Conducting Varicella Surveillance

Trends and Statistics

Before varicella vaccine became available in 1995, almost everyone in the United States got the disease. In the early 1990s, there was an average of 4 million cases of varicella, 10,500–13,000 hospitalizations (range, 8,000–18,000), and 100–150 deaths each year. Varicella affected mainly children. About 90% of people who got varicella were younger than 15 years old. In the 1970s and 1980s, the highest rates of varicella were reported among children 5–9 years old, followed closely by children 1–4 years old. In the 1990s, the highest rate of varicella was reported in preschool-aged children. This might have been due to increasing attendance at child care centers and preschools.

Two doses of varicella vaccine are now recommended for routine use. Children should receive the first dose at 12–15 months old and the second dose at 4–6 years old. Persons 13 years and older without evidence of immunity to varicella should also receive two doses of varicella vaccine 4–8 weeks apart. In 2009, 90% of children 19–35 months old in the United States had received one dose of varicella vaccine. State and city estimates ranged from 76% to 95%. As of 2009, varicella cases have declined by 41% to 81% compared to the pre-vaccine era in four states (West Virginia, Illinois, Texas, and Michigan) that consistently reported to the National Notifiable Disease Surveillance System (NNDSS).

From 2005 to 2007, compared with 1990-1994, varicella deaths in children and adolescents less than age 20 years went down 97%, and in adults age 20-49 years, deaths went down 90%. From 2000 to 2006, national varicella-related hospitalization rates declined by 71% compared with rates from 1988 to 1995.

Surveillance data are needed to

1. document and monitor the impact of vaccination programs on varicella incidence, morbidity, and mortality,
2. evaluate the effectiveness of prevention strategies, and
3. evaluate vaccine effectiveness under conditions of routine use.

With the increase in vaccine coverage and decline in varicella, surveillance is especially important for monitoring changes in epidemiology. All states should establish or enhance varicella case-based surveillance to monitor these changes.

Varicella Reporting

Case Reporting

In 2002, Council of State and Territorial Epidemiologists (CSTE) recommended that varicella be included in NNDSS. All states are encouraged to conduct ongoing varicella surveillance to monitor vaccine impact on morbidity. States are encouraged to report varicella cases to NNDSS via the National Electronic Telecommunications Surveillance System (NETSS) or the National Electronic Disease Surveillance System (NEDSS).

As of 2010, 36 states have been conducting case-based varicella surveillance. For state-specific reporting requirements, contact the state health department. Information can also be found on CSTE's web site on state reportable conditions.

Case Definition

The following varicella case definitions were approved by CSTE in June 1999 and updated in June 2009. Case definitions for varicella cases and deaths can be found on CDC's web site on Nationally Notifiable Diseases Surveillance System.

Varicella Clinical Case Definition

An illness with acute onset of diffuse (generalized) maculopapulovesicular rash without other apparent cause. In vaccinated persons who develop varicella more than 42 days after vaccination (breakthrough disease), the disease is usually mild with fewer than 50 skin lesions and shorter duration of illness. The rash may also be atypical in appearance (maculopapular with few or no vesicles).

Laboratory Criteria for Diagnosis

• Isolation of varicella-zoster virus (VZV) from a clinical specimen
• Detection of VZV DNA by direct fluorescent antibody (DFA) or by polymerase chain reaction (PCR) tests from a clinical specimen, ideally scabs, vesicular fluid, or cells from the base of a lesion (For more information, see Laboratory Testing) These tests are also useful for diagnosing breakthrough disease.
• Fourfold or greater rise in serum varicella IgG antibody titer by any standard serologic assay

For both unvaccinated and vaccinated persons, DNA detection methods (PCR, DFA) and culture are the methods of choice for laboratory confirmation. Of these, PCR is the most reliable method for confirming infection.

In unvaccinated persons, experience is limited with IgM antibody tests and with timing of the IgM response. In vaccinated persons, even less experience with serologic methods for laboratory confirmation is available. Therefore, DNA detection methods are the laboratory methods of choice for diagnosis. A negative IgM result should not be used to rule out the diagnosis. A positive IgM in the absence of rash should not be used to confirm a diagnosis.

A fourfold rise in IgG antibody may not occur in vaccinated persons.

For more information, see Interpreting Laboratory Tests.

Varicella Case Classification

Probable: A case that meets the clinical case definition but is not laboratory confirmed nor epidemiologically linked to another probable or confirmed case

Confirmed: A case that is laboratory confirmed or that meets the clinical case definition and is epidemiologically linked to a confirmed or a probable case

Note: Two probable cases that are epidemiologically linked are considered confirmed, even in the absence of laboratory confirmation.

Varicella Deaths

In 1998, CSTE recommended that varicella-related deaths be placed under national surveillance. As of January 1, 1999, varicella-related deaths became nationally notifiable to CDC.

Varicella deaths can be identified through death certificates, which may be available through state vital records systems. In states using electronic records, certificates may be more readily available soon after a person dies. State health departments may also request that local health departments, health care providers, and hospitals report varicella deaths that occur in their community.

Since varicella is preventable with vaccine, all deaths due to varicella should be investigated. Investigations may provide insight into risk factors for varicella mortality and may help identify missed opportunities for and barriers to vaccination. (For guidance on varicella death investigations, see the Varicella Death Investigation Worksheet [169 KB, 4 pages].)

Deaths should be reported to the Epidemiology Branch in the Division of Viral Diseases at CDC (404/639-8230) and to NNDSS via the NETSS or NEDSS.

The following case definitions for varicella deaths were approved by CSTE in 1998.

Varicella Deaths Case Classification

Probable: A probable case of varicella that contributes directly or indirectly to acute medical complications that result in death

Confirmed: A confirmed case of varicella that contributes directly or indirectly to acute medical complications that result in death

Resources for Varicella Surveillance

• Chapter on Varicella, Manual for the Surveillance of Vaccine-Preventable Diseases
• Surveillance worksheets
  • Varicella Surveillance Worksheet [282 KB, 3 pages]
  • Varicella Death Investigation Worksheet [169 KB, 4 pages]
  • Varicella Outbreak Reporting Worksheet [176 KB, 1 page]

References

1. Nguyen HQ, Jumaan AO, Seward JF. Decline in Mortality Due to Varicella After Implementation of Varicella Vaccination in the United States. N Engl J Med. 2005; 352(5):450-458.
2. Marin M, Zhang JX, Seward JF. Near Elimination of Varicella Deaths in the US After Implementation of the Vaccination Program. Pediatrics. 2011;128(2):214-20.
3. CDC. National and State Vaccination Coverage Among Children 19 – 35 Months—United States. MMWR. 2010; 60(34);1157-1163.
4. CDC. National and State Vaccination Coverage Among Adolescents Aged 13 Through 17 Years – United States. MMWR. 2011; 60(33):1117-1123.

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