Protecting individuals and communities
Infectious diseases move through populations by infecting people who are not immune to the disease and then spreading onwards. When a high percentage of the population is vaccinated, it is difficult for infectious diseases to spread because there are not many people who can be infected.
Immunisation programmes aim to protect individuals against disease and also prevent the onward spread of disease within the population as a whole. This way of controlling disease is called herd immunity. However, herd immunity depends on high vaccination levels, and cannot protect every individual.
Protecting our most vulnerable
The aim of immunisation programmes is to protect people when they are most at risk from particular diseases.
- babies are particularly vulnerable to infectious diseases, so the childhood immunisation schedule begins at 6 weeks of age.
- people aged over 65 are at greater risk from shingles and its complications and are therefore offered the shingles vaccine as part of the immunisation schedule.
Diseases change and evolve
For vaccines to work, the strain of bacteria or virus in the vaccine needs to be the same as, or very similar to, the strain that causes disease in the population. Some viruses and bacteria change over time, and this can have an impact on how effective vaccines are.
For example, the flu virus can change very rapidly, meaning that last year’s flu vaccine is unlikely to protect you from the virus strains that are circulating this year. That is why it is important to get the flu vaccine every year.
Some bacteria that cause disease come in many different types (such as pneumococcal bacteria). Vaccines are developed to protect against the main types that cause disease. However, sometimes new strains appear, or less common strains start to cause more disease. This can also have an impact on vaccine effectiveness.
Protection from some vaccines wanes over time
Most vaccines offer good protection for many years, however, levels of protection may naturally decrease over time, or may be reduced when the immune system is working less well because of medical conditions, medications, or ageing.
For some vaccines it is therefore necessary to give additional doses to provide continued protection.
Waning protection of the whooping cough (pertussis) vaccine
The immune response to pertussis vaccines is known to decrease over time and in New Zealand, most people aged 16 or over have not been vaccinated against whooping cough in the last 5 years. The same is true if you catch whooping cough – you do not become immune for life. Therefore, you can catch pertussis even if you have had it before, or if you have been vaccinated.
In immunised children who develop symptomatic infection, the disease course will tend to be less severe than in those who have not been vaccinated.
Effectiveness of vaccines
Some vaccines on the immunisation schedule are almost 100% effective. For example, 2 doses of the measles mumps and rubella vaccine (MMR) will protect 99% (99 out of 100) of people from measles and will protect about 88% (88 out of 100) of people from mumps.
However, there will always be a very small number of people who will not make an immune response to a particular vaccine and will therefore remain unprotected from that disease.
The effectiveness of some vaccines also wanes over time – such as whooping cough. This is why booster doses may be recommended for some people.