Flu Vaccination Important even if Vaccine Matching is Poor

Previously vaccinated adult patients who were hospitalized with influenza A had reduced morbidity and mortality compared to non-vaccinated patients despite the poor vaccine matching recognized for the 2014-2015 flu season, according to a study by physician-scientists at the University of Pittsburgh and the University of Michigan.

The findings, published in the journal Infectious Diseases in Clinical Practice, demonstrate a positive relationship between influenza vaccination status and attenuated illness severity in hospitalized patients with influenza A. The published research article was accompanied by an editorial written by Thomas File, MD, the president of the Infectious Diseases Society of America.

“Specifically, patients who were vaccinated were less likely to die and less likely to require mechanical ventilation,” said lead author Timothy Kaselitz, MD, clinical assistant professor in the Department of Critical Care Medicine and a third-year pediatric critical care fellow. He added that crucially, these findings were during a season with poor vaccine match to the circulating viral strains.

The question of vaccination and illness severity benefit during years of poor match is a relatively unexplored question. The paper discusses several plausible biological mechanisms to explain how vaccination could benefit illness severity despite poor vaccine matching:

  • The vaccine’s antibody response may have mediated the severity of the flu, even if it did not prevent infection;
  • The well-described “back-boost” phenomenon whereby contemporary vaccination prompts broad antibody responses to previously encountered antigens;
  • This could have provided a sufficiently broad antibody repertoire in response to the vaccine hemagglutinin;
  • Antibodies to neuraminidase, an unexplored component of 2014-2015 vaccine, have also been suggested to protect against symptomatic or severe disease, more so than the infection itself; and
  • Vaccination behavior is correlated from year to year, so that vaccinated individuals may have had more frequent previous annual vaccinations, affording them a broader antibody repertoire.

This research supports the already strong recommendation that all adults and children without a contraindication obtain a yearly flu shot.

Dr. Kaselitz described his experience encouraging patients to have a flu vaccine when he was working in a primary care clinic during his residency, “My patients would often refuse their yearly flu vaccine because they believed they still got the flu despite getting the shot.”

There is now a growing body of evidence that infection prevention is only part of the story and that it is especially important that those at increased risk—elderly, pregnant, children, and those with chronic illnesses—get vaccinated, regardless of vaccine match for that particular season.

Dr. Kaselitz began this research project during his residency in Internal Medicine and Pediatrics at the University of Michigan, and later completed the analysis and manuscript during the first year of his combined Adult/Pediatric Critical Care Medicine fellowship at the University of Pittsburgh.