The most effective way to reduce the cases of a virus, or stop a pandemic, is to reach herd immunity. This is something we have been hearing a lot as vaccination against COVID-19 progresses, but what exactly does it mean?

In order for a virus to spread, it needs a host to infect. Those vulnerable to infection are known as the threshold proportion. If a large number of people are not immune to the virus, it will spread from person to person, often very easily. If a large number of people in the population are immune, transmission rates will decline, and maybe even stop. This is known as the herd immunity threshold. In order to become immune, your body must have antibodies specific to the antigen, and there are two ways to get them: either from an infection or from a vaccine.

If your immune response is strong enough, B-cells (an immune response cell) will produce antibodies that bind to the antigen and prevent them from infecting other cells. A much less risky method of achieving immunity is through a vaccine. Most vaccines will deliver a dead or weakened version of the antigen, which triggers an immune response and forms antibodies because your body recognizes it as foreign material.

Recent vaccine technology has created mRNA (messenger ribonucleic acid, a molecule that codes for proteins) vaccines. These vaccines will deliver a strand of mRNA that codes for a spike protein found on the surface of the virus. The body’s cells begin to produce the protein, then the body recognizes it, which triggers an immune response and builds antibodies. The COVID-19 vaccines use mRNA technology. After infection or immunization, the antibodies will stick around in case the body is exposed to the same virus again, and the duration of the antibody’s stay depends on the virus.

Herd immunity is essential, not only to slow the spread of a virus, but also to protect the vulnerable. For medical reasons, some people are unable to take vaccines, but if enough people living around them are immune, their chances of infection go down. Herd immunity significantly lowers the risk a virus poses to a population, and over time can even eradicate it. The safest way for everyone to achieve herd immunity is to take a vaccine, because they pose a significantly lower health risk than immunity acquired by infection. The percent needed for reliable herd immunity depends on the virus. For COVID-19 it is estimated that at least 80% will need to be immune.