Repeated flu vaccination may better protect children – article review

A recent peer-reviewed article by Huong Q. McLean et al., published in JAMA Network Open, showed that repeat flu vaccination does not reduce the subsequent season’s flu vaccine effectiveness in children, ages 2-17 years. In fact, the study may show that children who received the flu vaccine in the prior flu season may benefit from the same vaccine in the current season.

This research may help settle a concern among vaccine experts that previous year’s flu vaccination may reduce current-season flu vaccine effectiveness. A 2017 study from Canada showed that if flu strains used in two vaccines are the same (or very similar), the second season’s flu vaccine effectiveness may be diminished.

This lowered effectiveness in subsequent years is described as the “antigenic distance hypothesis” (ADH). It describes the possibility that vaccine effectiveness of the current vaccine is lowered when the difference, or antigenic distance, between the current vaccine and prior year vaccine, is small, but the actual difference between the vaccine strains and wild circulating strains is large.

For example, the recent issues with the H3N2 influenza A outbreak, where the available flu vaccines had lower effectiveness against the H3N2 strain. The H3N2 portion of the 2017-18 flu vaccine was the same as in the 2013-14 vaccine, but it no longer matched with the H3N2 flu in circulation. The H3N2 variant had “drifted” genetically.

The repeated flu vaccine study – results

The study by McLean et al. took place over three flu seasons – 2013-14, 2014-15, and 2015-16. The study enrolled 3,369 children at four US centers, of whom 1,674 (about 50%) were vaccinated during the flu season. The study looked at repeated flu vaccine effectiveness for both the live attenuated influenza vaccine (LAIV) and inactivated influenza vaccine (IIV). The LAIVs are usually the nasal spray flu vaccines, whereas the IIVs are the injected versions.

The results for the LAIV were as follows:

  • 50.3% vaccine effectiveness against the H3N2 strain among children who were vaccinated in one of the three flu seasons and one prior season.
  • -82.4% vaccine effectiveness against H3N2 for children who had not received a prior season vaccine.
  • 47.5% effectiveness against the H1N1 strain for children who were previously vaccinated.
  • 7.8% effectiveness for those without a prior year vaccine.

The results for those who received the IIV injection:

  • 38.7% vaccine effectiveness against the H3N2 strain among children who were vaccinated in one of the three flu seasons and one prior season.
  • 23.2% vaccine effectiveness against H3N2 for children who had not received a prior season vaccine.
  • 72.4% effectiveness against the H1N1 strain for children who were previously vaccinated.
  • 67.5% effectiveness for those without a prior year vaccine.

The repeated flu vaccine study – takeaways

The authors concluded that:

The antigenic distance hypothesis predicts negative interference from repeated vaccination when vaccine strains are identical [vaccine strain 1 is the same as vaccine strain 2] and circulating viruses are drifted, as occurred in the 2014-2015 season. However, the antigenic distance hypothesis was based on simulations of serologic response to inactivated vaccines, and differences in the adaptive immune response to LAIV and IIV might lead to different repeated vaccination effects in children.

Influenza VE varied by influenza type and subtype and vaccine type, but prior-season vaccination was not associated with reduced VE. These findings support current recommendations for annual influenza vaccination of children.

In an accompanying commentary on the study, also published in JAMA Network Open, Sarah Cobey, PhD of the University of Chicago, stated that the results of the study:

In no case was repeated vaccination associated with lower effectiveness than vaccination in the current season only. In other words, there was no evidence of diminished vaccine effectiveness in frequent vaccinees, even though the study included seasons in which such effects had been reported elsewhere.

…suggest additional support for the current Advisory Committee on Immunization Practices’ recommendation that children be vaccinated annually against influenza. Although annual vaccinations may do little to enhance protection against influenza B, regular vaccinations might improve performance of the influenza A(H3N2) and influenza A(H1N1)pdm09 components.


Both the injected and nasal flu vaccines were very effective against the H1N1 flu strain. The effectiveness against the H3N2 strain was also higher than what has been pushed in anti-vaccine tropes.

This research provides us with strong evidence that repeated flu vaccination actually boosts children’s immunity against various strains of the flu. And remember, the flu is a deadly and dangerous disease, killing thousands of people every year.

Why wouldn’t you protect your children against the flu?


The Original Skeptical Raptor
Chief Executive Officer at SkepticalRaptor
Lifetime lover of science, especially biomedical research. Spent years in academics, business development, research, and traveling the world shilling for Big Pharma. I love sports, mostly college basketball and football, hockey, and baseball. I enjoy great food and intelligent conversation. And a delicious morning coffee!