Let’s be clear: immunisations matter. They matter a lot.
We all have a complex and ever-changing ecology of microorganisms and parasites inside our bodies and in our community.
A recent fatal case of diphtheria in Brisbane and an ongoing grumbling epidemic of pertussis (whooping cough) around Australia are reminders of our interdependence in relation to infectious diseases.
Community immunity, often referred to as herd immunity, is the protection individuals gain from others in their community being protected against an infectious disease.
This immunity can come after exposure to natural infection (“wild” organisms), from exposure to related organisms which provide some level of cross-protection, or from immunisation.
Vaccine protection
Vaccines protect against disease by presenting parts of “pathogenic organisms” or “subdued whole organisms” to our immune system. This allows our bodies to develop protective immunity against the disease, without paying the price of the disease the full-blown wild organism can cause.
Some vaccines provide stronger immunity than the wild virus – the human papilloma virus (HPV) vaccine given to girls to prevent cervical cancer, for instance.
Even if we’re susceptible to a disease, our chances of being exposed to a disease-causing pathogen are substantially reduced if the people we’re in close contact with have immunity.
Diphtheria
The role of vaccinations and community immunity differs from bug to bug.
For diphtheria, the organism causes severe and commonly fatal disease only when a particular toxin is present.
The diphtheria vaccine protects against the toxin, not infection with the organism per se, which can occur in the skin, throat or airways.
It is estimated that in a fully susceptible population, a person with diphtheria is likely to infect six or seven other people.
Based on how readily diphtheria spreads, around 85% of a population need to be immune in order to effectively control the disease.
Experience bears this out. In many countries, coverage rates for diphtheria immunisation of around 90% have reduced the number of cases by more than 99.99%.
But while toxin-producing diphtheria strains are around in some regions and can be imported, the disease can recur if high levels of immunisation aren’t maintained.
Following the break-up of the former Soviet Union, disruption to immunisation led to an epidemic of more than 150,000 cases and spread to other regions.
The reportedly unimmunised 22-year-old Brisbane woman who recently died of diphtheria is likely to have been infected by a friend who had recently returned from overseas.
Whooping cough
Pertussis, or whooping cough, typically occurs in cyclical epidemics every three to five years, when population immunity wanes enough for an outbreak to be sustained.
The current Australia-wide epidemic began in 2008 and has caused the deaths of four infants so far; it shows little sign of waning.
Pertussis is one of the most highly contagious diseases.
For a susceptible population, each infected person infects around 12 to 17 others. More than 92% of people need to be immune for community immunity to be strong enough to provide protection to the remainder of the population.
But pertussis vaccines are typically 80-90% effective in preventing disease. So pertussis can’t be eradicated through immunising children alone.
Nevertheless, if an immunised person is infected with pertussis, the cough and illness tend to be milder and shorter than in the unimmunised.
Unfortunately, immunity against pertussis (from both natural infection and vaccine) wanes after a decade or so. This is why pertussis occurs in people of all ages.
In adolescents and adults, it is one of the most common causes of cough illnesses lasting more than three weeks.
Infant pertussis immunisation (in a combination vaccine) is scheduled at two, four and six months of age, and it takes two doses before protection becomes substantial.
But most severe pertussis infections and almost all deaths occur in infants under three months, before they can be protected through immunisation.
Parents and other household contacts are the most frequent sources of pertussis infection for infants. That’s why governments recommend boosting parental immunisation: to “cocoon” young infants and provide protection against the disease.
Hib and pneumococcal bacteria
There are other diseases for which community immunity powerfully augments individual protection.
The Hib bacterium (Hemophilus influenzae type b) causes severe infections in young children, such as meningitis.
Following introduction of Hib vaccine in the Gambia in Africa, Hib disease almost disappeared despite only 60% of children receiving the three vaccine doses that provide full immunity.
Pneumococcal bacteria also causes meningitis and other serious infections. Worldwide, these bacteria are the most common cause of severe and fatal pneumonia at all ages.
Within a few years of the US introducing its infants pneumococcal conjugate vaccination program, the number of severe infections in young children plummeted, as hoped. But by reducing transmission of the bacteria across the community, more than twice as many infections were prevented in older people – who did not receive the vaccine – as in children who did.
Vaccines against Hib, pneumococci and meningococcal bacteria are so effective in building community immunity because they reduce carriage of the organisms in the nose and throat (where they normally live), thereby reducing transmission.
Hepatitis A
One of the most powerful examples of the benefits of herd immunity is the story of hepatitis A in Queensland.
The deaths of several indigenous children from hepatitis A in north Queensland prompted the introduction of hepatitis A immunisation for Indigenous toddlers in north Queensland in 1998.
Despite only 15% of children there being Indigenous, cases of hepatitis A rapidly plummeted, not only among the whole population of north Queensland (Indigenous and non-Indigenous) but across the state.
Personal responsibility
By contrast, one disease where community immunity plays no role is tetanus.
Tetanus spores are widespread in soil and the gut of animals, such as horses. Disease occurs when the causative bacterium grows in contaminated wounds and produces a potent toxin.
We can only be protected against tetanus if we are immunised ourselves. Even having tetanus disease doesn’t protect against subsequent attacks.
But for most diseases targeted by vaccines, immunisation substantially reduces not only our individual vulnerability but also our shared exposure.
Keeping our immunisations up to date, at home and when we travel, not only protects ourselves but helps protect our children, partners, friends, patients, work colleagues and wider community.