Why Seasonal Flu Vaccines Don’t Stay Long Term

For immune cells bone marrow is the home base that produces antibodies. While the seasonal flu vaccine may increase the number of antibody-producing cells for the strain being inoculated for in the bone marrow, the majority of these cells will be lost within one year, according to the researchers. 

Vaccine studies require samples of participant’s blood where these antibody-producing cells can be found for a few weeks after vaccination, this study took an extra step of obtaining bone marrow samples which is a more invasive procedure. 

Most people will have some flu specific plasma cells, this study needed to distinguish between the antibodies made by pre-existing cells and the antibodies that were created by the strains present in the seasonal flu vaccine:

"We were able to follow the specific cells produced by the vaccine because they produced unique antibodies that can be identified using sequencing techniques," says Carl Davis, PhD, first author of the paper and a postdoctoral fellow in Ahmed's laboratory.

"We could see that these new antibodies expanded in the bone marrow one month after vaccination and then contracted after one year. On the other hand, antibodies against influenza that were in the bone marrow before the vaccine was given stayed at a constant level over one year."

"What this shows is that just getting to the bone marrow is not enough," Ahmed says. "A plasma cell has to find a niche within the bone marrow and establish itself there, and undergo gene expression and metabolism changes that promote longevity."

For this study, 53 healthy volunteers provided bone marrow samples one month before the vaccine and again one month after, with another follow up about a year later with some of the participants. According to the researchers, vaccination increased the proportion of flu specific cells after one month, and the follow up visits months later revealed those numbers had declined back to baseline; up to 99% of the newly generated plasma cell lineages were lost after one year. 

The levels of antibody-secreting cells in the blood were found to correlate with long term responses in the bone marrow. According to the researchers vaccine additives may be able to increase the long term bone marrow homing for antibody-secreting cells, which may promote the formation of germinal centers in the lymph nodes where plasma cells producing high-affinity antibodies are generated.