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(c) The need for immunisation
(i) Isn’t natural immunity better than immunity from vaccination?
While vaccine-induced immunity may diminish with time without boosters (vaccine or contact with wild-type infection), ‘natural’ immunity, acquired by catching the disease, is usually life-long, with the exception of pertussis. The problem is that the wild or ‘natural’ disease has a higher risk of serious illness and occasionally death. Children or adults can be revaccinated (with some but not all vaccines) if their immunity from the vaccines falls to a low level or if previous research has shown that a booster vaccination is required for long-term protection. It is important to remember that vaccines are many times safer than the diseases they prevent.
(ii) Diseases like measles, polio, whooping cough and diphtheria have already disappeared from most parts of Australia. Why do we need to keep vaccinating children against these diseases?
Although these diseases are much less common now, they still exist. The potential problem of disease escalation is kept in check by routine vaccination programs. In countries where vaccination rates have declined, vaccine-preventable diseases have sometimes reappeared. For example, Holland has one of the highest rates of fully vaccinated people in the world. However, in the early 1990s, there was a large outbreak of polio among a group of Dutch people who belonged to a religious group that objected to vaccination. While many of these people suffered severe complications like paralysis, polio did not spread into the rest of the Dutch community. This was due to the high rate of vaccination against polio, which protected the rest of the Dutch community.
There have been recent outbreaks of whooping cough, measles and rubella in Australia, and a number of children have died. Cases of tetanus and diphtheria, although rare, still occur. Thus, even though these diseases are much less common now than in the past, it is necessary to continue to protect Australian children, so that the diseases cannot re-emerge to cause large epidemics and deaths.
Also, many of the diseases which we vaccinate our children against are still common in other areas of the world. For example, measles still occurs in many Asian countries and many people take holidays or travel for business to these areas. Therefore, it is possible for non-immune individuals to acquire measles overseas, and with the speed of air travel, arrive home and be able to pass measles onto those around them if they are unprotected. Measles is highly infectious and can infect others for several hours after an infected person has left a room. Vaccination, while not 100% effective, can minimise a person’s chance of catching a disease. The more people who are vaccinated the less chance of a disease, such as measles, spreading widely in the community. This is referred to as herd immunity.
(iii) Why do some children get the disease despite being vaccinated?
This is possible, since no vaccine is 100% effective. A small proportion of those who are vaccinated will remain susceptible to the disease. However, in the cases in which illness does occur in vaccinated individuals, the illness is usually much less severe than in those who were not vaccinated. The protection provided by the same vaccine to different individuals can differ. For example, if 100 children are vaccinated with MMR, 5 to 10 of the fully vaccinated children might still catch measles, mumps or rubella (although the disease will often be less severe in vaccinated children). If 100 children are vaccinated with a full schedule of pertussis-containing vaccines, 20 of the children might still get whooping cough but, once again, the disease is often less severe in these vaccinated children. To put it another way, if you do not vaccinate 100 children with MMR vaccine, and the children are exposed to measles, all of them will catch the disease with a risk of high rates of complications like pneumonia or encephalitis. The reason why fewer children become infected than these figures suggest is due to the high vaccine coverage rates in the community. If there are high coverage rates, there is less chance of contact with the infection and, although children may be susceptible, they have a low chance of contact with the infection.
This should answer your question. Most people who are vaccinated develop immunity which means that if they are exposed to the disease their own immune system is able to stamp it out, the pathogen can not therefore multiply and spread to others. This is the reason that they have been able to stop vaccinating us all for smallpox, it has effectively been stamped right out of common existence by mass immunisation programs in the past.