Varicella Breaks Out Despite Double Doses of Vaccine

ATLANTA – Children who had received two doses of varicella vaccine comprised nearly a third of the cases in a varicella outbreak among elementary school children in Arkansas.

“This was one of the largest varicella outbreaks investigated in recent years, and it was the first U.S. outbreak reported with a significant number of two-dose vaccines,” said Adriana Lopez, an epidemiologist with the Centers for Disease Control and Prevention's division of viral diseases.

A total of 85 cases were identified during the outbreak, which occurred among children in prekindergarten through sixth grade in the fall of 2006. Of these, 25 (29%) had received two doses of varicella vaccine, 54 (64%) had received one dose, and 6 (7%) had not received any varicella vaccination. “The moderate level of two-dose coverage was insufficient to prevent the outbreak,” said Ms. Lopez, who presented the results at a meeting of the CDC's Advisory Committee on Immunization Practices (ACIP).

Overall, the school's varicella vaccination coverage was 97% among 758 children and 41% of the vaccinated children had received two doses. The CDC was invited to investigate the outbreak in order to confirm varicella in the children who had received two doses of the vaccine and to characterize the vaccine's effectiveness among one- and two-dose recipients. “Disease in vaccinated persons is generally mild, with fewer than 50 lesions and a shorter duration of illness,” noted Ms. Lopez.

For purposes of the investigation, a varicella case was defined as an acute macular papular vesicular rash without another apparent cause. Because of the frequency of insect bites in the region of the outbreak, a child needed at least three lesions to be considered a case.

A total of 27 cases were sampled for testing. Six of these were confirmed as positive using five polymerase chain reaction tests on lesion samples and one using an IgM serum spot test. The positive results for the PCR tests included one child who had received two doses of vaccine, three children who had each received single doses, and one unvaccinated child who had a history of varicella. The positive IgM serum spot test was used for a child who had received one dose of vaccine.

In June 2006, ACIP recommended a routine two-dose varicella vaccination plan for children, with the first dose given at age 12-15 months, the second dose given at age 4-6 years, and catch-up doses given to anyone older than 6 years.

Varicella cases associated with the outbreak were identified by local health data, the school nurse, and through school surveys on vaccination status and disease history. Parents of the children who were identified as cases were contacted and asked for additional information about medical history and vaccination status. Vaccination history was verified using the state's immunization registry, local health records, and the parents' surveys. Children were considered unvaccinated if they had received their first dose of vaccine 42 days or less before they became ill.

“As next steps, additional vaccine effectiveness studies for two doses are needed,” said Ms. Lopez. Data from more than 30 postlicensure studies show effectiveness estimates for a single dose of varicella vaccine ranging from 44% to 100%.

ATLANTA – Children who had received two doses of varicella vaccine comprised nearly a third of the cases in a varicella outbreak among elementary school children in Arkansas.

“This was one of the largest varicella outbreaks investigated in recent years, and it was the first U.S. outbreak reported with a significant number of two-dose vaccines,” said Adriana Lopez, an epidemiologist with the Centers for Disease Control and Prevention's division of viral diseases.

A total of 85 cases were identified during the outbreak, which occurred among children in prekindergarten through sixth grade in the fall of 2006. Of these, 25 (29%) had received two doses of varicella vaccine, 54 (64%) had received one dose, and 6 (7%) had not received any varicella vaccination. “The moderate level of two-dose coverage was insufficient to prevent the outbreak,” said Ms. Lopez, who presented the results at a meeting of the CDC's Advisory Committee on Immunization Practices (ACIP).

Overall, the school's varicella vaccination coverage was 97% among 758 children and 41% of the vaccinated children had received two doses. The CDC was invited to investigate the outbreak in order to confirm varicella in the children who had received two doses of the vaccine and to characterize the vaccine's effectiveness among one- and two-dose recipients. “Disease in vaccinated persons is generally mild, with fewer than 50 lesions and a shorter duration of illness,” noted Ms. Lopez.

