What is a vaccine, anyway?

February 10, 2020

by Clair Geary

Over the last century, deaths and disability from infectious diseases have declined, largely due to improvements in sanitation, nutrition, and vaccines. These days, we often hear more about the hypothetical risks of vaccination rather than the very real dangers posed by the diseases these vaccines prevent. Increasing numbers of worried parents now choose to delay, or even forgo, vaccinating their children. A prime example is measles: while the disease was officially eliminated from the US in 2000 (meaning all cases were traceable to international travel), due to slipping vaccination rates, there were multiple outbreaks and nearly 1300 confirmed cases in 2019. This article will address common questions about vaccines and why they’re so important.

What is a vaccine?

A vaccine is something that can stimulate a person’s immune system to protect against a specific disease. It’s typically made of a similar looking but less dangerous version of the bacteria or virus it protects against. This can mean the same virus but “killed” so it can’t grow or make you sick (e.g. flu shot), a weakened “cousin” virus that grows a bit but won’t make you sick (e.g. measles), or something scientists have made by taking pieces of the virus/bacteria and sticking them to a harmless framework (e.g. HPV and whooping cough).

How do vaccines work?

Vaccines teach our immune system to fight off a particular biological attack. The defenders of our immune system come in many different forms, or cell types, the most important of which is a type of white blood cell called a B cell. B cells make antibodies – small molecules that recognize infectious agents with incredible specificity. At any given moment, you have approximately 10 billion B cells in your body; since each can make a slightly different antibody, you can, in theory, make antibodies specific for anything that might make you sick, whether it’s bacteria, a virus, or cancer. 

When antibodies recognize a potential pathogen, they do a few things. First, they stick to it like a piece of Velcro; if this is an important part of the pathogen, like maybe an “arm” of protein that helps it infect new cells, then the antibody can stop the infection directly. The second task is indirect: when antibodies recognize the pathogen, they sound an alarm, alerting the rest of the immune system to the danger. The call specifically alerts macrophages, another type of white blood cell, which quickly assemble to eat and destroy the invader. 

If this is something the body does naturally, why do we need vaccines?

Simply put: time. If you start with a single B cell that recognizes the pathogen, it takes days to weeks for that single cell to multiply and mature enough to make lots of antibodies. Meanwhile, the pathogen is also multiplying, and if it’s numbers get high enough, you get sick. Without vaccines, your body is in an arms race: can you make antibodies fast enough to fend off a brand-new enemy?

Luckily, your immune system has a way to deal with repeat offenders: memory B cells. These cells not only know how to make the needed antibody, they stick around and patrol your body, ready to make antibodies more quickly if the invader comes back. Think of memory cells as the sheriffs with a wanted poster in hand - as soon as they see the outlaw, they can spring into action, call the deputies (macrophages), and make an arrest. 

Getting a vaccine gives your body an opportunity to make memory B cells so that if you encounter the pathogen, your immune system can respond quickly. Even better: you make memory B cells without getting sick. 

How were vaccines invented?

Prior to vaccination, smallpox killed hundreds of thousands of people per year throughout Europe. It was common knowledge that survivors were protected from getting sick again, so they were often called upon to care for the ill. Some agricultural communities noticed that milkmaids who’d gotten a similar but milder disease, called cowpox, also seemed protected against smallpox. In 1796, Edward Jenner, a British physician and scientist, directly tested this theory by injecting material from a milkmaid’s cowpox lesions into a young boy. Although ill for several days, once the boy recovered, Jenner injected him with smallpox and he was completely protected. Jenner spread news of his experiment to other physicians, and within a few years, vaccination (from the Latin vaccinus meaning “from the cow”) became the preferred method of smallpox prevention throughout Europe. 

Are vaccines safe?

Yes, with rare exceptions. Vaccines go through extensive laboratory testing and clinical trials before becoming available to the public, and organizations including the CDC and FDA continuously monitor their safety. There are medical reasons to delay or avoid certain vaccinations, such as pregnancy, severe allergic reactions to previous vaccinations, or a compromised immune system (e.g. from chemotherapy, genetic causes, or HIV); always discuss your concerns with your healthcare provider.

Do vaccines cause autism?

No. There is no link between vaccines and autism. The article that first suggested a connection has been retracted, and the main author had his medical license revoked amid evidence of data manipulation, undisclosed conflicts of interest, and other ethical violations. Most of the remaining proponents of this anti-vaccine case are celebrities with no medical or scientific training. Notably, many large studies conducted since have found no link between vaccination status and autism spectrum disorders. 

Why do some vaccines work better than others?

The measles vaccine is up to 98% effective at preventing measles with just 2 shots. Most other childhood vaccines are 85% to 95% effective. The flu shot, however, is typically only 40-60% effective. Why? Some pathogens mutate a great deal, while others are more stable: influenza is good at changing and disguising itself, while the measles virus always looks the same, making it easier for the immune system to recognize and fight it. Because measles always looks the same, the measles vaccine (or an encounter with the virus itself) usually leads to life-long immunity, while changeable influenza can make you sick again and again. 

What is “herd immunity”? 

If you are immune to a contagious disease by virtue of a vaccine or previous encounter, it means: 1.) you won’t get sick if exposed, and 2.) you can’t spread the disease to others. In a community with little to no pre-existing immunity, a contagious infection will quickly spread from person to person, getting many people sick. However, if a sufficiently high percentage of the population is immune, there aren’t as many people to infect, so the pathogen doesn’t jump from person to person as well, ultimately blocking the path between the sick and the vulnerable- those who can’t be immunized (such as babies and cancer patients), or those for whom the vaccine hasn’t worked. (see this YouTube video)



In many ways vaccines are victims of their own success – exactly because cases of polio, diphtheria, or measles seem so unlikely, it can be easy to discount the risks and instead wonder if the vaccine is really necessary. However, as the recent resurgence of measles (almost 1300 confirmed cases and over 120 hospitalizations in the US in 2019) shows, rare is not eliminated. Recommended vaccines are available for free through most health insurance plans, Medicare, and Medicaid, and many state and local health departments offer free or low cost options for people without insurance. Even better, some grocery stores and pharmacies run promotions (offer coupons) so that they basically pay you to get your flu shot! So, getting vaccines is safe, easy, and protects you and those around you from getting sick: it’s a win-win-win. Why not make 2020 the year you and your family get up to date?

Clair Greary photoClair is a freshly minted PhD in Immunology and Microbial Pathogenesis from Weill Cornell Graduate School, where her thesis research explored natural killer cell responses to viral infection. She is currently doing the Mirzayan Science and Technology Policy Fellowship working with the Board on Science Education at the National Academies. In her free she time enjoys being out in nature and baking. Find her on Twitter!