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Immunization gives a certain amount of protection to an individual but a great deal more to the population as a whole. And whether somebody who has been immunized gets whooping cough or not depends on a great many other factors as well. Vaccine manufacturers tend to quote protection levels of about 80%.
Most people are surprised when an immunized individual gets it. But it should not cause surprise. It is a complex organism which needs to be attacked in several different ways at once to stop it infecting. The natural infection provides these ways for the future, but vaccine is a poor substitute for the real thing and it is only able to do a poor imitation of natural infection. Whether you get it or not depends primarily on whether you come into contact with it. If everybody else has been immunized then the bug never gets much of a chance to spread itself around, so you may never come into contact with it. If nobody is immunized then it will always be around and you will come into contact with it so often that even if you have been immunized you will get it sooner or later when your resistance is lowered by having a cold or something similar.
Nobody has been able to measure the effectiveness of the vaccine precisely because it depends on the ability of the bug to spread itself around. This will depend to an extent on how many people have natural immunity and how many have vaccine immunity which is not so good. The number of people with natural immunity is probably getting less as the pre-immunization generation gets older, but some of the immunized ones will perhaps get an unnoticed boost from natural infection if it comes back a bit. So it is all complex, and there is no good way of measuring susceptibility by measuring antibodies as it is also so complex.
What we do know is that when a population of children gets immunized the number of cases falls dramatically, and it is enough to ask of a vaccine that it should do that. It is also generally agreed that individual protection falls quite rapidly after the last shot, so that 5 years later the amount of protection may have fallen to quite a low level.
The main purpose of immunization is to stop young babies getting it because they can die. So as long as their older brothers and sisters are protected by immunization they are relatively safe. But because adults are not getting their faded immunity boosted by natural infection, some of them are getting it and passing it to their children! Most immunization programs now have 3 shots in infancy and another at about 5 years of age. There are now pertussis vaccines available for use in adults (Repevax). Some countries are recommending a booster shot at 10 year intervals throughout life. There is a problem however. The vaccine is against pertussis, diphtheria, tetanus and polio. It can be fine to give this once every 10 years, but it cannot be used to immunise people who have never has pertussis immunization because 3 shots are required and that would run the risk of a reaction to one or more of the other componants. A vaccine to pertussis alone would help to fill the gap, but so far there is no such vaccine available.
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