New evidence contradicts a federal panel's recommendations that found the nasal form of the influenza vaccine to be ineffective in fighting the virus.
The study published Monday in the Annals of Internal Medicine concluded that vaccinating children with the live attenuated version of influenza vaccine, which is delivered through a nasal spray, was as effective as the inactivated form that comes in a shot.
But the differing conclusions may have resulted from the way in which the data were collected.
Researchers conducting a randomized blinded trial of both vaccines among 1,186 children within an isolated, rural community in western Canada found that between 2012 and 2015, the rate of flu infection among those vaccinated with the nasal spray was 5.3% compared to 5.2% among the group who received the shot.
“The irony of the whole thing is that we originally intended the study to demonstrate whether the live vaccine was superior to the inactivated vaccine,” said the study's lead author, Dr. Mark Loeb, director of the infectious diseases division at McMaster University in Ontario. “I think it shows the importance of doing carefully designed, randomized controlled trials to answer these questions.”
The Canadian study also did not use the quadrivalent, or four virus-strains, form of the nasal vaccine, which is not available in the country. Rather, it used the trivalent or three-strain version of the spray. The CDC findings on the effectiveness of the nasal vaccine were based on the quadrivalent version.
In June, a federal advisory committee on vaccine efficacy said observational evidence collected by the Centers for Disease Control and Prevention over the past three flu seasons led the committee to advise against the use of the nasal spray, known as FluMist.
Results from the 2015-2016 flu season showed the nasal spray vaccine to be about 3% effective in protecting against the virus for children ages of 2 and 17 compared with an efficacy rate of 63% for the flu shot, according to the CDC.
The nasal flu vaccine was also found to have under-performed compared to the flu shot during the 2013-2014 flu season. During the 2014-2015 flu season, when the effectiveness of all flu vaccines was severely diminished because of a mutation in the predominant flu strain, FluMist was found to have an efficacy of -23% for children ages 2 through 8 compared with a rate of 15% for the flu shot.
FluMist is the only brand of the nasal flu vaccine approved to be sold on the market and makes up about 8% of the total annual flu vaccine supply in the U.S., so it is unlikely that its absence will cause a disruption in this year's vaccine distribution. AstraZeneca, the vaccine's manufacturer, generated more than $200 million in sales from it in fiscal 2015. The company estimates it will lose about $100 million as a result of the CDC's findings.
The advisory committee is scheduled to meet again in October.