Vaccines are powerful and effective tools for preventing and slowing the spread of disease.
When the body is invaded by a pathogen, such as the novel coronavirus, the immune system typically responds by attacking it with the help of white blood cells. One type of white blood cell, called a B-lymphocyte, produces proteins called antibodies, which bind to the pathogen and help to attack it.
The human immune system has evolved to remember pathogens it has fought off in the past. If the same viruses or bacteria reinvade, the immune system can often attack them quickly enough to prevent another bout of illness. This explains why some diseases only infect a person once.
It also explains how vaccination protects against infection: vaccines usually contain dead, weakened, or partial versions of the pathogens that cause a particular disease—enough to stimulate the immune response and create immunological memory but not enough to trigger harmful symptoms. In this way, vaccination allows us to develop immunity to a disease without having first been infected.
Why do we get some vaccines just once and others multiple times?
Vaccine dosages and administration schedules depend, in part, on how many strains of a virus exist and on the body's immunological memory of specific pathogens.
For example, each year, researchers predict which strains of influenza will be prevalent that season and design a vaccine to target them. This is necessary because the flu virus has a relatively high mutation rate, meaning that the virus changes as it replicates itself. Some new strains are different enough from older versions that they evade immunological memory. Viruses such as HIV mutate so rapidly that vaccine development is particularly challenging.
Scientists are more optimistic when it comes to SARS-CoV-2. The virus has a "proofreading" function as it replicates inside host cells, which leads to fewer mutations and therefore fewer strains. This makes SARS-CoV-2 a somewhat easier target for vaccine development.
The body's ability to remember how to fight a pathogen may diminish over time. For the common cold coronavirus, for example, immunological memory tends to be shorter, which is why many people get colds once a year or more often. Conversely, the immune system remembers the antigen for measles, which is why the measles vaccine gives lifelong protection.