The Science Behind Vaccines

For centuries, vaccines have safeguarded people against infectious disease. Today, roughly 7 out of 10 Americans believe vaccination is important—yet recent studies show that vaccination rates in more than half of U.S. states have been on the decline. And with the current push to develop and distribute vaccines in response to COVID-19—the disease caused by the coronavirus SARS-CoV-2—many people have questions about vaccine reliability, safety, and development.

Carolina Lucas and Felicia Goodrum, two experts in the fields of immunology and virology and members of the Pew Latin American Fellows Program and Pew Biomedical Scholars Program,  respectively, recently answered common questions about vaccines. Their responses, in interviews that took place shortly before a third vaccine against SARS-CoV-2 was authorized for emergency use in the U.S., have been edited for length and clarity.

Q:  Let’s start with the most basic question: What’s a vaccine and how does it work?

Lucas: A vaccine is a biologically based drug designed to elicit an immune response in the body to provide protection when exposed to a particular pathogen, or disease-causing agent. Vaccines are first sensed by, and activate components of, our innate immune system, which is the first line of defense after a pathogen appears in the body. This in turn induces adaptive immunity, which provides ongoing protection with antibodies and immune cells that work specifically against the pathogen.

Goodrum: Vaccines help the body “learn” to recognize a pathogen before an infection. That way, you have a protective immune response ready to prevent an infection if you encounter the pathogen. Vaccines provide protective immunity in a number of ways. For instance, they may present a pathogen in a weakened form, in what is known as a live-attenuated vaccine, or they can include parts or pieces of the pathogen, in what are called subunit vaccines. The two vaccines currently available in the U.S. against SARS-CoV-2, which causes the COVID-19 disease, contain messenger ribonucleic acid (mRNA), which instructs the body to make the SARS-CoV-2 spike protein. This protein is produced and presented to the immune system by cells that take up the RNA, helping to trigger a defense response.

Q: You say that vaccines can contain a weakened form of the virus, or pieces of proteins from the virus. Does this mean that, in theory, a vaccine could give someone the disease it’s supposed to prevent?

Lucas: No. Vaccines are safe and require rigorous steps during their development to ensure efficacy and safety. The possibility of a weakened virus within a vaccine to regain the ability to become infectious and cause disease is extremely unlikely, and vaccines are designed in a way to further reduce this risk. In rare cases, some live-attenuated vaccines—such as the MMR vaccine against measles, mumps, and rubella, and the chickenpox vaccines against varicella—can cause nonharmful and noncontagious mild immune reactions, such as a rash or low fever; however, severe forms of the disease have not been reported.

Goodrum: The science around vaccines has evolved significantly since the famous Cutter Incident in 1955, when a live-attenuated polio vaccine produced by Cutter Laboratories wasn’t completely inactivated, or killed, causing some people to actually contract polio. That led to a better system for vaccine regulation and new ways of creating safer and more effective vaccines—including the vaccines that present only pieces of a virus, rather than the whole virus. Most vaccines for SARS-CoV-2 that are available or in development, for instance, are based on a single viral protein, so there’s no chance for the vaccines to cause infection.

Q: So vaccines don’t cause autism or other developmental disorders?

Goodrum: There is no evidence in the scientific or medical communities of any link between vaccines and developmental disorders, including autism. This was a concern many years ago when one study made a claim based on a small handful of people, but that article has since been retracted and the findings have been refuted by a large body of scientific evidence from across the globe.

Lucas: The National Academy of Medicine has done an extensive review of past studies on this topic and, based on the evidence available, rejects the correlation alleged between vaccines and autism.

Goodrum: We have to be very careful that we don’t let anecdote become scientific evidence. Data from many countries with detailed vaccination records show that, if anything, there is actually a lower incidence of autism in people who get vaccinated.  

Q: How long does immunity from vaccines last, and why are boosters and additional immunizations sometimes required?

Goodrum: Vaccines prime an immune response, and sometimes boosters are needed to amplify or sustain this response. Immunity to other coronaviruses lasts for a few years. The immunity to SARS-CoV-2 may also last a few years, although we don’t know for sure yet. It’s possible that people will have to be revaccinated every few years because the immune response to the original vaccine has weakened, or because the virus has mutated, meaning that new variants will be able to escape the original vaccine.

Lucas: The longevity of a vaccine—and, in turn, whether booster shots are needed—depends on a range of factors. Those include a person’s age, immune status—meaning the capacity of an individual’s immune system to defend against infections—and the type of vaccine. With some vaccines against measles and yellow fever, for instance, more than 95% of people acquire lifelong immunity with one or two doses. On the other hand, the protection from vaccines for pertussis, or whooping cough, dwindles over time.

Goodrum: Smallpox is a good example. We don’t vaccinate many children today for smallpox because it’s been eradicated. However, if smallpox did reappear, scientists would need to try to figure out if people who were vaccinated many decades ago would need a booster.

Lucas: Another example is the flu vaccine, which generally induces protection in up to 60% of vaccinated individuals. However, the virus has a high mutation rate—so immunity can wane. That’s why annual flu vaccines are recommended to protect against new strains.

Q: We started by asking one basic question. Let’s end with another: Why should people get vaccinated?

Lucas: Vaccination is the major tool in the prevention of infectious diseases, and the most cost-effective lifesaving intervention available. The World Health Organization estimates that vaccination programs help prevent 2-3 million deaths each year.

Goodrum: Since the start of the COVID-19 pandemic, over 480,000 people in the United States have died. Millions of people have now been vaccinated, most with mild to no side effects. We hear people worrying about the safety and efficacy of vaccines. But the dangers and risks of long-term illness far outweigh any risk of a vaccine. It seems like people are forgetting when several diseases, such as polio, were a persistent life-threatening risk. Vaccines protect you and the community around you.

Lucas: There are many benefits when people get vaccinated, including reducing the risk of disease transmission, as well as its severity and mortality. When most of a population is vaccinated against an infectious disease, there’s a better chance of achieving herd immunity, which helps to protect the very young and immunocompromised who may not be able to get vaccinated.

Goodrum: It’s incredibly important that people trust in science—an evidence-based, data-driven approach to problems that constantly weighs risks versus benefits. Viruses, and the diseases they cause, don’t naturally disappear. It’s because of strong vaccination campaigns that they’re kept at bay or eradicated.