THURSDAY, Aug. 18 (HealthDay News) -- Children younger than 3 years old receive roughly the same amount of protection from the flu vaccine regardless of which method (injection, nasal spray, or one of each) is used to give the two recommended doses to them, according to a new federal study.

Researchers from the U.S. National Institute of Allergy and Infectious Diseases (NIAID) revealed, however, that young children who receive at least one dose of the nasal spray vaccine had the broadest immune response, which could boost their protection against many diverse flu strains. The nasal spray contains a live, attenuated influenza virus vaccine, meaning that its live virus is highly weakened or modified.

Young children are given two doses of flu vaccine, in what's known as a prime-booster combination. The first dose is intended to prime their immune system to produce antibodies, while the second dose is meant to "boost" or trigger an immune response.

"Severe complications from seasonal influenza can be devastating to young children," said NIAID director Dr. Anthony S. Fauci, in a news release from the institute. "This study provides initial evidence that the prime and booster doses for these young children can be different types of flu vaccines and still provide adequate protection against matching seasonal flu strains."

In conducting the study, researchers examined 53 children ranging in age from 6 months to 35 months over the course of two years.

During this time, all of the children received a prime dose of the seasonal flu vaccine as well as a booster dose one month later. The children were divided into four groups, receiving the doses in one of the following manners: two injections of flu vaccine; two doses of the live, attenuated nasal spray; nasal spray followed by an injection; injection followed by the nasal spray.

The investigators found that all four groups of children were safe and produced similar amounts of protective antibodies.

Only children who received the live, attenuated virus, however, produced significant amounts of three important protective T-cells specific to influenza, the researchers added. They noted the effects were similar if the children received just one or both doses of the nasal spray. For children who received one injection and one nasal spray vaccine, the order of the doses made no difference.

The study pointed out that the live, attenuated flu virus has been linked to wheezing in the youngest recipients. As a result, the researchers concluded that the best regimen for children younger than 24 months might be one injected dose followed by one live, attenuated dose of the seasonal flu vaccine.

More information

The U.S. Centers for Disease Control and Prevention has more on children and the flu vaccine.