Vaccine Safety: Are Vaccines Safe?

The first definition of the word safe is "harmless." This definition would imply that any negative consequence of a vaccine would make the vaccine unsafe. Using this definition, no vaccine is 100 percent safe. Almost all vaccines can cause pain, redness or tenderness at the site of injection. And some vaccines cause more severe side effects. For example, the original pertussis (whooping cough) vaccine could cause persistent, inconsolable crying, high fever or seizures associated with fever. Although none of these severe symptoms resulted in permanent damage, they could be quite frightening to parents.

But, in truth, few things meet the definition of "harmless." Even everyday activities contain hidden dangers. For example, every year in the United States, about 350 people are killed in bath- or shower-related accidents, about 5000 people are killed when food lodges in their windpipe, and about 50 people are struck and killed by lightning. However, few of us consider taking a bath, eating solid food or walking outside on a rainy day as unsafe activities. We just figure that the benefits of the activity outweigh the risks.

The second definition of the word safe is "having been preserved from a real danger." Using this definition, the danger (the disease) must be significantly greater than the means of protecting against the danger (the vaccine). Or, said another way, a vaccine's benefits must clearly and definitively outweigh its risks.

To better understand the definition of the word safe when applied to vaccines, let's examine a few different vaccines and the diseases they prevent.

Hear Dr. Offit explain the risks and benefits of vaccines by watching this short video, part of the Talking About Vaccines with Dr. Paul Offit video series.

Is the hepatitis B vaccine safe?

The hepatitis B vaccine has few side effects. However, one side effect is serious. About 1 of every 1 million doses of hepatitis B vaccine is complicated by a severe allergic reaction, called anaphylaxis. The symptoms of anaphylaxis are hives, difficulty breathing and a drop in blood pressure. Although no one has ever died because of the hepatitis B vaccine, the symptoms of anaphylaxis caused by the vaccine can be quite frightening.

On the other hand, every year about 1,700 people die in the United States soon after being infected with hepatitis B virus. And, millions of people throughout the world live with chronic hepatitis B infection. Between 1 and 2 million of these people live in the United States. Up to 1 in 4 chronically infected people will suffer severe liver damage (called cirrhosis) or liver cancer caused by hepatitis B virus. People are much more likely to develop these severe, and often fatal, consequences of hepatitis B virus infection if they get infected when they are very young children. For this reason, the hepatitis B vaccine is recommended for newborns.

Some parents wonder whether it is necessary to give the hepatitis B virus vaccine to newborns. They ask: "How is a baby going to catch hepatitis B?" But, before the hepatitis B virus vaccine, every year in the United States about 18,000 children less than 10 years of age caught hepatitis B virus from someone other than their mother. Some children caught it from another family member, and some children caught it from someone outside the home who came into contact with the baby.

Although about 1 to 2 million people in the United States are infected with hepatitis B virus, many of these are “silent infections,” meaning without obvious symptoms. As a result, many people who are infected with hepatitis B virus don't know that they have it. So it can be hard to tell from whom you could catch hepatitis B virus.

Worse yet, many people don't realize that you can catch hepatitis B virus after coming into contact with very small quantities of blood — small enough that they are not visible to the naked eye. This means that someone can be infected through relatively casual contact with someone who is infected, such as sharing washcloths or toothbrushes. In fact, a milliliter of blood (about one-fifth of a teaspoon) from someone who is infected can contain as many as 1 billion infectious viruses.

Because the benefits of the hepatitis B vaccine clearly and definitively outweigh its risks, the hepatitis B vaccine is safe.

Was the old pertussis vaccine safe?

The old pertussis vaccine had far more risks than the hepatitis B vaccine. The old pertussis vaccine was called the "whole-cell" vaccine and had a high rate of severe side effects. Persistent, inconsolable crying occurred in 1 of every 100 doses, fever greater than 105 degrees Fahrenheit occurred in 1 of every 100 doses, and seizures with fever occurred in 1 of every 1,750 doses. Due to negative publicity related to this vaccine, its use decreased in many areas of the world.

