For three consecutive years, AstraZeneca's FluMist, its flu vaccine's nasal-spray version, failed to protect children. This caused the Advisory Committee on Immunization Practices (ACIP) of the U.S. Centers for Disease Control and Prevention (CDC) to recommend flu shots to children this flu season instead of the painless alternative.

Health experts said that while conventional flu shots worked well in protecting vaccinated individuals last year, AstraZeneca's FluMist didn't work at all. As a result, a significant number of people got sick.

What A Difference Two Years Can Do

In 2014, the CDC recommended that doctors use FluMist instead of the conventional flu shots whenever possible.

According to CDC flu expert Dr. Joseph Bresee, the expert panel couldn't find evidence that the FluMist was effective. The initial study findings were presented to the federal advisory committee on immunization.

As a result, the immunization committee voted to withdraw its FluMist endorsement on Wednesday, June 22. The government typically adopts the committee's recommendations which are then sent to doctors.

"We agree with [the Advisory Committee on Immunization Practices'] decision today to recommend health care providers and parents use only the inactivated vaccine," said American Academy of Pediatrics President Dr. Benard Dreyer.

The U.S. Food and Drug Administration (FDA) approved AstraZeneca's flu nasal spray in 2003. FluMist is produced by MedImmune, the subsidiary of the pharmaceutical company.

Inactivated Vs. Attenuated Vaccine

Traditional flu shots are made up of killed (inactivated) influenza viruses. In contrast, FluMist contains weakened (attenuated) but live viruses and works by stimulating a person's immune system.

FluMist has two versions. The trivalent vaccine protects people from three strains of the influenza virus while the quadrivalent vaccine protects against four strains.

Bresee added it remains unclear why the nasal spray didn't work. However, the suspicion is that the failure stems from the decision to include all four influenza virus strains instead of only three. The inclusion might have lowered the body's response to the additional strain.

"AstraZeneca is working with the CDC to better understand its data to help ensure eligible patients continue to receive the vaccine in future seasons in the US," wrote the pharmaceutical company in a press release.

A significant number of hospitals, clinics and doctors make their flu vaccine orders early in the year. The current decision and recommendation could become a problem for the flu vaccination programs set for children this coming fall.

Dr. William Schaffner, a vaccine expert at Vanderbilt University, said it could "really disrupt the vaccine supply."