New data from the U.S. Flu Vaccine Effectiveness Network has shown that a live attenuated influenza vaccine (“LAIV”), delivered using a nasal spray, was not effective against the H1N1 virus (also known as swine flu) when compared to inactivated influenza vaccine (“IIV”), delivered via injection, in children two to eight years old.

MedImmune, the manufacturer of the LAIV in question, suggested that the poor performance of their vaccine against H1N1 last year may have been related to the instability of the hemagglutinin protein in the vaccine, which led to degradation when exposed to heat.

MedImmune data indicates that the vaccine's low effectiveness against H1N1 was specific to certain production lots – in a presentation to the CDC Advisory Committee on Immunization Practices (“ACIP”), the company noted that vaccine delivered in Weeks 4 - 9 of the shipping season was significantly less effective than lots shipped before or after.Update on Effectiveness of Live-Attenuated Versus Inactivated Influenza Vaccines in Children and Adolescents Aged 2–18 Years After a comprehensive investigation, MedImmune found a correlation between the effectiveness of vaccine lots and the ambient temperature when the lots were unloaded from trucks for delivery. The vaccine lots out on hot days were the least effective, possibly because they degraded and lost potency.

On its website, MedImmune states that its LAIV must be stored and transported at temperatures between 2° - 8°C. However, they also note that, generally, a single temperature excursion (up to 25°C for 12 hours) has not been shown to have an “adverse impact” on the vaccine. However, this may not be the case with the H1N1 strain MedImmune used. MedImmune plans to replace the H1N1 strain with a similar strain that has a more stable hemagglutinin protein (the key antigen, or active ingredient, in most flu vaccines) in the hopes of removing the potential for degradation in the event of limited temperature excursions.

The MedImmune vaccine for the 2014 - 2015 season was intended to provide protection against four flu viruses: an influenza A (H1N1) virus, an influenza A (H3N2) virus, and two influenza B viruses.

CDC Revises Previous Guidance Favoring LAIV

In late February, ACIP chose not to reaffirm 2014 - 2015 guidance that favored LAIV over IIV in healthy children two to eight years old. This countered a recommendation ACIP had made last June after a review of data suggested that LAIV offered better protection than IIV for children in this age group. ACIP now recommends that children six months and older get a flu vaccine once a year, with no preference stated for the nasal spray vaccine or the injection.