People sometimes ask: if vaccines work so well, why do some vaccinated people still catch the diseases they have been vaccinated against? Why do we get outbreaks of infectious disease in populations where most people get their routine vaccinations?
This is a complex issue and there are several linked factors that are involved, listed below. Further down the page are four case studies that explain how these factors work in practice for some different diseases and vaccines.
1. The effectiveness of vaccines
Vaccines are more effective than almost any other medicine we use on a daily basis. Most people will be protected by most of the vaccines they receive, and some vaccines in the routine UK schedule are almost 100% effective against some diseases. For example, after two doses of MMR vaccine up to 99% of individuals will be protected from catching measles. However, there will always be a small number of people who fail to make an immune response to a particular vaccine. If their body has not made an immune response, then those people remain vulnerable to the disease.
2. Long-term protection of vaccines
Most vaccines offer good protection for many years. For some vaccines it is necessary to give repeated doses of vaccines or booster doses to provide continued protection. But vaccines do not usually provide protection ‘for ever’. Levels of protection may naturally decrease over time, or may be reduced because of medical conditions, medications or ageing, when the immune system may work less well.
3. Who is at risk, and when?
The aim of vaccine programmes is to protect different groups in the population through the times that they are most at risk from particular diseases. Babies are particularly vulnerable to infectious diseases, so most countries begin their vaccine programmes in young babies. Some vaccines are targeted at other groups who are at higher risk from particular diseases: teenagers have a higher risk of developing meningococcal disease; people aged over 70 are at greater risk from shingles and its complications; pregnant women are more at risk from flu.
4. Diseases change and evolve
For vaccines to work, the strain of bacteria or virus in the vaccine needs to be the same as the strain that causes disease in the population (or very similar to it). Some viruses and bacteria change and evolve 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. In contrast, the measles virus hardly changes from year to year, and so the measles vaccine that forms part of the MMR is as likely to protect you today as it was ten years ago. Some bacteria that cause disease come in many different types (such as pneumococcal bacteria or meningococcal bacteria). Vaccines are developed to protect against the main types that cause disease, and different vaccines may be needed for different types. 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 (or population immunity)
Many 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. This makes it harder for the disease to keep passing from person to person. Vaccination 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. See the Herd Immunity page for more detail.
Herd immunity and vaccine effectiveness: Measles and the MMR vaccine
The measles component of the MMR vaccine is highly effective. A single dose of the MMR vaccine protects at least 9 out of 10 of vaccinated children against measles. A second dose protects about 90% of those who failed to make an immune response to the first dose. So in total about 99 out of 100 of children are protected against measles by two doses of the MMR vaccine.
This leaves about one child in 100 not protected against measles. It is not known why this happens, and researchers are working to understand why people’s immune systems react in different ways so that they can develop more effective vaccines. But as long as about 95 out of every 100 people get the MMR vaccine, there is a very high probability that the ‘one in 100’ child will be protected by herd immunity. This is because they are effectively surrounded by a protective ‘ring’ of people who are immune to measles, so they cannot catch it and pass it on to the child who is not immune.
However, measles is extremely contagious (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 lots of disease cases in a community or geographical area (an epidemic). Measles can be passed to unvaccinated individuals (including babies who are too young to be vaccinated, or people who are too ill). But it is quite possible that some of those who failed to make an immune response to the MMR vaccine may catch measles too. Because measles is so contagious it can spread very fast, so a serious outbreak can quickly develop in a community or population which has pockets of low vaccination coverage. This happened in the Swansea (UK) outbreak in 2013, and in the Disneyland California (USA) outbreak in 2015.
Protection over time: Whooping cough (pertussis) vaccines
In 2004 the UK changed the type of pertussis vaccine it used in its vaccination programme. Until 2004, the pertussis vaccine used in the UK was made using whole killed pertussis bacteria. This vaccine provided good protection, but was known to cause side effects in some cases. The current pertussis vaccine is called ‘acellular’: it is made using a small number of individual proteins from the pertussis bacteria rather than the whole bacteria. This type of vaccine gives a good immune response and also reduces the chances of severe side effects.
The immune response to pertussis vaccines tends to fade over time. (The same is true if you catch the disease: you do not become immune for life, and so you can catch pertussis again.) With ‘acellular’ vaccines, the immune response tends to fade more quickly than with ‘whole cell’ vaccines. This means that children may be able to catch pertussis even if they have been vaccinated. This in itself is not too much of a problem, because older babies and children are not at serious risk from pertussis. However, it does become a problem if children pass pertussis on to newborn babies who are too young to be vaccinated, because they are at risk of serious illness and even death from the disease.
In 2012 rates of pertussis in the UK increased dramatically and several newborn babies died. It is not known exactly why disease rates increased, but it is possible that the ‘fading immune response’ associated with the new pertussis vaccine may have played a part. The UK government immediately introduced a programme of pertussis vaccination for pregnant women. This quickly led to a fall in rates of pertussis in newborn babies, even though there is still a lot of pertussis circulating amongst children and adults in the UK.
How diseases change: Pneumococcal disease and PCV vaccines
Pneumococcal bacteria cause a range of problems including ear infections, chest infections and meningitis (see page on Pneumococcal disease for more information). There are over 90 different types of these bacteria, and vaccines have been produced to protect against the types that cause the most disease.
The vaccine used to protect infants (the PCV) was introduced in 2006 with a vaccine protecting against seven of the types of bacteria. This resulted in a big reduction in the number of cases of pneumococcal disease in babies caused by these seven types. However, there was an increase in the number of cases caused by other types of the bacteria. Six strains in particular were identified as causing most of the new cases of pneumococcal disease. So in 2010 the PCV vaccine was changed to one that protected against all 13 types of bacteria. The additional six types of pneumococcal bacteria are now also starting to disappear from the UK because of the vaccination programme. Vaccination of babies has reduced the amount of disease in the whole population, because infants and children are no longer carrying the pneumococcal bacteria and spreading them around.
It is possible that other strains of pneumococcal bacteria may become more common in future and start to replace the strains that are disappearing. If this happened, a new vaccine covering more of the strains would need to be developed.
Deciding who is most at risk: Shingles vaccine
As people get older their chance of developing shingles increases, and complications also become more common. However, as people get older their immune system also gets weaker. It becomes less able to mount a response to a vaccine, and the response is likely to fade more quickly. If the shingles vaccine is given too early in later life, the protection may not last long enough to still be working when people are most likely to develop shingles and its complications (age 70-80). If the shingles vaccine is given too late, the body may not mount an immune response at all. The NHS has calculated that giving the shingles vaccine to 70-year-olds will protect the greatest number of people for the longest possible time. Some vaccinated people will still get shingles, but the effects of the disease are likely to be less severe that they would usually be.