[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe how vaccines can reduce the spread of infectious diseases you should then be able to describe what's meant by herd immunity and finally you should be able to describe the potential issues around people choosing not to be vaccinated now vaccinations are widely used in medicine and people in the UK are given a range of different vaccines both during childhood and as an adult vaccines can be used to prevent both epidemics and pandemics an epidemic is when an infectious disease spreads rapidly in a population either in a specific location such as a city or across a whole country for example in 2019-2020 New Zealand experienced an epidemic of measles in a pandemic an infectious disease spreads rapidly across several countries a continent or even the whole world and we saw this with the covid pandemic so the use of vaccines is a powerful tool in our fight against epidemics and pandemics let's take a look at how vaccines work vaccines are usually given via the mouth or as an injection into the bloodstream vaccines contain antigens from the pathogen that we want to protect the body against for example some vaccines contain an attenuated or weakened strain of a bacterium or virus in this case the bacterium or virus can infect the patient but it's easily fought off by the immune system other vaccines contain bacterial cells which have been killed or virus particles which have been inactivated in these cases the pathogen cannot cause an infection but the antigens can trigger an immune response other vaccines only contain the antigen molecules these antigens may be extracted from the pathogen or they can be manufactured using genetic engineering and finally some vaccines provide protection against a bacterial toxin in this case the vaccine contains toxin molecules which have been modified these modifications make the toxin harmless but still allow it to act as an antigen okay so once the vaccine enters the human body this stimulates a primary immune response this leads to the production of antibodies and memory B cells T lymphocytes have also activated if the person later comes into contact with the pathogen then the secondary immune response is triggered in the secondary immune response a large amount of antibodies is produced rapidly and in this case the pathogen is destroyed before any symptoms of the disease develop so as we've seen vaccination triggers a person to develop immunity to a pathogen and remember that this is an example of artificial active immunity which we saw in a previous video now a key idea with vaccination is the concept of herd immunity in any population some people will not have been vaccinated this includes for example very young children or people with a weakened immune system for whom vaccines may not work effectively I'm showing the unvaccinated person in red here the people in green represent those who have been vaccinated under immune to the disease as you can see the unvaccinated person is surrounded by people who have been vaccinated because vaccinated people cannot catch the disease they cannot pass the pathogen onto the unvaccinated person so this means that the unvaccinated person is unlikely to catch the Infectious Disease so if enough of the population are being vaccinated then this also provides some protection to people who have not been vaccinated in scientists call this herd immunity now herd immunity only works if a high enough percentage of people have been vaccinated if a significant number of people are not vaccinated as I'm showing you here then herd immunity breaks down and this can lead to an epidemic of an infectious disease this can happen for example if a large number of people choose not to be vaccinated now in the UK many vaccines are provided routinely to the population for example the flu vaccine is provided free to anyone over 50 or to people with certain health issues on children in the UK are routinely vaccinated against a range of diseases however vaccines can also be used globally in the case of pandemics and we saw this with covid now the effectiveness of vaccines depends on the disease for example the influenza virus mutates regularly and changes its surface antigens scientists call this antigenic variability and because of this people need to be re-vaccinated every year with an updated influenza vaccine now the common cold is also caused by a virus but in this case there are over a hundred types of the cold virus each of these types of the common cold virus has different surface antigens and this is why there's no effective vaccine for the common cold okay so hopefully now you should be able to discover how vaccination can protect against infectious disease [Music]