As a family physician, I thought I could shed some light on this discussion.
I certainly recommend getting flu shots. These will prevent most forms of influenza and have NO physical side effects (except perhaps a sore arm).
The Lyme disease vaccine has not been available for people for more than 10 years. I practice in an area endemic for Lyme. I had my series and even got a booster when they were pulling the vaccine off the market. The vaccine is only currently recommended for dogs and horses. The company took the vaccine off the market because of financial reasons. The marketing for this was a bit excessive and sparked some lawsuits. It may very well become available as an orphan drug when the patent expires. It remains available for dogs and horses because it is reasonably effective (and because dogs and horses don’t sue vaccine manufacturers after some unusual or psychologically-based illness shows up). I would not give the versions for dogs and horses to humans. However, if it did become available for humans again, I would certainly get another booster myself.
There are two separate pneumonia vaccines. They both work against the same bacteria - streptococcus pneumoniae. Strep pneumonia is the most common form of bacterial pneumonia but only accounts for ~40% of pneumonias in non-smokers (and an even lower percentage in smokers). Most pneumonias will follow a viral illness so the best way to prevent pneumonia is keeping yourself healthy to be able to prevent complications from developing after you get the inevitable viral infection. The best things to do are to not smoke and to stay away from smokers (as there seems to be far too much smoking on the trails and even second or third hand (exposure to a smoker who smokes outdoors away from you) smoke exposure will cause disease). A good well-rounded diet is also helpful (as many hikers give up on veggies and fruit). BTW, exposure to wood smoke from campfires does not seem to carry the same risk for disease as tobacco (which is far more dangerous).
The older Pneumovax covers 23 different strains but doesn’t work as well as the newer vaccine. Boosters are recommended after 10 years (or at least once after until age 65). This is only recommended for senior citizens, smokers, or people with lung diseases or poor immune systems. There can be significant side effects if you get a booster too soon (I’ve seen several people hospitalized after getting a booster within a year with what was initially thought to be cellulitis (a skin infection).
The newer pneumonia vaccine, Prevnar only covers 13 strains of pneumonia but seems to have much better and longer immunity. This has been given to babies for ~17 years and has been highly effective. Within the last several years, it has been recommended for senior citizens and there are starting to be recommendations to give this to people with poor immune systems. The best sites don’t recommend this yet for smokers or people with lung disease yet but I would suspect that we may eventually give this to everyone (particularly if they come up with a vaccine which covers even more strains (as the original version only covered 7 strains).
The only other vaccine which I would make sure that every hiker gets is a tetanus shot.
To summarize, if you are otherwise healthy, making sure your tetanus vaccine is up to date (within 10 years) and getting a flu shot if you are hitting the trail before April are really the only vaccines necessary before hitting the trail.