Bordetella pertussis bacteria | Recovery | Immunity | Vaccine | Boosters | Vaccine effectiveness | Vaccine safety | Herd immunity | Take home message
Whooping cough activity in NSW
Whooping cough (or pertussis) is a notifiable disease in all Australian States and Territories. This means that doctors, hospitals and laboratories are required by law to let the local public health unit know about each new case that is diagnosed. Information is collected about each case and this is entered onto a confidential database to enable health departments to track how whooping cough is moving through the community.
As many cases never see a doctor or are never tested for whooping cough, health departments only see the tip of the iceberg! So there are many more cases in the community than are officially captured in the cases reported.
What we do know is that whooping cough is always around in the community and we know that it's often under-recognised in adults and in older children.
Highly-skilled public health workers track how whooping cough infection is spreading through the population and can pick out significant changes, for example if there is a large increase in numbers of cases, or if new age groups are being infected more frequently or if the infection is taking hold somewhere in the State.
The last big outbreak in NSW was in 2010/11 and saw over 22,000 cases reported to NSW public health units. For recent case numbers see the Pertussis notifications data page.
Bordetella pertussis bacteria
Whooping cough is caused by bacteria called Bordetella pertussis. The bacteria are spread from one person to another when someone with the infection coughs and fine droplets that contain the bacteria spread into the surrounding air. The bacteria are breathed in by others who are nearby. If that person's immune system doesn't recognise the whooping cough bacteria, it can't start to fight the infection.
Once the whooping cough bacteria are breathed in, they stick to cells in the nose and throat and the bacteria start to reproduce rapidly. As the bacteria reproduce, they produce toxins that paralyse cells in the windpipe and lungs that clear the airways of mucus and debris. Another bacterial toxin paralyses some of the body's immune cells and prevents them from attacking the bacteria.
Whooping cough bacteria can also multiply inside the lung tissue and cause pneumonia it can also lead to other pneumonias with different bacteria and viruses. This also stops oxygen in the air from getting into the blood.
With time, the body's immune system develops antibodies which are specialised immune proteins that recognise that whooping cough bacteria are foreign. The antibodies stick to the bacteria and help the body's immune cells to respond to fight the infection.
Initially, there are so many live bacteria in the throat that each cough can spread bacteria to others. With time, the numbers of bacteria decrease and by 3 weeks, the numbers of live bacteria have normally been reduced so that the person is no longer able to easily pass the infection on to others, although the cough can linger for a number of weeks, until the airways recover.
Specific antibiotic drugs are sometimes prescribed to help kill off the whooping cough bacteria that are reproducing in the respiratory tract early in the infection. The result is that the numbers of bacteria fall more quickly. After 5 days of therapy, the numbers of bacteria are low and the person can no longer easily spread the infection to others. Unfortunately, antibiotics often don't stop the cough.
During infection, the immune system learns to recognise the whooping cough bacteria as abnormal and this allows it to respond immediately if the person is exposed to more whooping cough bacteria next time. A more rapid immune response next time the body sees whooping cough bacteria means that the infection is less likely to take hold and cause disease.
Whooping cough vaccination is the single best thing to prevent whooping cough infection. Modern "acellular" whooping cough vaccines work by priming the body's immune system by using tiny amounts of proteins that have been purified from the whooping cough bacterium. The immune system sees this as if it's an infection and begins to produce antibodies in readiness to fight whooping cough bacteria. It is important that pregnant woman get vaccinated in the third trimester of each pregnancy to provide protection against pertussis in the six weeks of the infant’s life, prior to the infant receiving the their own vaccine. The "primary course" of whooping cough immunisation involves four doses of whooping cough vaccine given at 6 weeks, 4 months, 6 months and 18 months. This vaccine also contains proteins that cause the immune system to recognise tetanus and diphtheria. Some infant vaccines also contain proteins against polio, hepatitis B and Hib infections too.
Because a baby's immune system needs 4 spaced-out doses of vaccine to recognise whooping cough, it's very important for parents to get their baby immunised on time. This gives the baby the best opportunity to strengthen his or her defences against the infection at the earliest opportunity, when they are most at risk of severe disease.
A slightly different whooping cough vaccine is used for the booster, given to 4 year olds, to teenagers at high school and to some adults. Booster vaccines also contain whooping cough, tetanus and diphtheria proteins but smaller amounts of purified proteins are needed to remind the immune system about whooping cough.
Whooping cough vaccination with modern acellular whooping cough vaccine is effective in preventing typical whooping cough around 85% of the time and prevents mild whooping cough around 75% of the time.
Unfortunately the human immune system doesn't remember the abnormal bacteria forever, either after an episode of whooping cough infection or after vaccination or booster vaccination. This means you can still get whooping cough again but the infection will often be milder.
Modern whooping cough vaccines (sometimes called "acellular" vaccines) are made using purified proteins rather than killed whooping cough cells that were used in old vaccines (sometimes called "whole-cell" vaccines). Modern whooping cough vaccines cause fewer side effects than the older vaccines and are far safer. About one in five people will experience fever and about one in 10 experience mild local reactions following vaccination with modern whooping cough vaccines.
People who have a serious allergic reaction (anaphylaxis) to whooping cough (pertussis) vaccine or anaphylaxis to any component of the vaccine previously should not be vaccinated with an acellular whooping cough vaccine.
Some diseases spread through populations more effectively if there are lots of people around who are vulnerable to infection and who get infected and pass the infection on to other vulnerable people. Herd immunity is when most of the community are immune to an infection (either through immunisation or from immunity from a past infection). This situation provides some protection to any people who are not immune as there is less opportunity for the infection to spread.
Because immunity to whooping cough fades with time and the effectiveness is not 100%, we need a much higher proportion of people to be immunised to get the protective effect of herd immunity compared to many other vaccine preventable diseases.
The take home message...
Vaccination is still the single best way to prevent whooping cough although not everyone will get an immune response and immunity wanes with time. This means that people can still get whooping cough if they have been vaccinated or if they have had previous infection.