How much protection does immunisation give?
Good personal protection for roughly 5 or more years with acellular vaccine (DTaP, TdaP).
Between 5 and 15 years of personal protection with whole cell vaccine or natural infection.
But these numbers vary enormously from person to person because we do not understand all the factors that cause protection.
More important than personal protection is herd protection. Herd protection (herd immunity) exists when so many individuals are immunized that an infected person is unlikely to pass it on.
Immunization gives a certain amount of protection to an individual but a great deal more to the population as a whole. So the more people that are immunized the better the protection for the individual is. It is a bit like paying taxes. If lots of people don’t pay their taxes, everyone loses out.
The minimal expected individual protection for any vaccine is 80%. A vaccine would never get on the market without this level at least. Although calculations show that individual protection can wear off quite quickly, especially after acellular vaccine, this is not the way to judge whether it is worthwhile, because immunity is boosted frequently by coming into contact with whooping cough bacteria although we are not usually aware of it. This keeps immunity high in the whole population and is the reason very few people get whooping cough without getting boosters. The immunization is vital to protect children. After childhood, natural boosting keeps herd immunity high.
Less severe in the immunized
Whether somebody who has been immunized gets whooping cough or not depends on a great many other factors as well. Pertussis vaccine manufacturers tend to quote protection levels of about 80%, but this is an average and it falls as time passes. But if immunization fails to protect an individual the severity is always less than if unimmunized.
Immunized people often seem to get it.
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.
Whether you get it or not depends primarily on whether you come into contact with it. If everybody 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 everybody is immunized and the vaccine is not perfect, all the cases will be in immunised individuals.
For that reason you can never say a vaccine is ineffective because an immunised person gets it. As long as a smaller proportion of immunised than unimmunized people get it, then it is effective
It is all too complex to measure or know an individual’s risk.
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 possibly not so good.
The number of people with natural immunity is probably getting less as the pre-immunization generation (born before 1958) gets older, but many of the immunized ones will perhaps get an unnoticed boost from natural infection if it comes back. So it is all complex, and there is no good way of measuring susceptibility. We do not even know what antibody levels are protective, even though we can measure some of them.
The more people are immunized the less there is.
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 individual protection may have fallen to quite a low level.
Acellular vaccine not as good.
Research suggests that acellular pertussis vaccines do not give as good protection as the older whole cell vaccines. As a very rough rule of thumb, you could say the old vaccine is effective for up to 10 to 15 years and the newer ones up to 5 or more years. But this is a great simplification of a complex issue. It is also likely that the newer vaccines are not so good at preventing colonisation of the respiratory tract by pertussis and this may create greater risk of transmission.
Pertussis vaccine can prevent the disease but still allow some infection.
It seems that to a significant extent it we can say that immunization can prevent the disease but not necessarily the infection. This area is being extensively researched.
The main purpose of immunization is to stop young babies getting it because they can die.
So as long as their mother and older brothers and sisters are protected by immunization they are relatively safe.
Most immunization programs now have 3 shots in infancy and another at about 5 years of age. Some have a booster in early teens too, then every 10 years. It varies from country to country.
Unfortunately there is no vaccine against whooping cough alone.
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 immunize 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 components.
A vaccine to pertussis alone would help to fill the gap, but so far there is no such vaccine available.
There is also considerable doubt about whether repeated boosters will prevent spread, given that natural reinfection and probable boosting is quite common. Much research is ongoing in this area.