For example, the Japanese Ministry of Health decided to stop using the pertussis vaccine in 1975. In the three years before the vaccine was discontinued, 400 cases of pertussis and 10 deaths from pertussis occurred in Japan. In 1979, four years after the pertussis vaccine was discontinued, 13,000 cases of pertussis and 41 deaths were reported. It should be noted that although the side effects of the pertussis vaccine were real, children didn't die from pertussis vaccine; however, they did die from pertussis infection. The Japanese Ministry of Health, realizing how costly their error had been, soon reinstituted use of pertussis vaccine.

The decisions in Japan proved that, despite the side effects, the benefits of the old pertussis vaccine clearly outweighed the risks.

Scientific progress eventually led to the creation of another version of the pertussis vaccine. Known as the “acellular” pertussis vaccine, it was more purified, so instead of containing about 3,000 immunogenic proteins, it only contained two to five proteins. The ("acellular") pertussis vaccine became available in the United States in 1996. Children who received this vaccine had a much lower risk of severe side effects than those who received the old "whole-cell" vaccine. Therefore, the new pertussis vaccine is safer than the old pertussis vaccine. But because the benefits of the old pertussis vaccine outweighed its risks, it too was safe.

Is the pneumococcal vaccine safe?

The pneumococcal vaccine was licensed in the United States in 2000 and was recommended for use in all children less than 5 years of age. Some parents chose to take a "wait-and-see" attitude. They reasoned that because the problems with the rotavirus vaccine were not revealed until the vaccine was given to 1 million children, why not wait and see what happened after the pneumococcal vaccine was given to several million or more children.

However, the choice not to give the pneumococcal vaccine was not a risk-free choice. Before pneumococcal vaccine was first given to infants in 2000, every year in the United States about 700 children (less than 5 years old) got meningitis, 17,000 got bloodstream infections, 5 million children developed ear infections, and 200 died from pneumococcus. So the choice not to get a pneumococcal vaccine was a choice to risk the consequences of pneumococcal infection, which were occasionally fatal.

Parents should be reassured by two facts. First, the pneumococcal vaccine was tested in about 40,000 children before it was licensed for use. Second, the Haemophilus influenzae type b (Hib) vaccine is made in a manner almost identical to the pneumococcal vaccine (see How Are Vaccines Made?) and has been given safely to millions of children since 1990.

Detecting rare side effects

The first rotavirus vaccine (RotaShield) is an example of how rare side effects can be detected quickly. The rotavirus vaccine was tested in about 11,000 children before it was submitted to the FDA for licensure. After the vaccine was licensed and recommended for use (see "Who licenses, recommends and requires vaccines?"), the vaccine was given to about 1 million children.

A system called the Vaccines Adverse Event Reporting System (VAERS) initially found about 15 cases of an intestinal blockage, called intussusception, soon after administration of the vaccine. This was worrisome enough to the Centers for Disease Control and Prevention (CDC) to cause them to temporarily suspend use of the rotavirus vaccine until it could be determined whether the vaccine did, in fact, cause intussusception. Their analysis showed that intussusception occurred in about 10 of every 100,000 children who received the vaccine. Because only 11,000 children were tested before the vaccine was licensed, it was really not possible to pick up such a rare side effect. The result of the rotavirus vaccine experience is that at least 60,000 children had to be tested before the next vaccine was licensed.

Several other systems of study to understand the rate of vaccine side effects, such as the Vaccine Safety Data Link (VSD), are also available. The VSD also allows one to determine the background rate of side effects, meaning the rate of adverse events in children who don't receive a vaccine. So, in many ways, systems like the Vaccine Safety Data Link are better than VAERS because they allow one to determine whether a particular vaccine is the cause of a rare side effect.

In A Look at Each Vaccine, we discuss the risks and benefits of every vaccine in order to determine whether these vaccines are safe.


Offit PA and Moser CA. "Vaccines and Your Child: Separating Fact from Fiction.2011. Columbia University Press.

Reviewed by Paul A. Offit, MD on May 19, 2020

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.