Public Health Response to Varicella Outbreaks --- United States, 2003--2004

Since introduction of varicella vaccine in 1995, incidence of varicella has decreased as vaccination coverage has increased (1,2). Nevertheless, varicella outbreaks continue to occur, even among populations with high vaccination coverage (3--5). Although varicella typically is mild, the outbreaks can last for several months and be challenging and costly for health departments to control. In 2005, CDC conducted a national survey to determine the distribution and extent of reported varicella outbreaks during 2003--2004 and the public health response. This report summarizes the results of that survey, which indicated that varicella outbreaks are still common and that health jurisdictions are responding to these outbreaks, although they have varying definitions and guidelines for varicella-outbreak management.

During April--May 2005, a voluntary, Internet-based survey was sent by e-mail to 59 CDC immunization program grantees:* 50 states and the District of Columbia (DC), five cities (Chicago, Illinois; Houston, Texas; New York City, New York; Philadelphia, Pennsylvania; and San Antonio, Texas), and three U.S. territories (Guam, Puerto Rico, and the U.S. Virgin Islands). These health jurisdictions were asked questions about varicella outbreaks identified during 2003--2004, including local definitions of outbreaks, existence of outbreak management guidelines, and outbreak response strategies used.

Fifty-five (93%) of 59 jurisdictions responded to the survey; Alaska, New Mexico, Utah, and the U.S. Virgin Islands did not participate. In 2003, 43 (78%) of the 55 jurisdictions were notified of at least one varicella outbreak, and 10 (18%) were notified of six or more; in 2004, 45 (82%) were notified of at least one varicella outbreak, and 13 (24%) were notified of six or more. Additional information regarding 190 outbreaks that occurred in 2004 was obtained from 24 jurisdictions. Of the outbreaks reported to these jurisdictions, 67% occurred in elementary schools, and 43% included patients with a median age of 5--9 years; 64% had <10 cases, 26% had 11--40 cases, and 10% had 41--50 cases.

When asked whether their health jurisdiction had a formal program definition for varicella outbreaks, approximately 70% of the 55 responding jurisdictions indicated that they did, and the definitions varied (Figure 1). Forty-four percent of jurisdictions reported that they had guidelines for managing varicella outbreaks. When asked whether their health jurisdiction responds to an outbreak if notified,^† 47 of the 55 jurisdictions reported that they did. Thirty (55%) of these jurisdictions stated that their outbreak response involved both local and state health departments, 10 (18%) said that it involved the state health department only, and seven (13%) said that it involved the local health department only; eight jurisdictions did not respond. Among the 47 jurisdictions that respond to outbreaks, the most commonly reported criteria used to determine whether to respond were the population affected (57%), the outbreak setting (68%), and the size of the outbreak (68%); the age of patients was a less frequently used criterion (40%) (Figure 2). Approximately 35% of jurisdictions identified at least one other criterion.

Among the jurisdictions that respond to outbreaks, frequently used response strategies included verifying vaccination history, excluding patients from the outbreak setting, and documenting relevant outbreak information, such as age of patients. Vaccinating or excluding susceptible contacts from the outbreak setting and conducting laboratory testing to assess susceptibility were not frequently used strategies (Table).

Reported by: J Leung, MPH, A Lopez, MHS, F Averhoff, MD, R Harpaz, MD, D Guris, MD, JF Seward, MBBS, MPH, Div of Viral Diseases, National Center for Immunization and Respiratory Diseases (proposed), CDC.

Editorial Note:

The results of this survey indicate that varicella outbreaks are still common despite increasing vaccination coverage. During 2004, 45 jurisdictions were notified of varicella outbreaks, and 18 reported them to CDC. More comprehensive information about occurring outbreaks would be useful for monitoring the effects of the vaccination program and evaluating vaccination policies.

Approximately two thirds of jurisdictions have formal, but varying, definitions for a varicella outbreak, and almost half of jurisdictions have existing varicella-outbreak management guidelines. Creating a standard definition for varicella outbreaks would be useful for monitoring the outbreak trends among jurisdictions, and establishing outbreak management guidelines might facilitate a more uniform response, especially at the local level, and ensure that critical procedures are not overlooked. CDC is working with state and local health departments to develop a standard definition and national guidelines for management of varicella outbreaks to be published in a future MMWR.

