The long term efficacy of flu vaccines are well documented, however I cannot seem to find a good source showing how long immunity lasts when contracting the disease.
I suspect it can't be much longer than vaccines since the virus mutates so quickly.
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$\begingroup$ I you feel I can improve my question, please let me know. I don't quite understand why the downvote. $\endgroup$– niobeApr 11, 2017 at 14:42
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The effect on adaptive immunity are often stronger when contracting the flu than for getting the flu vaccination. The backside of this is that the flu is a quite dangerous disease which could end fatally, so getting the vaccine is always preferrable.
Getting infected by a specific flu strain (or receive a vaccination against it) generates specific immunity against this strain. If it mutates to strong or you get infected with another strain, you will have no immunity (and will have to develop a new immune response.
It seems that immunity against the flu can be livelong (against this strain) after you went through an infection and survived it. There are two studies showing long term response against antigens from the 1918 flu pandemic. The first (summarized in reference 1, the original article is reference 2) shows specific antibodies against the reconstructed haemagglutinin protein (H1) from the 1918 virus strain.
Serum from people born 1915 or before showed specific activity against this virus antigen, even more than 90 years after the infection.
The second study analyzes cross-reactivity of antibodies against the 2009 A/H1N1 pandemic flu strain. Here the cross reactivity of people born before 1930 was the highest - the 1918 strain (with which these people most likely had contact) was also a H1N1 virus. Although these viruses mutated between 1918 und 2009, there is still enough similarity that they are recognized by H1N1 specific antibodies (see reference 2).
A study which compared antibodies responses of people who contracted the 2009 flu against people who where vaccinated against it, showed that the antibody response was more longlasting in the people who got the infection (see reference 3).
So I think it is safe to say that the immune response is stronger after the infection compared to the vaccination, but the vaccination is much safer.
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$\begingroup$ Thanks for the well thought out reply. I came across the exact same references you did in my preliminary searching. Just to be clear, immunity from influenza is less than a year due to how much it mutates between flu seasons? I know this is true for vaccines, so it stands to reason that it will be similar when contracting the disease. $\endgroup$– niobeApr 11, 2017 at 19:38
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$\begingroup$ @niobe Nope, immunity for a certain strain seems to last for a long time, possibly life long. But the strains change from season to season, thats why they need to be adapted. Surface antigens (haemagglutinin and neuraminidase) don't change too much between two seasons, but different strains might get into circulation. Over time they change, this is why we have different strains. $\endgroup$– Chris ♦Apr 11, 2017 at 20:25
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$\begingroup$ @niobe You may want to have a look at "The Pathology of Influenza Virus Infections". $\endgroup$– Chris ♦Apr 11, 2017 at 20:28
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$\begingroup$ Perhaps I misspoke, I am aware of antigen drift, which is why I was curious whether immunity to contracting the disease(taking antigen drift into account) is significantly different between the two groups of people as mentioned in the above post. Due to antigen drift, I suspect immunity is seasonal for both groups. $\endgroup$– niobeApr 12, 2017 at 9:34