Complete influenza vaccination crucial in young children

May 2008

ATLANTA — Preventing influenza-associated hospitalizations in young children begins with ensuring that the children are fully vaccinated, according to a presentation made at the Advisory Committee on Immunization Practices meeting in February.

“The ACIP recommended annual influenza vaccination for all children aged 6 to 23 months beginning with the 2004-2005 season,” David K. Shay, MD, MPH, said during the influenza vaccine session. “Much of this was motivated by the burden of disease among children in this age group, specifically that hospitalization rates in this age group are similar to those that have been seen among the elderly for many years.”

Shay, of the National Center for Immunization and Respiratory Diseases’ influenza division, presented data from an eight-state, case-control study in which he and colleagues estimated the effectiveness of trivalent inactivated vaccine in preventing influenza-associated hospitalization in children aged 6 to 23 months.

The researchers enrolled 93 of 191 eligible patients who were hospitalized with lab-confirmed influenza and 334 age- and ZIP-code-matched controls in the first two seasons.

Influenza was diagnosed through rapid testing in 50% of the cases, direct fluorescence antibody in 40%, viral culture in 11%, real-time polymerase chain reaction in 2% and through multiple tests in 8%. Influenza A was the most commonly diagnosed influenza type (85%).

Children were considered immunized 14 days after the receipt of each vaccine dose. For the purposes of the data reported, children were categorized as fully immunized after receiving two doses in the current season if it was their first season being vaccinated, or after one dose in the current season if the child had received two doses during the previous season, based on the 2007 ACIP recommendations for receipt of influenza vaccine. Children who received only 1 dose of vaccine when 2 were recommended were classified as partially immunized.

Vaccine efficacy

Crude vaccine effectiveness estimates showed that full immunization was 74% protective, whereas partial immunization was only 39% protective.

After adjusting for high-risk conditions, very low birth weight and lack of a source of private insurance, Shay and colleagues found that vaccine effectiveness slightly increased among fully immunized children and slightly decreased among children who were partially immunized.

“It’s critical to ensure that children aged 6 to 23 months are fully immunized if we seek to prevent hospitalizations among children,” Shay said.

Dr. Shay is a medical officer at the CDC.

For more information:

• Shay DK. Multistate case-control study of the effectiveness of influenza vaccine in preventing laboratory-confirmed influenza hospitalizations among children aged 6 to 23 months during the 2005-06 and 2006-07 seasons. Presented at: the Advisory Committee on Immunization Practices meeting; Feb. 27-28, 2008; Atlanta.