That is precisely the question, and I confess that I don’t know the answer for certain. I think, though, that both factors are important.
The issue you’re talking about is definitely a thing: influenza evolves rapidly enough that any given vaccine will become less effective over time simply because the dominant strain of the virus has drifted.
However, I believe it is also the case that the immune response drops off fairly quickly. I haven’t found a definitive source (I confess that I didn’t look hard), but the closest I came is this article, with this quote:
“My informal sense of the literature [is] that the suggestion is strong enough that if people could reliably get vaccinated the week or two before the flu season starts, they’d be better protected,” Marc Lipsitch, PhD, a professor of epidemiology at Harvard University, told CIDRAP News. Lipsitch also penned a commentary on this study. “The more complicated thing is the trade-off between putting it off and not doing it at all,” he said.
My interpretation of that is that he’s talking about a benefit from getting the identical vaccine closer to the start of flu season, so that flu season hits while the immune system is at maximum activation.
“Can vaccinating someone twice provide added immunity?
In adults, studies have not shown a benefit from getting more than one dose of vaccine during the same influenza season, even among elderly persons with weakened immune systems. Except for children getting vaccinated for the first time, only one dose of flu vaccine is recommended each season.”
Since they say that “studies have not shown” rather than “we don’t have studies that show,” I’m more inclined to believe them.
That is precisely the question, and I confess that I don’t know the answer for certain. I think, though, that both factors are important.
The issue you’re talking about is definitely a thing: influenza evolves rapidly enough that any given vaccine will become less effective over time simply because the dominant strain of the virus has drifted.
However, I believe it is also the case that the immune response drops off fairly quickly. I haven’t found a definitive source (I confess that I didn’t look hard), but the closest I came is this article, with this quote:
“My informal sense of the literature [is] that the suggestion is strong enough that if people could reliably get vaccinated the week or two before the flu season starts, they’d be better protected,” Marc Lipsitch, PhD, a professor of epidemiology at Harvard University, told CIDRAP News. Lipsitch also penned a commentary on this study. “The more complicated thing is the trade-off between putting it off and not doing it at all,” he said.
My interpretation of that is that he’s talking about a benefit from getting the identical vaccine closer to the start of flu season, so that flu season hits while the immune system is at maximum activation.
I may have found the answer to my lazy question on the CDC website:
https://www.cdc.gov/flu/prevent/misconceptions.htm
“Can vaccinating someone twice provide added immunity?
In adults, studies have not shown a benefit from getting more than one dose of vaccine during the same influenza season, even among elderly persons with weakened immune systems. Except for children getting vaccinated for the first time, only one dose of flu vaccine is recommended each season.”
Since they say that “studies have not shown” rather than “we don’t have studies that show,” I’m more inclined to believe them.