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Birth of a vaccine （疫苗誕生記） How to make a vaccine？
   1-Isolating the virus strain （分類病毒株） .
   First，scientists need to take some virus strains out of patients' bodies.
   2-Making a vaccine.Scientists use a few special ways to weaken （減弱） the virus strain.
   3-Animal test （實(shí)驗(yàn)） .To make sure the vaccine is safe （安全） for people，scientists first give it to animals.If the animals are well，scientists will go on to the next step （下一步） .
   4-Clinical test（臨床實(shí)驗(yàn)）.Scientists give the vaccine to about 20 patients，then to even more patients if they feel better.If they all feel better，the vaccine will be safe for most people to take.
   How long？From making the vaccine in a lab to giving most people the vaccine，this process usually takes a few years.
   How do vaccines protect people？
   When a vaccine goes into our body，our immune system（免疫系統(tǒng)）thinks it is an enemy（敵人） .The immune system practices fighting this enemy.It becomes stronger and stronger.When the real enemy comes，the immune system can beat it and protect us.
（1）Making a vaccine needs four steps．

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（2）In clinical test，scientists give the vaccine to about 20 animals．

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（3）It usually takes a few months to give most people the vaccine．

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（4）When a vaccine goes into our body，our immune system becomes weaker and weaker．

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（5）Vaccines can protect people without the immune system．

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