Flu viruses, however, evolve so fast that this "immune memory" provides only partial protection.

然而，流感病毒进化如此之快，以致于这种“免疫记忆”只能提供部分保护。

youdao

We can also say that once most people have been infected by swine flu and become immune, a variant that can beat that immunity will replace the current strain.

我们还可以确定的是，一旦大部分人群感染猪流感并获得免疫力，疾病的变种将会打破目前的免疫性并成为当下的压力。

youdao

People tend to mount the best immune response to the first kind of flu virus they experience.

人们往往对他们经历的第一种流感病毒有最好的免疫应答。

youdao

The end result is a virus with the exterior proteins of the H1N1 swine flu -- so the immune system recognizes it -- but with the inner mechanics -- the fast-growth ability -- of X-157.

CNN: Candidate virus for H1N1 vaccine arrives at CDC

Some doctors paint a scary worst-case scenario: A flu pandemic breaks out and beats down the immune systems of millions of people, and then staphylococci--the hospital strain and the nastier variant in the community--run wild on a killing spree.

FORBES: Germ Warfare

Finally, the flu shot does not always work -- particularly among the elderly and people with weak immune systems.

CNN: Flu shot myths addressed