Influenza Vaccine Effectiveness

Influenza can be pretty bad for the elderly, for most people, it is a pretty bad cold.  I have known for some time that the flu vaccines are not particularly effective, generally because you have to guess which of the many varieties of influenza virus will dominate for the flu “season”.  The CV-19 epidemic made me re-look at some of those effectiveness studies and a recent post by the authors of the Trust the Evidence blog brought the most recent Cochrane Review studies of flu vaccine effectiveness to my attention.  Cochrane used to be the gold standard, but it has lost some of its objectivity and tended to favor some agendas other than just what does the research say and what are the limitations of the research.

Now, I am not in any manner suggesting that people not get a flu vaccine, especially the elderly.  Whether you do or don’t get any particular health care is solely between you and your doctor.  But you should also have full and complete information about any treatment, including a flu vaccine.  I did not get a flu vaccine last year and may not this year, but it was partly because I wanted to see what would happen.  It was also due to the research suggesting that we don’t completely understand our immune system’s response to repeated proddings with the same or similar antigens.  That may actually impair the response to a particular strain of virus.  So skipping a flu vaccine every now and then may not be the worst thing.  We will see how sick I get.

The Cochrane reviews reflect the modest effectiveness of these vaccines.  And of course the effectiveness analysis is compromised, as it was with CV-19, by the fact that everyone has been “infected” with a flu virus multiple times over their lives and so have some natural immune response as well as that prompted by vaccines.  Now these reviews were done in 2018 and apparently have not been updated because Cochrane considers the research to be “stable”, and there aren’t a lot of new studies.  I would say it is time for some updated work, with more sophisticated approaches.

There are three reviews, one covering the research on healthy adults, one for healthy children and one for the elderly.  They all find what I would consider pretty modest effectiveness at best.  For healthy adults, 52 trials were covered.  The vaccines reduced the likelihood of getting flu from 2.3% to .9% and had some effect on flu-like illnesses.  Vaccines appeared unlikely to reduce flu hospitalizations, maybe because there are very few of those.  The evidence for effectiveness was considered of moderate strength.  There was small risk of minor fevers after vaccination.

For children there were 41 trials and the risk of flu was reduced from 18% to 4%.  There was a lot of variation in trial results, which raises questions.  Inactivated vaccines performed better than active ones.  There was no good data on hospitalizations but children are rarely hospitalized for flu.  The evidence here was also of moderate strength.  For the elderly, there were only 8 trials, which is a bit shocking.  Equally disturbing is that was basically no data for the effect of vaccination on hospitalizations or deaths.  The likelihood of getting flu was reduced from 6% to 2.4%, on low strength of evidence.  I am stunned by the lack of more research among the elderly.  (Cochrane Reviews)

We clearly do need more and better research on flu vaccines.  Among other things that might help guide efforts to make a more effective vaccine.  And none of these studies appear to give good guidance on how effectiveness might change over time.  Do we really need a new vaccination every year?