccine were one-third as contagious as unvaccinated people with varicella. However, people with breakthrough varicella with 50 or more lesions were just as contagious as unvaccinated people with the disease.

Disease rates

Prior to vaccine introduction:

Varicella used to be very common in the United States and contributed significantly to the burden of childhood disease. In the early 1990s each year:

  • More than 4 million people got varicella
  • 10,500 to 13,500 were hospitalized
  • 100 to 150 died

More than 90% of cases, 70% of hospitalizations, and about half of the deaths occurred in children.

Clinical features

A mild prodrome of fever and malaise may occur 1 to 2 days before rash onset, particularly in adults. In children, the rash is often the first sign of disease.

In unvaccinated people, varicella progresses rapidly from macular to papular to vesicular lesions before crusting.

The most common chickenpox complications are bacterial infections of the skin and soft tissues in children and pneumonia in adults.

Breakthrough varicella occurs less frequently among those who have received 2 doses of vaccine; compared with those who have received only 1 dose.


CDC recommends two doses of varicella vaccine for all children, adolescents, and adults without evidence of immunity to varicella. Those who previously received one dose of varicella vaccine should receive their second dose for best protection against the disease.

Nosocomial transmission of VZV is well-recognized and can be life-threatening to certain groups of patients. Reports of nosocomial transmission are uncommon in the United States since the introduction of varicella vaccine.

Patients, healthcare providers, and visitors with varicella or herpes zoster can spread VZV to susceptible patients and healthcare providers in hospitals, long-term care facilities, and other healthcare settings. In healthcare settings, transmissions have been attributed to delays in the diagnosis or reporting of varicella and herpes zoster and failures to implement control measures promptly.

Although all susceptible patients in healthcare settings are at risk for severe varicella and complications, certain patients without evidence of immunity are at increased risk.

Testing and diagnosis

Varicella-zoster virus (VZV) laboratory testing information applies to testing and diagnosis of primary VZV infection (varicella) and reactivation (herpes zoster). Polymerase chain reaction (PCR) is the most helpful laboratory test for confirming cases of varicella. PCR can detect VZV DNA rapidly.

Patient management

Routine testing after 2 doses is not recommended‎

Routine testing for varicella immunity after two doses of vaccine is not recommended. Available commercial assays are not sensitive enough to detect antibodies after vaccination in all instances. Documented receipt of two doses of varicella vaccine supersedes results of subsequent serologic testing.

Healthcare providers should follow standard precautions, airborne precautions (negative air-flow rooms), and contact precautions until lesions are dry and crusted. If negative air-flow rooms are not available, patients with varicella should be isolated in closed rooms. They should have no contact with people without evidence of immunity. Patients with varicella should be cared for by staff with evidence of immunity.

Evidence of immunity to varicella includes any of the following:

  • Documentation of age-appropriate varicella vaccination.
    • Preschool-age children (i.e., age 12 months through 3 years): one dose
    • School-age children, adolescents, and adults: two doses
  • Laboratory evidence of immunity or laboratory confirmation of disease
    • Commercial assays can be used to assess disease-induced immunity, but they lack sensitivity to detect vaccine-induced immunity (i.e., they might yield false-negative results).
  • Birth in the United States before 1980 (should not be considered evidence of immunity for healthcare personnel, pregnant women, and immunocompromised people)
  • Diagnosis or verification of a history of varicella or herpes zoster by a healthcare provider

Verify history of varicella

Healthcare providers should inquire about either:

  • An epidemiologic link to another typical varicella case or to a laboratory confirmed case.
  • Evidence of laboratory confirmation, if testing was performed at the time of acute disease.

People who have neither of the above criteria should not be considered as having a valid history of disease. For these people, a second dose of vaccine is recommended if they previously received only one dose. If a healthcare provider verifies the diagnosis based on the above criteria, then vaccination is not needed.

Impact of the U.S. Varicella Vaccination Program

Since the start of the U.S. varicella vaccination program in 1995, chickenpox cases have declined overall by more than 97%.
The vaccine program has reduced chickenpox cases by 97% since 1995.

25 years of the program‎

‎Summarizing the first 25 years of the vaccination program, CDC and its collaborators published 14 manuscripts in a special supplement in the Journal of Infectious Diseases—highlighting the success of this important vaccine.

The varicella vaccine program has dramatically decreased virus circulation and increased community protection.

  • Varicella hospitalizations and deaths declined 94% and 97%, respectively, among people 50 years old and younger.
  • The greatest decline in cases was among people 20 years old and younger who were born during the program, with 97% drop in hospitalizations and more than 99% drop in deaths.
  • The risk of shingles is significantly lower among vaccinated children, including those who are immunocompromised.
  • There is about an 80% lower risk of shingles among healthy vaccinated children compared to unvaccinated children who had wild-type varicella.

Disease prevented and dollar saved

During the first 25 years, the U.S varicella vaccination program saved 118,000 life-years, at net societal savings of $23.4 billion. and prevented an estimated:

  • 91 million cases
  • 238,000 hospitalizations
  • 1.1 million hospitalization days
  • 2,000 deaths

For every $1 spent, the estimated return on investment is $1.70; however, the benefits of the investment will continue to accrue beyond 2020.

Case definitions