100 days. That’s over 3 months. A relatively long time, but an especially long time to have a terrible cough. Whooping cough, also known as the 100 days cough, is caused by the bacteria Bortedella pertussis. It is highly infectious and, like measles, spread by coughing and sneezing. One of the major causes of childhood illness and death during the 20^th century, whooping cough is still causing outbreaks, distress and even death in Australia and elsewhere in the world.

First, let’s look at what happens during a pertussis infection. After the bacteria get into the airways, they start releasing toxins that cause irritation and inflammation, eventually leading to the characteristic cough. Unfortunately, for the first week or so, the symptoms of infection are just like a regular cold, making a whooping cough diagnosis unlikely at this stage. It is during this early phase of the infection that a person is most likely to pass on the infection to others.

During the second phase of infection, usually lasting 1-6 weeks, bursts of intense, rapid coughing begin as the body attempts to get rid of the mucous accumulating in the airways. The violent coughing attacks occur mostly at night, are exhausting and can even cause the person to vomit, turn blue or pass out. They are particularly distressing for babies and children, who account for the majority of complications and deaths from whooping cough. Up to half of all babies under 1 year of age who are infected with B. pertussis will need hospitalisation, and of those around 10% will develop pneumonia, some will have breathing difficulties, some will experience convulsions and about 1 in every 125 infants will die. The hospitalisation and complication rates for teens and adults are much lower, with less than 5% of those infected requiring hospitalisation. For older people, the most common complications are passing out, fractured ribs from coughing and pneumonia. The final, though often lingering, phase of infection is a gradual diminishing of the frequency and intensity of the coughing attacks which can take weeks.

There are a few characteristics of pertussis infection that make it challenging to control and especially to eliminate. While it is an infection that only occurs in humans (there is no animal species also passing it around), even natural infection doesn’t protect a person from catching pertussis again. It is estimated that natural infection with pertussis will provide protection from reinfection for 4-20 years. Vaccination against pertussis usually protects from infection for 4-12 years, although around 20% of people will not be protected from infection even when fully vaccinated. This is a critical point, because it means that even when very high levels of vaccination coverage are reached, there are individuals who are still susceptible. The symptoms of infection are generally milder in vaccinated people who do become infected, but of course these people can still transmit the infection to others. An extra challenge to protecting babies from infection is that mothers do not transfer immunity to their babies, even if they are immune themselves, and so until they can be vaccinated themselves, babies need to be protected by being surrounded by families and carers who have been vaccinated.

Now let’s have a closer look at the vaccines available. The first pertussis vaccine developed was a whole cell vaccine, made of killed whole bacteria. It made a huge impact once used on a population-wide scale from the 1950s onwards, reducing pertussis cases and deaths by over 90%. Newer vaccines, called acellular pertussis vaccines (containing only purified, inactive parts of the bacteria and not the whole cell), are marginally better tolerated, but the level of protection is similar to the whole cell vaccine (both prevent disease in about 80% of vaccinated people). In Australia, the acellular pertussis vaccine is used and is usually given to children in a formulation that also includes vaccines against 5 other diseases (Hepatitis B, diptheria, tetanus, haemophilus influenzae and polio), and to teens and adults in a formulation with diptheria and tetanus. Because the vaccine effect wanes after a few years, the vaccine dosing schedule recommends 3 doses in infancy (at 2, 4 and 6 months), followed by boosters at around 4 years and at 10-15 years. Adults are generally recommended to get booster doses every 10 years, but especially women in their third trimester of pregnancy, new parents and those coming into contact with young babies should make sure their vaccination is up to date.

As with all vaccines, the acellular pertussis vaccine can cause side-effects. Most are mild, such as fever or redness and tenderness at the injection site, though these are more likely after the 4^th or 5^th vaccine dose. Moderate side-effects include seizures and high fever, but only occur in a tiny percentage of vaccine recipients (around 0.007%). Aside from these side effects, the biggest drawbacks to pertussis vaccination are its low success rate and short-lived effect. Until a better one comes along, however, the current vaccine will have to do. The Bortedella pertussis bacteria is too prevalent to think about not vaccinating.

Check out the Council on Foreign Relations Outbreak map. http://www.cfr.org/interactives/GH_Vaccine_Map/#intro