Flu vaccine is quite unlike that of many childhood vaccines. While majority of childhood vaccines last for years, vaccine for the influenza virus (flu) gives us protection for a limited period. At the beginning of each flu season, we need a new influenza virus shot.
Why do we need the seasonal flu shot?
There are multiple reasons which compel us to go for seasonal flu shot. One lies in the structure of the virus’s genome of eight RNA segments. The RNA pieces are prone to exchanging when more than one virus enters a cell. In doing so, antigenic shifts create seemingly ever-changing strains that require new vaccines adapted to fight against them.
However, the vaccine gives only short-lived protection. Researchers from Emory Vaccine Center has published a study in Science, which gives fresh insights into the flu vaccine’s effectiveness.
The study, titled, “Influenza vaccine–induced human bone marrow plasma cells decline within a year after vaccination”, focuses its attention on bone marrow. According to the study, home base for immune cells that produce antibodies is the bone marrow.
Long-term serum antibody levels are maintained by bone marrow plasma cells (BMPC), says the study.
How do these cells behave after influenza vaccination?
Number of anti-body-producing cells multiply after the flu vaccination. These cells are specific for flu in the bone marrow.
However, these cells remain in the bone marrow for a very period. They disappear within one year, the researchers found.
Most vaccine studies acquire samples of participants’ blood, which is where antibody-producing cells can be found for a few weeks after vaccination.
What differentiates the latest study from previous one is the fact in the current study researchers went beyond acquiring participant’s blood. They collected marrow samples (conducted from 2009–2018) from the participants, which is a more invasive procedure.
In this study, 53 healthy volunteers agreed to provide bone marrow before seasonal flu vaccination and then one month after, with follow-ups for some about a year later. Vaccination increased the proportion of flu-specific cells (from an average of 0.8% to 1.9%) after a month.
Yet the follow-up visit months later revealed that number had declined to baseline. This decline, the authors wrote, “was driven by the loss of BMPC induced by the vaccine, while pre-existing BMPC were maintained.”
The researchers found that most people already have some flu-specific plasma cells: the type of immune cells that secrete large amounts of antibodies. So the Emory researchers needed to distinguish between antibodies made by pre-existing cells and antibodies whose production was spurred by the strains present in the seasonal vaccine.
For most of the newly-generated plasma cell lineages, between 70–99% of the cells were lost after one year.
“We were able to follow the specific cells produced by the vaccine because they produced unique antibodies that can be identified using sequencing techniques,” said 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.”
The researchers found that the levels of antibody-secreting cells in blood in the participants correlate with long-term response in the bone marrow. So vaccine researchers can continue to monitor immune responses by looking for antibody-secreting cells in blood.
The findings are expected to inform the design of proposed longer-lasting “universal” flu vaccines.
