Live-attenuated vaccines

Live-attenuated vaccines are prepared from viruses or bacteria that are whole, active and able to cause infection, but they have been weakened in the laboratory. The term ‘attenuated’ (pronounced ‘at-ten-you-ay-ted’) means ‘made weak’, so the infectious agents in the vaccine should cause no disease at all.

Measles vaccine and oral polio vaccine (OPV) are live-attenuated antiviral vaccines. Bacillus of Calmette and Guerin (BCG) is a live attenuated antibacterial vaccine (named after its French inventors) that protects infants and young children against severe forms of tuberculosis (TB).

Live-attenuated vaccines generally activate the immune system very effectively, because they cause a similar reaction in the body as if to a natural infection. For example, a single dose of measles vaccine produces lifelong protection against measles because it is highly immunogenic, i.e. it has a very high ability to produce immunity.

If a mild fever and small rashes appear in a child you have vaccinated against measles, tell the mother not to worry. Reassure her that her child will be protected against the more serious measles disease.

However, live-attenuated vaccines can sometimes produce a weakened disease pattern in a small proportion of vaccinated children. For example, measles vaccines can induce fever and an occasional rash, but this is very unusual and is nothing to worry about. The live-attenuated oral polio vaccine (OPV) can very rarely cause a type of paralysis, but on average this happens in only one child in every 1–10 million vaccinated children.