For purposes of the investigation, a varicella case was defined as an acute macular papular vesicular rash without another apparent cause. Because of the frequency of insect bites in the region of the outbreak, a child needed at least three lesions to be considered a case.

A total of 27 cases were sampled for testing. Six of these were confirmed as positive using five polymerase chain reaction tests on lesion samples and one using an IgM serum spot test. The positive results for the PCR tests included one child who had received two doses of vaccine, three children who had each received single doses, and one unvaccinated child who had a history of varicella. The positive IgM serum spot test was used for a child who had received one dose of vaccine.

In June 2006, ACIP recommended a routine two-dose varicella vaccination plan for children, with the first dose given at age 12-15 months, the second dose given at age 4-6 years, and catch-up doses given to anyone older than 6 years.

Varicella cases associated with the outbreak were identified by local health data, the school nurse, and through school surveys on vaccination status and disease history. Parents of the children who were identified as cases were contacted and asked for additional information about medical history and vaccination status. Vaccination history was verified using the state's immunization registry, local health records, and the parents' surveys. Children were considered unvaccinated if they had received their first dose of vaccine 42 days or less before they became ill.

“As next steps, additional vaccine effectiveness studies for two doses are needed,” said Ms. Lopez. Data from more than 30 postlicensure studies show effectiveness estimates for a single dose of varicella vaccine ranging from 44% to 100%.

ATLANTA – Children who had received two doses of varicella vaccine comprised nearly a third of the cases in a varicella outbreak among elementary school children in Arkansas.

“This was one of the largest varicella outbreaks investigated in recent years, and it was the first U.S. outbreak reported with a significant number of two-dose vaccines,” said Adriana Lopez, an epidemiologist with the Centers for Disease Control and Prevention's division of viral diseases.

A total of 85 cases were identified during the outbreak, which occurred among children in prekindergarten through sixth grade in the fall of 2006. Of these, 25 (29%) had received two doses of varicella vaccine, 54 (64%) had received one dose, and 6 (7%) had not received any varicella vaccination. “The moderate level of two-dose coverage was insufficient to prevent the outbreak,” said Ms. Lopez, who presented the results at a meeting of the CDC's Advisory Committee on Immunization Practices (ACIP).

Overall, the school's varicella vaccination coverage was 97% among 758 children and 41% of the vaccinated children had received two doses. The CDC was invited to investigate the outbreak in order to confirm varicella in the children who had received two doses of the vaccine and to characterize the vaccine's effectiveness among one- and two-dose recipients. “Disease in vaccinated persons is generally mild, with fewer than 50 lesions and a shorter duration of illness,” noted Ms. Lopez.

For purposes of the investigation, a varicella case was defined as an acute macular papular vesicular rash without another apparent cause. Because of the frequency of insect bites in the region of the outbreak, a child needed at least three lesions to be considered a case.

A total of 27 cases were sampled for testing. Six of these were confirmed as positive using five polymerase chain reaction tests on lesion samples and one using an IgM serum spot test. The positive results for the PCR tests included one child who had received two doses of vaccine, three children who had each received single doses, and one unvaccinated child who had a history of varicella. The positive IgM serum spot test was used for a child who had received one dose of vaccine.

In June 2006, ACIP recommended a routine two-dose varicella vaccination plan for children, with the first dose given at age 12-15 months, the second dose given at age 4-6 years, and catch-up doses given to anyone older than 6 years.

Varicella cases associated with the outbreak were identified by local health data, the school nurse, and through school surveys on vaccination status and disease history. Parents of the children who were identified as cases were contacted and asked for additional information about medical history and vaccination status. Vaccination history was verified using the state's immunization registry, local health records, and the parents' surveys. Children were considered unvaccinated if they had received their first dose of vaccine 42 days or less before they became ill.

“As next steps, additional vaccine effectiveness studies for two doses are needed,” said Ms. Lopez. Data from more than 30 postlicensure studies show effectiveness estimates for a single dose of varicella vaccine ranging from 44% to 100%.