_____Because the oral polio vaccine contains live attenuated viruses, mutations of these viruses can cause polio.
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Understand the nature of the oral polio vaccine (OPV): it contains live attenuated (weakened) poliovirus strains that can replicate in the intestine without causing disease in most cases.
Recognize that because these viruses are live and replicate, they can undergo genetic changes or mutations during replication.
Know that some mutations may revert the attenuated virus back to a more virulent form capable of causing polio, which is why vaccine-derived poliovirus can sometimes cause outbreaks.
Identify the term that fits the blank as 'Because the oral polio vaccine contains live attenuated viruses, mutations of these viruses can cause polio.' The missing word is likely 'Reversion' or 'Reversion to virulence'.
Summarize that the key concept is the risk of 'reversion' of the attenuated virus to a pathogenic form due to mutations during replication in the vaccinated individual.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Live Attenuated Vaccines
Live attenuated vaccines use weakened forms of a virus that can still replicate but do not cause disease in healthy individuals. These vaccines stimulate a strong immune response by mimicking natural infection, but carry a small risk of reverting to a virulent form.
Viruses, especially RNA viruses like poliovirus, can mutate during replication. In live attenuated vaccines, mutations may cause the weakened virus to revert to a more virulent form, potentially leading to vaccine-derived infections.
Oral Polio Vaccine (OPV) and Vaccine-Derived Poliovirus
The oral polio vaccine contains live attenuated poliovirus strains that can rarely mutate and regain neurovirulence, causing vaccine-derived poliovirus outbreaks. This risk is why some regions use inactivated polio vaccines instead.