Who catches whooping cough?
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 illness by the time they are five years old and most will remain immune for the rest of their lives because the illness builds up good antibody levels, and because these antibodies are being boosted by frequent contact with the organism.
But in the developed world we now live in an environment where most children are immunized early in life against whooping cough, giving 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.
So nowadays in developed communities there are three groups of people who are susceptible.
It used to be children under 5 who caught it before about 1950. At the present time (2016) 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 age it is most common in.
It is very likely the situation in Australia, New Zealand and North America is similar.
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 testing has suggested that in school age children perhaps 30% of coughs lasting between 2 and 8 weeks can be due to pertussis. It is also probable that pertussis bacteria can infect people with no, or minimal symptoms. The exact significance of this phenomenon is not known, but it is thought more likely to occur in people immunized with acellular pertussis vaccine.
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 to not change from whole cell vaccine to acellular types.