With the rollout of vaccines across the world, people are asking when things will return to normal. At what point will the vaccine work to stop the virus? How do Vaccines protect everyone, even those who reject them? To answer these questions you first need to understand exactly how vaccines work:
How do Vaccines protect everyone?
The first goal of a vaccine is to protect the person receiving the vaccine. But there is a far bigger goal of any vaccination program: to protect everyone, even those who have not had the vaccine. The way this is done is by vaccinating as many people as possible so that the population as a whole is immune. This is called herd immunity, and it is surprisingly effective.
The term herd immunity has been thrown around social media a lot, negatively, but the theory was first developed by veterinarians who were trying to protect herds of cattle from common diseases. It was quickly established that it works with any group of animals and doctors started to used it for human diseases.
Today scientists work to establish a herd threshold; the herd threshold is the percentage of a group required to prevent transition within that group. The threshold percentage changes for each different disease based on its infection rate:
If one person can pass on the infection to 4 people on average, that disease has a basic reproduction number of 4, or ‘R0=4′ for short. (This is where we get the ‘R Number) and this means the number of infections will go up in a factor of four: 1, 4, 16, 64, 256, 1024 etc.
Vaccines work by bringing down the R Number of a virus. In simple terms, if your vaccine is 100% effective, and you vaccinate 50% of a group of people, you have halved its R Number. So with the example of the R0=4, your new infection rate will only go up in twos: 1, 2, 4, 8, 16, 32.
To prevent the number of infections from growing, you have to ensure that the virus is only being passed onto 1 or fewer people each time it is transmitted. So with our R0=4 example, 3 out of 4 people, or 75% would need to be immune to prevent cases growing out of control. But vaccines are not usually 100% effective. For example the measles vaccine protects 97% of the people who receive it.
When will the Vaccine stop COVID-19?
COVID-19 has been estimated to be around R0=2 to 3 and COVID vaccines are only 90% effective, so we would need 75% of the population to get vaccinated giving us 68% protection to see numbers come down. But in the end the more people that get vaccinated the better.
So a vaccination program in the end is all about reducing the number of possible new transitions. Each day that passes more and more vaccines are administered, we are moving ever closer to a return to normal. And that is how vaccines protect everyone, even those who reject them.
As more people are immunised, there are fewer people who can pass on a virus, and so it (in most cases) will not find its way to people who have no resistance to it.
For more information on vaccination, I highly recommend the following sites:
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