How do vaccines work?

• Vaccines vary in how they generate protection against disease and in how they are administered (usually as injections, but some can be taken by mouth or given as a nasal spray).
• But every vaccine stimulates the immune system to produce antibodies—proteins that help protect the body against a specific disease. The immune system can generate countless types of antibodies; vaccines aim to trigger the production of antibodies that attack a specific “pathogen”—the agent, such as a bacterium or virus, that causes the targeted disease.
• Some vaccines require only one dose, while others require two or more doses (booster shots) to deliver stronger and longer lasting protection. Protection against influenza requires annual re-vaccination to protect against ever-changing variants of the flu virus.

What kinds of vaccines are there?

In various ways, vaccines introduce “antigens” into the body—substances that stimulate the immune system to produce disease-specific antibodies in the blood. These antigens may be inactivated versions of toxins, viruses, or bacteria—or versions that are still active but weakened so they can’t cause disease. Some vaccines contain only certain pieces of the relevant virus or bacterium, or laboratory-synthesized particles that resemble those pieces—such as synthetic versions of the “spike protein” that is part of the virus that causes COVID-19. Some examples:

• Inactivated vaccines contain pathogens that have been deactivated using chemicals, heat, or radiation. These pathogens can’t sicken people anymore, but they can still cause the body to produce antibodies that protect against future infections caused by the same kind of pathogen. Some cholera vaccines, for example, are made from inactivated versions of the causative bacterium.
• Live-attenuated vaccines use a weakened version of a pathogen. These pathogens are still active, but they aren’t dangerous like the original pathogen. The MMR vaccine is a live-attenuated vaccine.
• While inactivated and live-attenuated vaccines include entire pathogens, some vaccines only contain pieces of pathogens. These subunit vaccines often have the advantage of causing even fewer side effects than inactivated or attenuated vaccines, yet still stimulate the body to generate disease-fighting antibodies.
• Nucleic acid vaccines deliver genetic material such as messenger RNA (mRNA) or DNA into the body’s cells. This material acts as instructions, telling the cells to produce parts of a virus—viral subunits—that serve as antigens to stimulate antibody production. The COVID-19 vaccines produced by Moderna and Pfizer-BioNTech, for example, use mRNA that directs cells in the body to produce pieces of viral spike proteins, which in turn stimulate the immune system to produce antibodies that attack SARS-CoV-2, the virus that causes COVID-19.
• Several other COVID-19 vaccine candidates, including the one produced by AstraZeneca, use viral vectors—such as common cold viruses—that cannot reproduce or cause disease in the body but serve as delivery vehicles to carry laboratory synthesized genetic material into a person’s cells. As with other nucleic acid vaccines, that genetic material directs the cells produce SARS-CoV-2 spike proteins, which stimulate the production of antibodies.

How can vaccines prevent disease spread?

• Vaccination and natural infection both result in people having antibodies in their blood that, along with an array of disease-fighting cells, help prevent subsequent infection.
• When enough of a population is immune to a disease, whether by vaccination or past infection, that disease’s spread becomes severely limited by the lack of vulnerable people to infect—a phenomenon called “herd immunity,” where even those who have not developed immunity are to a large extent protected because the odds of coming in contact with an infected person are so low. Once herd immunity is reached, cases steeply decline. Just how high a fraction of the population must become immune for herd immunity to start reducing a disease’s spread varies based on characteristics of the disease-causing pathogen and the community in which it spreads—such as how infectious the pathogen is, the rates of personal contact among individuals in a population, the use of mitigation strategies such as masks, and how long an infected person remains contagious.
• Individuals who decline recommended vaccinations can interfere with the development of herd immunity within a community. This heightens risk for all members of the community, but especially for individuals who are more vulnerable to a disease’s effects or who may not be able to get vaccinated safely, such as those with certain medical conditions. In 2013, measles outbreaks occurred in communities in New York, Texas, and other states, as low vaccination rates caused breakdowns in herd immunity and accelerated the spread of the virus.

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