Vaccine effectiveness

Why do some people who have been vaccinated still catch a disease?

The majority of people in New Zealand are fully immunised, with very high rates among children born since 2005. When there is a disease outbreak, in a highly immunised population, there might appear to be significant numbers of vaccinated people that still get the disease, but that is simply because vaccinated people make up a larger proportion of the population. For example, in a population of 1000 with a 90% vaccination rate, if there are 50 mumps cases and half the cases were vaccinated, that's still only 2.7% of the vaccinated population (25/900) compared with 25% of the unvaccinated population (25/100), meaning the vaccine is highly effective.

Vaccines are highly effective, however, as they are not 100% effective some vaccinated people will still catch the disease they are vaccinated against and there may also be outbreaks of infectious diseases in populations where most people receive their scheduled vaccinations. The reasons for this are described below.

1. The effectiveness of vaccines

Some vaccines on the immunisation schedule are almost 100% effective. For example, two 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. For measles, with two doses of the MMR vaccine this is about 1 out of every 100 people vaccinated, for mumps this is about 12 out of every 100 people vaccinated.

2. Long-term protection of vaccines

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 of vaccines to provide continued protection.

Waning protection over time: Whooping cough (pertussis) vaccines

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 whopping cough in the last five 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.

3. Who is at risk, and when?

The aim of immunisation programmes is to protect people when they are most at risk from particular diseases.

For example:

  • 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.

4. 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.

5. Achieving herd immunity

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.

Herd immunity and vaccine effectiveness: Measles and the MMR vaccine

The measles component of the MMR vaccine is highly effective. Two doses of the MMR vaccines protects 99 out of 100 vaccinated people from measles. That leaves one out of every 100 vaccinated people not protected against measles.

Provided there are high levels of vaccination in a community (eg, 95 out of every 100), there is a very high probability that the one person out of 100 vaccinated people who does not develop immunity from the vaccine, will be protected by herd immunity. This is because they are surrounded by people who are immune to measles, so they cannot catch it and pass it on to the one person who is not immune.

However, measles is extremely easy to catch and pass on. In a population that is not vaccinated at all, someone with measles will infect between 14 and 18 other people. This can rapidly lead to high levels of disease cases in a community or geographical area (an epidemic). If there are high levels of disease in a community, due to lower vaccination levels, it is quite possible that some of those who failed to make an immune response to the MMR vaccine may catch measles too.

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