Annual Flu Vaccine May Weaken Kids' Pandemic Immunity

Laird Harrison

November 28, 2011

November 28, 2011 — The annual flu vaccine can weaken children's resistance to other types of influenza virus, including those that could cause a pandemic, according to a study published in the November issue of the Journal of Virology.

"Annual vaccination against influenza is effective but may have potential drawbacks that have previously been underappreciated and that are also a matter of debate," write lead author Rogier Bodewes, DVM, from Erasmus Medical Center in Rotterdam, the Netherlands, and coauthors.

They don't suggest that annual childhood vaccinations should be halted, but they call for research into better vaccines that eliminate this adverse effect.

To study the effects of seasonal immunization, the researchers collected blood samples from 27 healthy unvaccinated children (median age, 6.0 years; range, 2.0 to 8.8) and 14 children with cystic fibrosis who had received annual influenza vaccinations (median age, 6.2 years; range, 3.1 to 9.0). In the Netherlands, only special populations of children, such as those with cystic fibrosis, are routinely vaccinated against influenza.

Of the unvaccinated children, 24 of 27 (89%) had antibodies against at least 1 A/H3N2 virus, and 20 (74%) had antibodies against at least 1 A/H1N1 virus. All vaccinated children had antibodies against at least 1 influenza virus, including A/H1N1(2009). However, unvaccinated children had significantly higher geometric mean titers for several viruses, including the influenza A/Panama/07/99 (H3N2) and A/Solomon Islands/3/2006 (H1N1) viruses (P = .04 and P = .01, respectively).

They found that the percentage of virus-specific ICD8+ T cells — that is, T cells with a memory for influenza viruses — ranged from 0.00% to 2.32% in unvaccinated children and from 0.06% to 1.56% in vaccinated children.

They noted an age-dependent increase in the virus-specific CD8+ T cell response in the unvaccinated children (P = .040), but not in the vaccinated children (P = .714).

Finally, in children older than 5 years, the investigators found a significantly higher percentage (P = .038) of CD8+ T cells in the unvaccinated group (mean, 0.86%; standard deviation [SD], 0.67%) than in the vaccinated group (mean, 0.37%; SD, 0.45%).

The study results are consistent with the team's previous mouse experiments. In those studies, Dr. Bodewes and colleagues found that vaccination against the seasonal influenza A virus prevented the development of influenza A virus-specific CD8+ T cell immunity, which would otherwise be induced by infection.

During the 2009 pandemic, Canadian children who had been vaccinated against the seasonal flu were more likely to be infected with A/H1N1 than those who had not been vaccinated. (These results were not confirmed by studies in other countries.)

It is a small study, so the results have to be taken with a grain of salt, Christopher Harrison, MD, director of the Infectious Disease Research Laboratory at Children's Mercy Hospitals and Clinics in Kansas City, Missouri, who was not involved with the study, told Medscape Medical News. But, he said, it lends weight to the argument that children should be immunized with the live virus instead of the killed virus.

"We have known for a long time that most vaccines given in the muscle don't produce the ideal response," said Dr. Harrison.

Killing the virus changes key proteins, which immune cells must recognize to prepare for another influenza infection, he explained. In addition to probably providing better protection against unexpected flu viruses, immunization with the live virus is 10% to 15% more effective than immunization with the killed virus against the seasonal flu, he added.

But Walter Orenstein, MD, a member of the American Academy of Pediatrics Committee on Infectious Diseases, said there isn't enough evidence yet to make changes in the US Centers for Disease Control and Prevention guidelines for vaccinations, which explicitly state no preference for live or killed vaccines.

This study only shows a difference in biomarkers for immunity, not in immunity itself, Dr. Orenstein emphasized. "We have to look at the actual disease," he said. "Do people who get repeated vaccination have as much disease as people who don't? That's the ultimate question."

The study was supported in part by TI Pharma. The study authors and Dr. Orenstein have disclosed no relevant financial relationships. Dr. Harrison reports receiving research support from GlaxoSmithKline.

Journal of Virology. 2011;85:11995-12000. Abstract

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