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December 2007 | Back to Table of Contents

Clinical and Health Affairs

Varicella Surveillance and Policy in the United States and Minnesota

By Andrew Murray, M.P.H.

Abstract
An estimated 82.7% of Minnesota children 19 months to 35 months of age were vaccinated against varicella in 2006 as a result of accepted national recommendations, provider education, and vaccination mandates. Despite the wide-ranging acceptance of vaccination, outbreaks of varicella continue to occur in schools and childcare centers, even those with a high percentage of vaccinated children. As a result, national recommendations have been modified to include a universal second dose of vaccine. Starting in September 2008, students entering kindergarten or 7th grade in Minnesota will be required to provide documentation of having had 2 doses of varicella vaccine, a history of the disease, or legal exemption.

Varicella is a rash illness caused by the varicella zoster virus (VZV). It is extremely infectious with transmission rates as high as 90%.1 Varicella zoster virus is transmitted through direct contact with fluid from skin lesions or inhalation of aerosolized viral particles. The symptoms of varicella are fever, malaise, and a generalized vesicular rash of 250 to 500 lesions; it resolves without treatment and generally lasts 4 to 5 days.2 The disease primarily affects children 3 years to 6 years of age.3 Most children recover without complications; however, infants, individuals 13 years of age and older, and people who are immunocompromised are at higher risk for complications.2

A vaccine against varicella was first licensed in this country in 1995. Before the vaccine became available, varicella infected an estimated 4 million people each year, resulting in 11,000 hospitalizations and 100 deaths annually in the United States.2 After the vaccine was licensed, recommendations for its use were published by several advisory committees, including the American Academy of Pediatrics, the American Academy of Family Physicians, and the national Advisory Committee on Immunization Practices (ACIP). The ACIP first published recommendations for use of Varivax, a single-antigen vaccine, in 1996. The recommendations called for routine vaccination with 1 dose for children 12 months to 18 months of age and susceptible children 19 months to 12 years of age.4 Two doses given 4 weeks to 8 weeks apart were recommended for persons 13 years of age and older; immunogenicity studies in initial clinical trials indicated a need for 2 doses in this age group.

Compliance with the recommendations was initially slow in part because of health care providers’ concerns about waning immunity following vaccination, their preference for natural infection, and the stringent storage and handling requirements for the vaccine, for which some clinics were not equipped.5 Provider education and gradual introduction of mandates helped to modestly increase vaccination rates by 1998. That year, the Council of State and Territorial Epidemiologists (CTSE) encouraged states to establish case-based reporting systems in which individual cases are reported to assess both the impact and safety of varicella vaccination.6 The CTSE also pushed the ACIP to recommend that states begin adding varicella vaccination to childcare and school entry requirements by 1999.7 The ACIP did so the following year, albeit without specifying a deadline for implementation.2 As a result of those efforts, an estimated 76.3% of children in the United States aged 19 months to 35 months had received at least 1 dose of varicella vaccine by 2001 (Table).8

National Varicella Disease Surveillance and Policy
Prior to the licensure of the vaccine, most states did not have surveillance programs to monitor varicella. As vaccination coverage increased in the United States, national surveillance systems showed engouraging trends, as did the monitoring programs in states that had them in place. Four states (Illinois, Michigan, Texas, and West Virginia) that had reported cases equivalent to 5% or more of their birth cohort before 1995 and consistent reporting methods recorded reductions in varicella disease ranging from 67% to 82% when comparing the average incidence between 1990 and 1994 with that from 2001.9 Vaccination rates among children aged 19 months to 35 months ranged from 57% to 84% in those states. Between 1995 and 2000, varicella incidence declined 71%, 84%, and 79% in 3 federally funded active surveillance areas; the decline corresponded with vaccination rates of 82%, 74%, and 84% among children aged 19 months to 35 months, respectively, in 2000.10 By 2005, incidence rates fell to 9% and 10% of what they originally were in 2 sites that continued surveillance. Vaccination coverage in those sites had reached 90% by then.3 Hospitalization rates fell between 50% and 88%, and varicella-related deaths decreased from an average of 145 per year in the United States between 1990 and 1994 to 66 per year between 1999 and 2001.3,10-13 In 2003, only 16 varicella-related deaths were reported.1

New Minnesota School Immunization Requirement for Varicella

  • Currently, 2 doses of varicella vaccine are recommended for all children, the first at 12 to 15 months and the second at 4 to 6 years of age.
  • Starting in September 2008, students entering kindergarten or 7th grade will be required by Minnesota law to provide documentation of having had 2 doses of varicella vaccine, a disease history, or legal exemption.
  • Health care providers are urged to take advantage of opportunities for varicella immunization during routine office visits (eg, sports physicals).
  • The Minnesota Immunization Information Connection (MIIC) registry allows participating health care providers to identify children needing immunizations. Information regarding MIIC and enrollment is posted on the Minnesota Department of Health website: www.health.state.mn.us
    /divs/idepc/immunize/
    registry/index.html    .
Despite the success of the 1-dose vaccination policy in reducing morbidity and mortality in the United States, outbreaks continued to occur at schools in which a high percentage of students were vaccinated. An investigation of 1 Minnesota school outbreak suggested that breakthrough infection, which is defined as an infection with wild-type VZV (ie, circulating VZV) occurring more than 42 days after vaccination, was associated with increased time since vaccination.1,14 Among vaccinated students, those inoculated at least 5 years prior to the outbreak were at significantly greater risk of developing a breakthrough infection than students vaccinated less than 5 years prior to the outbreak. Those factors led the CSTE to recommend in 2006 that the ACIP consider a universal 2-dose varicella vaccination policy.15

