Vaccines: What are they and how do they prevent disease?

Vaccines are one of the greatest advancements in the history of public health. Vaccines are responsible for eliminating smallpox and reducing by 95 percent or more many of the common childhood diseases like diphtheria, measles and polio, which once killed hundreds of thousands. Even now, there are 1.5 million people—mostly children in the developing world—who die from vaccine-preventable illnesses annually. However, with the decline of the disease rate in the United States comes a resulting lack of appreciation of their severity, and myths abound about the dangers of vaccines. It’s no wonder there is confusion.

Rather than lecturing about how you should vaccinate your kids (although you should) or get the recommended adult vaccinations for your situation (although that’s a good idea too), it’s important to know about vaccines themselves—their history, how they work and exactly why they can keep you healthy.

Vaccines were first developed—at least in a rudimentary way—in the 1700s, when people used cow pox to build immunity to smallpox. Then, in the 1800s, Louis Pasteur found a way to create vaccines in the lab, and the modern age of vaccines had begun.

In the 1950’s, Jonas Salk created a vaccine for polio using killed versions of the polio virus itself. In 1957, two years after a mass immunization campaign, polio cases in the United States dropped from 57,628 in 1952 to about 5,600 and according to Centers for Disease Control and Prevention (CDC) estimates, in 1952, 3,145 Americans died as a result of polio. Since 1996, that number has been zero. In 1958, 763,094 people in the United States contracted measles, and 552 died. In 2015, there were 188 measles cases and one death.

“Vaccines are safe, and they save the lives of children and adults every day,” said Carrie L. Byington, MD, dean of the Texas A&M College of Medicine, senior vice president of the Texas A&M University Health Science Center and vice chancellor for health services at The Texas A&M University System. “The science of vaccines is sound, and their safety is established.”

Vaccines work because of a remarkable property of the human immune system: Once infected with a particular pathogen, your antibodies get to work. “’Memory’ cells develop when you get sick with a new virus so you don’t get the viral infection again,” said Van G. Wilson, PhD, associate dean for research and graduate studies and professor of microbial pathogenesis and immunology at the Texas A&M College of Medicine. “The second infection is dealt with quickly, before you even feel sick.”

There are three major types of vaccines: Live (but attenuated, or weakened), inactivated and subunit. Live virus vaccines, like the one for chickenpox (varicella), typically only require one or two injections for lifelong immunity, but since the virus isn’t neutralized, there is a tiny possibility that this type of vaccine can cause a mild version of the very disease it’s designed to prevent. Inactivated vaccines, on the other hand, can’t replicate or give someone the illness, but they typically require multiple doses—or booster shots—to be fully effective. Many commonly thought of vaccines, from those that protect against polio and influenza to diphtheria and tetanus, are inactivated.

The third type of vaccine, subunit, only uses a subset of the pathogen. “This type of vaccine uses just pieces of the virus, so you need to know what the important parts are so you know what to include,” Wilson said. “These important pieces are the ones required to teach the body to make the necessary antibodies.” Examples of this type are the hepatitis B and the human papilloma virus vaccines, both of which prevent cancer as well as infection.

Vaccines are not 100 percent effective and no vaccine is without side effects, but vaccines are tested meticulously for safety. Science has shown over and over that vaccines do not cause autism. “Some people thought that the mercury preservative in vaccines was causing a problem, but that’s just not the case,” Wilson said. “There is more mercury in one can of tuna than in all of the childhood vaccines combined.”

Still, there can be other side effects from some vaccines. “The ideal vaccine is effective, safe and convenient,” Wilson said. “We don’t yet have that perfect vaccine, but the ones that have been approved are very, very safe.” The most common side effects are mild, like soreness and redness at the injection site, or just generally not feeling well, possibly with a low grade fever or headache.

Very rarely, people have more serious allergic, autoimmune or neurological reactions. “Still, the rate of consequences is so low that the benefit of vaccination far outweighs the risk,” Wilson said. “Whether or not a child is vaccinated, they will be exposed to the disease. Those with natural disease face the risk of severe illness, permanent injury such as blindness, deafness, developmental delay, paralysis or even death. They will either get very sick without the vaccine or get the antigen with the vaccine and stay well.”

High immunization rates, particularly in school age children, have a benefit to the community through “herd immunity” and protect those vulnerable members of the population who cannot be routinely immunized for medical reasons, such as in the case of babies in their first months of life and people with suppressed immune systems. When vaccination rates are high, there are fewer unvaccinated persons for the pathogen to infect, which stops a disease from spreading. In general, at least 95 percent of the population needs to be immunized to stop the spread of measles.

“Health organizations all over the world need to work together to make sure that vaccines are available and being used to their full potential,” said Byington, who also chairs the American Academy of Pediatrics’ Committee on Infectious Diseases. “That’s the only way we’re going to be able to make these preventable diseases truly a thing of the past.”