attenuated
Meaning of attenuated: Weakened deliberately: an organism bred until it can no longer cause illness but still looks threatening enough to train the body's defenses.
Definition of attenuated
Attenuated means weakened. In vaccines it describes a living virus or bacterium deliberately altered so that it multiplies poorly in people and cannot produce the illness it came from, while still carrying the surface features the body learns to recognize. The word is also used more loosely for any weakened beam, signal or effect, and for milder forms of inherited conditions.
How is a strain weakened?
By making it unfit for its usual host. Growing a virus over and over in animal cells, or in eggs, or at a temperature lower than the human body, allows the variants that thrive in those odd conditions to take over the population. Those same variants grow badly in human tissue. Some vaccine strains are cold-adapted, multiplying in the cool nose but not in the warmer lung. Newer strains are attenuated by deleting a gene the organism needs to cause damage, which is more precise than repeated passage. Measles, mumps, rubella, chickenpox, yellow fever, rotavirus, oral polio and the tuberculosis vaccine all use living weakened organisms, whether viruses or bacteria.
How do live and killed vaccines compare?
| Feature | Live attenuated | Inactivated or subunit |
|---|---|---|
| What is given | A whole organism, still alive | Killed organism or a piece of one |
| Response produced | Broad and long-lasting | Narrower, often needs boosting |
| Doses usually needed | One or two | Several |
| Severe immune deficiency | Can cause disease | Cannot cause the disease |
What are the limits?
A living organism can change again. Where population coverage is low, the weakened polio strain given by mouth has occasionally regained the ability to spread and cause paralysis, which is why programs move to the injected killed form as eradication approaches. Living vaccines also need careful cold storage, and they can cause genuine infection in someone whose immune system is severely suppressed. None of that applies to vaccines built from a fragment: the human papillomavirus vaccine, for example, contains empty shells assembled from a single viral protein, with no genetic material inside and nothing capable of multiplying.
Also known as
Used in a sentence
The vaccine contains a live attenuated strain of the virus.
Good to know
A live attenuated organism remains capable of multiplying, so it can cause disease in someone with severe immune suppression and can, rarely, regain virulence, neither of which is possible with an inactivated or subunit preparation.
Word origin
Where “attenuated” comes from
From the Latin attenuare, meaning to make thin or to weaken, built from ad, toward, and tenuis, thin.