Forty-seven of the jurisdictions indicate that they respond to varicella outbreaks if notified, although response strategies vary. A standard response for all varicella outbreaks should include a letter from the local health department or outbreak setting to inform the affected population of the outbreak. In addition, persons without evidence of immunity^§ (6) should be vaccinated either by their primary-care physician or at a vaccination clinic in outbreak settings. Vaccination of susceptible populations who might have been exposed during an outbreak or who have been exposed to infection is important to prevent illness and decrease disease severity.

In June 2005, the Advisory Committee on Immunization Practices (ACIP) provisionally recommended a second dose of varicella vaccine in outbreak settings for persons who have had only 1 dose of varicella vaccine and no disease history (provided that an appropriate interval has elapsed since the first dose) (6). On the basis of a 10-year follow-up prelicensure study of the vaccine, a 2-dose vaccination regimen has been determined more effective than a 1-dose regimen (7).

In a 2006 position statement, the Council of State and Territorial Epidemiologists (CSTE) supported a routine 2-dose varicella vaccination policy to improve varicella control and outbreak prevention. In June 2006, ACIP approved a routine 2-dose varicella vaccination policy for children (first dose at 12--15 months, second dose at 4--6 years) and catch-up vaccinations for children, adolescents, and adults who had previously received only 1 dose. Establishing a routine 2-dose vaccination regimen might make the 2-dose outbreak response for susceptible populations more feasible to implement.

In 2002, CSTE also recommended that states should establish individual case-based varicella surveillance by 2005 (8). Case-based reporting should improve detection of varicella outbreaks, the quality of the data reported from outbreaks, and the evaluation of outbreak-control measures.

The findings in this report are subject to at least two limitations. First, the data are taken from reports from jurisdictions and are subject to reporting biases such as recall bias. Second, varying outbreak definitions and reporting methods might have led to underestimation of the extent and distribution of varicella outbreaks.

As national outbreak guidelines are being created, jurisdictions are encouraged to contact CDC for assistance with investigating and responding to varicella outbreaks. Guidance on outbreak management and investigation also can be found in CDC's Manual for the Surveillance of Vaccine-Preventable Diseases (9).

Acknowledgments

The data in this report are based, in part, on information supplied by health departments of states, cities, and U.S. territories. The survey was conducted with assistance from the Immunization Svcs Div, National Center for Immunization and Respiratory Diseases (proposed), CDC

References

1. CDC. National, state, and urban area vaccination coverage among children aged 19--35 months---United States, 2004. MMWR 2005;54:717--21. Available at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5429a1.htm.
2. Seward JF, Watson BM, Peterson CL, et al. Varicella disease after introduction of varicella vaccine in the United States, 1995--2000. JAMA 2002;287:606--11.
3. Dworkin MS, Jennings CE, Roth-Thomas J, Lang JE, Stukenberg C, Lumpkin JR. An outbreak of varicella among children attending preschool and elementary school in Illinois. Clin Infect Dis 2002;35:102--4.
4. Tugwell BD, Lee LE, Gillette H, Lorber EM, Hedberg K, Cieslak PR. Chickenpox outbreak in a highly vaccinated school population. Pediatrics 2004;113:455--9.
5. Lopez AS, Guris D, Zimmerman L, et al. One dose of varicella vaccine does not prevent school outbreaks---is it time for a second dose? Pediatrics 2006;117:e1070--7.
6. CDC. ACIP provisional recommendations for prevention of varicella. Advisory Committee on Immunization Practices. Available at http://www.cdc.gov/nip/vaccine/varicella/varicella_acip_recs_prov_june_2006.pdf.
7. Kuter B, Matthews H, Shinefield H, et al. Ten year follow-up of healthy children who received one or two injections of varicella vaccine. Pediatr Infect Dis 2004;23:132--7.
8. Council of State and Territorial Epidemiologists. Position Statement 02-ID-06: varicella surveillance. Kansas City, MO: Council of State and Territorial Epidemiologists; 2002. Available at http://www.cste.org/position%20statements/02-ID-06.pdf.
9. CDC. Varicella [Chapter 14]. In: Wharton M, ed. Vaccine preventable disease. Surveillance manual. 3rd edition. Atlanta, GA: US Department of Health and Human Services, CDC; 2002. Available at http://www.cdc.gov/nip.

^§ Definition is available at http://www.cdc.gov/nip/vaccine/varicella/varicella_acip_recs_prov_june_2006.pdf.

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Date last reviewed: 9/14/2006