In June 2007, the ACIP replaced its 1-dose policy with one that requires 2 doses.1 The first dose was rescheduled for children 12 months to 15 months of age, with a universal second dose added for children 4 years to 6 years of age. Catch-up vaccination for anyone who previously received 1 dose was also included in the guidelines. In addition, the ACIP recommended that school-entry requirements for varicella vaccination be expanded to include middle schools, high schools, and colleges.

Varicella Vaccine Surveillance and Policy in Minnesota
The Minnesota Department of Health adhered to the national recommendations by including varicella vaccination, in accordance with medically accepted standards, in the Minnesota childcare and school immunization law in September 2004. The law currently requires that children 18 months of age or older enrolling in childcare, kindergarten, or 7th grade provide proof of having had 1 dose of varicella vaccine, a history of the disease, or legal exemption.16 However, starting in September 2008, students entering kindergarten or 7th grade will be required to provide documentation of having had 2 doses of varicella vaccine, a disease history, or legal exemption in order to adhere to the recent change in the standards.

In order to monitor the frequency of varicella outbreaks in Minnesota, the Minnesota Department of Health asked all schools throughout the state to report outbreaks of varicella and information on varicella vaccination history for students involved in those outbreaks beginning in September 2005. An outbreak of varicella in a school is defined as 5 or more cases within a 2-month period in persons younger than 13 years of age or 3 or more cases within a 2-month period in persons 13 years of age and older. An outbreak is considered to be over when no new cases occur within 2 months of the last case no longer being contagious.

During the 2006-07 school year, the Department of Health received reports of 73 outbreaks from 69 schools (4 schools reported 2 outbreaks) in 30 counties throughout Minnesota involving 1,230 students and no staff members. Cases per outbreak ranged from 5 to 96 (median: 13). Outbreaks took place almost entirely in elementary schools, and the highest disease burden was in 3rd- and 4th-grade students. The majority of cases involved vaccinated students: 907 (74%) of 1,230 children were vaccinated, 289 (23%) were not, and the vaccination status of 34 (3%) was unknown. Almost all of the cases involving vaccinated students occurred at least 3.5 years after the date of vaccination (Figure); 867 (96%) of 907 vaccinated students had a recorded date of vaccination and 835 (96%) of 867 cases occurred more than 3.5 years after that date. This further suggests the possibility of waning immunity of the 1-dose vaccine.

In addition to having all schools report outbreaks, the Department of Health monitors individual cases from selected sentinel schools and childcare centers. Sentinel surveillance in schools and childcare centers helps estimate the burden of varicella in Minnesota, as it identifies sporadic cases that may not be part of an outbreak. Epidemiologists use information from that subset of schools and childcare centers to predict what they would expect to see in the state’s entire school-aged population. Eighty schools and 40 childcare centers were randomly selected to report individual cases of varicella during the 2006-07 academic and 2006 calendar year, respectively. Department of Health staff followed up to collect case-specific information for comparing severity of illness (eg, fever, number of lesions, complications, and hospitalizations) between vaccinated and unvaccinated children.

During the 2006-07 school year, the department received 192 reports of varicella from 29 (36%) of 80 sentinel schools. Cases occurred almost entirely in elementary schools, with the highest disease burden among 2nd- and 3rd-grade students. A vaccine history was reported in 154 (80%) of 192 cases: 131 (85%) of 154 cases involved children who had been vaccinated, 18 (12%) were not vaccinated, and the vaccination status of 5 (3.2%) was unknown. Information on both vaccination history and number of lesions was reported in 135 (70%) of the 192 cases: 90 (76%) of 119 vaccinated children had mild illness (fewer than 50 lesions), and 12 (75%) of 16 unvaccinated children had moderately severe illness (50 to 499 lesions). The majority of these cases were involved in 7 outbreaks identified among sentinel schools; 131 (68%) of 192 reported cases were included in these outbreaks. Cases per outbreak ranged from 5 to 57 (median: 8). The 61 cases not associated with an outbreak represent sporadic varicella incidence.

Based on sentinel school data, an estimated 951 sporadic cases of varicella would have been expected to occur during a school year among the 883,181 school-aged children in Minnesota, representing 0.11% of this population, for an incidence rate of 107.7 per 100,000 population. This represents a decline from the estimated annual U.S. incidence of 1,500 per 100,000 population in the prevaccine era.17 During the 2006-07 school year, estimated grade-level specific annual incidence rates were 166.4 per 100,000 (686 of 412,111) for elementary school students; 75.8 per 100,000 (112 of 148,267) for middle school students; and 51.2 per 100,000 (153 of 299,320) for high school students.

