Disease in vaccinated populations
|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.
|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.
|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.
|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.
|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.