Study finds nasal, injectable flu vaccines similarly effective

Some experts question the study's methodology

The trivalent influenza vaccine is as effective as an injectable version, according to a study published Tuesday in the Annals of Internal Medicine.

Background

FluMist, developed by AstraZeneca's MedImmune, is the only nasal spray flu vaccine on the market and is approved for use on healthy people ages two through 49. The spray vaccine differs from the traditional shot-based flu vaccinations because it is made using a live but weakened virus, as opposed to a killed virus.

CDC research has consistently found the nasal vaccine, which featured three strains of the flu, to be highly effective for children. However, CDC deemed a newer version of the vaccine, featuring an additional flu strain, ineffective. As a result, CDC's Advisory Committee on Immunization Practices (ACIP) earlier this year recommended against the quadrivalent nasal spray.

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Study design

For the study, researchers compared the effectiveness of FluMist's trivalent nasal vaccine and the trivalent injectable vaccine among children ages 3 to 15 in Hutterite communities in rural Canada. According to NPR, researchers chose to conduct the experiment in Hutterite communities because their isolation allows for fewer confounding factors.

To compare the vaccines, the researchers randomly assigned 1,186 Hutterite children to receive either the nasal vaccine or the injectable vaccine. The researchers reviewed data on three flu seasons, spanning from 2012 to 2015.

Key findings

The researchers found the trivalent nasal spray and the trivalent injectable vaccines were equally effective over the three observed flu seasons. According to the study, the rate of children who contracted the flu was 5.3 percent among those who received the nasal vaccine and 5.2 percent among those who received the injectable vaccine. Those rates were consistent with previous research that found flu infection rates of about 4 to 5 percent among vaccinated Hutterite children.

Mark Loeb, the study's lead author and director of McMaster University's division of infectious diseases, said the new findings showed no difference in the herd immunity associated with either version of the vaccine, given that the immunizations' efficacies did not differ significantly.

Flu shots prevented millions of cases since 2005

Implications for CDC's ACIP decision

Experts say the new study's findings might not directly contradict the CDC advisory committee's latest recommendation because the study used the trivalent nasal vaccine and not the quadrivalent vaccine.

William Schaffner, professor and chair of the preventive medicine at Vanderbilt University, said the study also focused on different years than the CDC advisory committee data.

Kawsar Talaat, an assistant scientist at the Johns Hopkins Bloomberg School of Public Health's Center for Immunization Research, said comparing the trivalent and quadrivalent vaccines is like "comparing oranges and tangerines." She added that comparing any two flu vaccines involves a "quagmire of issues," in part because the vaccines differ from year to year.

However, Litjen Tan, chief strategy officer for the Immunization Action Coalition, said he "could argue it both ways." He cited an earlier FDA statement indicating ACIP had not considered the "totality of the evidence" when making its decision to rescind its endorsement of the quadrivalent nasal spray. However, he questioned, "In light of this data, are they going to come back and revisit this recommendation before flu season starts—even though it's already starting?" (Haelle, "Shots," NPR, 8/15; Smith, MedPage Today, 8/15).

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