How do they predict what viral strain(s) to make a flu shot for, and why was this year’s flu shot only 40% effective?
They just take their best guess as to the strains that are predominate at the time. They failed badly this year. I had two impatient stays on my own and my wife and daughters had a combined four outpatient emergency room visits. My small town doctor who usually has same day emergency visits had to shut down his practice for anything but the most serious symptoms lasting more than 3 days. The same was true for many coworkers. This is for the Boston area.
I read that the day after i spent 20 bucks on flu shots.
I found out it wasn’t 100% effective this year when I got the flu. :mad:
I have gotten a flu shot at my job for the last seven years and never gotten the flu, but this year I got the shot in October and got the flu for two weeks in November and then again in December. 
That’s unusual. Did the doctor who diagnosed flu in Nov and Dec tell you what kind you had (e.g., A(H1N1), A(H3N2), or B)?
Actually I just assumed it was the flu due to the duration and the symptoms and the fact that no medication helped, but I guess it could have been something else one or both times- not sure what else it could have been though. I didn’t go to a doctor either time.
From this link:
"As the U.S. flu season soars, the CDC says this year’s flu vaccine doesn’t match two of the three main types of flu bugs now in circulation.
That doesn’t mean the vaccine won’t work. Vaccinated people who catch the flu get a much milder disease. This can make a life-or-death difference to people who are at high risk of flu complications, such as the elderly."
I haven’t been able to find a source for this online, but my understanding is that infectious disease specialists have had a very good track record in recent years for predicting which flu strains will be the most prevalent.
Influenza is a very tricky bug, what with the annual genetic drifts that make it hard to protect against (plus the infrequent more dramatic shifts in the genome that can result in pandemics).