Study: Yearly Flu Vaccines Don’t Lose Effectiveness

The debate about whether someone should be vaccinated for influenza every year will probably continue, but researchers with Kochi Medical School Hospital, Nankoku, Japan, have come up with a strong argument that people should get vaccinated against flu each year. The investigators—who published their findings in a study in the American Journal of Infection Control—conclude that flu vaccinations do not lose their effectiveness if given each year over 5 years.

“This is a controversial point, and to the best of our knowledge, this is the first time that this issue has been addressed with objective measures of immune response to vaccination,” the study states. Investigators cite previous studies that contradict one another on this point, saying that “the impact of repeated seasonal vaccination remains to be clarified.”

They focused on antibody responses after flu vaccination to measure the ongoing effectiveness of repeated vaccination. In each of the 5 years, investigators found that hemagglutination inhibition (HI) antibody titers for influenza A/H1N1, influenza A/H3N2, and influenza B were elevated at 4 weeks after vaccination. And in each of the 5 years, the titers of all decreased to their original levels 8 months later. “PRs (the proportion of individuals with HI antibody titers ≥1:40 (or 40)) at 4 weeks after vaccination were similar among most of the 5 seasons evaluated,” the study states.

Investigators say that vaccination is necessary to have high titers during flu seasons.

“Our results suggest that annual vaccination is necessary in order to re-establish antibody responses to both influenza A and B even if vaccine strain does not change as overall antibody titiers will have dropped,” the investigators state.

The five flu seasons in which data were collected were: 2005-2006, 2006-2007, 2007-2008, 2008-2009, and 2009-2010.

As in every study, there were some limitations. For instance, there was no unvaccinated cohort involved. “While HI antibody titer is a meaningful parameter that adequately describes the immune response to vaccination, vaccine effectiveness is more clinically important,” the study states, adding that “our cohort included only those older than 61 years of age. Immune responses are different among different age groups; it is therefore not possible to generalize the results of the current study.”

Nevertheless, the findings offer support to those who argue that the flu vaccine should be gotten every year. “Taken together, our results suggest that annual vaccination is necessary to maintain humoral immunity for the elderly population,” the study states. “Furthermore, our findings revealed that annual seasonal vaccination was not associated with reduced vaccine effectiveness, and that the reformation of the vaccine resulted in amplified immune responses among those undergoing yearly vaccination in the elderly population.”