Who catches whooping cough?

Most cases (about 80%) in the western world are in teens and adults because children up to that age are protected by the immunizations they get at as infants which are boosted after about 3 years.

Explanation

It all depends on the environment you live in. Medically and economically developed, or otherwise.

Where there is no immunization against whooping cough in a population, most will have had the infection by the time they are five years old. Not all will have been ill with fully blown clinical whooping cough. Some will have had it mildly and got immune that way.

Immunity after natural infection is thought to last perhaps 10 to 15  years. Nobody is sure because immunity probably gets boosted by reinfection that may cause no symptoms at all.

In the developed world we now live in an environment where most children are immunized early in life against whooping cough. This gives them important protection against it at an age when it would otherwise be so easily spread by them to their newborn unimmunized siblings whom it could kill.

We must remember that immunization has drastically reduced the impact of whooping cough on our populations, and giving a booster in pregnancy can very effectively prevent babies dying of it.

Misunderstandings

Some people mistakenly claim that immunization gives poor protection and therefore is not worth bothering with. They do not appreciate that herd immunity drastically reduces its ability to spread and therefore reduces the number of cases.

Neither do they understand that immunization is much better at preventing severe cases than mild cases. Nobody sees the cases that are prevented so it is easy to get the wrong impression.

Which groups are susceptible?

So nowadays in developed communities there are three groups of people who are susceptible. 

1. Newborns until they have had their primary whooping cough shots (maybe under 4 months). It is very dangerous for this age group. One in a hundred die. 
2. Children who have not been immunized.
3. People whose last whooping cough immunization was more than a decade previously.

It used to be children under 5 who caught it before about 1950. At the present time in the UK, where the immunization rate in children is about 94% (2011). Official figures show that although most cases are in the over fifteens, ie adults, the first year of life is the year of age it is most common in. Maternal pregnancy boosters change this statistic very favourably.

The situation in Australia, New Zealand and North America is similar. Probably in many other countries too. 

It needs to be noted that as well as what we can recognise as whooping cough, pertussis bacteria can also cause a milder form of coughing illness that can be very similar to milder coughing illnesses such as caused by viruses. Modern antibody testing has suggested that in school and university age children perhaps 6% of coughs lasting between 2 and 8 weeks can be due to Bordetella pertussis, without it taking on the recognisable paroxysmal nature. Pertussis bacteria can also infect people with no, or minimal symptoms and they can get immunity from it. 

This field is being investigated thoroughly and may result in improved vaccine strategy in the future. One consequence is that less developed countries are being urged by the WHO to not change from whole cell vaccine to acellular vaccine as the latter is less effective.