In 2006, the Department of Health received 16 reports of varicella from 4 (10%) of 40 sentinel childcare centers. Eleven (69%) of 16 cases occurred in the same facility. Based on sentinel childcare data, an estimated 1,343 (1.3%) cases of varicella would have been expected to occur during a calendar year among the 107,070 children enrolled in Minnesota childcare centers.

As Minnesota moves from a 1-dose to a 2-dose vaccine requirement, sentinel reporting data will become more important as outbreaks are expected to diminish to only a handful each year. Eventually, reports of all individual cases will be the best way for Department of Health officials to monitor the true burden of the disease. Such a system will allow health officials to more quickly respond if they start to see evidence of the efficacy of the 2-dose schedule waning.

Conclusion
The incidence of varicella is declining in the United States and Minnesota. The 1-dose vaccination policy was successful in significantly decreasing varicella morbidity and mortality; however, school outbreaks continue to occur. A 2-dose requirement for school entry in Minnesota will be implemented in September 2008 in order to continue to comply with medically accepted standards. Surveillance efforts will now assess the impact of the 2-dose vaccination policy on varicella incidence. The accuracy of these efforts relies on vigilant reporting, and the success of the varicella vaccine relies on high rates of routine vaccination with 2 doses of varicella vaccine. MM

Andrew Murray is an epidemiologist in the vaccine-preventable disease unit at the Minnesota Department of Health.

Acknowledgement
The school and childcare surveillance programs in Minnesota were developed and implemented by former vaccine-preventable disease epidemiologist Gary Wax. Varicella surveillance data was reported by school health personnel and childcare facility administrators across the state. The author also acknowledges Claudia Miller, Kristen Ehresmann, Lynn Bahta, and Susan Ersted for their assistance in reviewing this article.

References
1. Centers for Disease Control and Prevention. Prevention of varicella: recommendations of the Advisory Committee on Immunization Practices (ACIP). Morb Mortal Wkly Rep. 2007;56(No. RR04):1-37.
2. Centers for Disease Control and Prevention. Prevention of varicella: update recommendations of the Advisory Committee on Immunization Practices (ACIP). Morb Mortal Wkly Rep. 1999;48(No. RR06):1-5.
3. Guris D, Jumaan AO, Mascola L, et al. Changing varicella epidemiology in active surveillance sites-United States, 1995-2005. J Infect Dis (Suppl). 2007; in press.
4. Centers for Disease Control and Prevention. Prevention of varicella: recommendations of the Advisory Committee on Immunization Practices (ACIP). Morb Mortal Wkly Rep. 1996;45(No. RR11):1-25.
5. Ehresmann KR, Mills WA, Loewenson PR, Moore, KA. Attitudes and practices regarding varicella vaccination among physicians in Minnesota: implications for public health and provider education. Am J Public Health. 2000;90:1917-20.
6. Council of State and Territorial Epidemiologists. Position Statement: Varicella surveillance. 2002-ID-06. Available at: http://www.cste.org/position statements/searchdbYear1new.asp Accessed September 24, 2007.
7. Council of State and Territorial Epidemiologists. Position Statement: Varicella surveillance and control. 1998-ID-9. Available at: www.cste.org/ps/1998/1998-id-09.htm Accessed November 5, 2007.
8. Centers for Disease Control and Prevention. Statistics and Surveillance: Immunization Coverage in the U.S. Available at: www.cdc.gov/vaccines/stats-surv/imz-coverage.htm#nis. Accessed November 5, 2007.
9. Centers for Disease Control and Prevention. Decline in annual incidence of varicella-selected states, 1990-2001. Morb Mortal Wkly Rep. 2003;52(37):884-5.
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12. Davis MM, Patel MS, Gebramariam A. Decline in varicella-related hospitalizations and expenditures for children and adults after introduction of varicella vaccine in the United States. Pediatrics. 2004;114(3):786-92.
13. Nguyen H, Jumaan AO, Seward JF. Decline in mortality due to varicella after implementation of varicella vaccination in the United States. New Engl J Med. 2005;352(5):450-8.
14. Lee BR, Feaver SL, Miller CA, Hedberg CW, Ehresmann KR. An elementary school outbreak of varicella attributed to vaccine failure: policy implications. J Infect Dis. 2004;190(3):477-83.
15. Council of State and Territorial Epidemiologists. Position Statement: Varicella vaccine policy and varicella surveillance. 2006-ID-13. Available at: www.cste.org/position statements/searchbyyear2006c.asp. Accessed November 5, 2007.
16. Minnesota Office of the Revisor of Statutes. Minnesota Rules, Chapter 4604. Available at: http://www.revisor.leg.state.mn.us/bin/getpub.php?pubtype=RULE_CHAP&year=current&chapter=4604. Accessed November 5, 2007.
17. Wharton M. The epidemiology of varicella-zoster virus infections. Infect Dis Clin N Amer. 1996;10:571-81.

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