Why is FLU vaccine a crap shoot?

This year's Influenza epidemics major strain  is  H3 N2

mainly found in Pigs and humans  

1. How effective is the 2018 flu shot?

Early findings from Canada indicate a 17 percent effectiveness against the viral strain, known as H3N2, that’s been the main culprit of flu in the U.S. this winter.

2. Why are vaccines less protective against some strains of the flu?

One reason has to do with the most-common way flu shots are made. They typically contain killed (or “inactivated”) flu viruses that are grown in chicken eggs. Flu viruses that spread easily among humans tend not to grow as well in chicken eggs as bird flu strains do. This is especially the case for H3N2 viruses, which have been circulating continuously in humans since 1968 and are, therefore, very well adapted to humans as a host. When replicating inside eggs, H3N2 viruses are prone to undergoing adaptive changes that make them better-suited to the egg environment, but less likely to prompt the right response in humans. A mutation in the H3N2 strain meant most people receiving the egg-grown vaccine didn’t have immunity against H3N2 viruses that circulated last year, leaving the vaccine with only about 30 percent effectiveness in the U.S. According to preliminary results from Australia, the flu shot was only about 10 percent effective against H3N2 there during the last winter in the southern hemisphere.

3. How effective should the flu vaccine be in a typical year?

The U.S. Centers for Disease Control and Prevention, which conducts studies annually to gauge the vaccine’s protective ability, has found immunization reduces the risk of flu illness by 40 percent to 60 percent during seasons when most circulating flu viruses are well-matched to the flu vaccine. Older people with weaker immune systems often have a lower protective immune response after flu vaccination compared to younger, healthier people.