Flu vaccines work by introducing a killed version of circulating virus strains, which trains the body's immune system to recognize and attack similar invaders in the future.

流感疫苗通过导入一种杀死的流感病毒变种起作用，来训练身体的免疫系统识别这些变种并在将来遇到类似的入侵者时作出反击。

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The vaccine proved safe and a close match with circulating virus.

VOA: standard.2010.01.18

Surprisingly, the child had no detectable circulating virus, no detectable anti-HIV antibodies, and was clinically healthy.

WHITEHOUSE: Prevention and Treatment of HIV Infection in Infants Born to Infected Mothers: Need for a Fresh Look | The White House

So far this year, however, there's little sign of seasonal flu in the U.S., where nearly all of the circulating flu virus is H1N1, or swine flu, Frieden said.

MSN: Seasonal flu vaccine could run short, CDC says

Their dalliances with younger women keep the virus circulating between generations.

ECONOMIST: Women and HIV