What Are Vaccines and How Do They Work?

This article was reviewed by our Baystate Health team to ensure medical accuracy.

Andrew W. Artenstein, MD Andrew W. Artenstein, MD View Profile
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A female doctor places a small bandage on a young girl’s arm after giving her an immunization.

Vaccines. These often-debated substances have been protecting us from diseases including smallpox, measles, mumps, rubella (German measles), and chickenpox, starting with the smallpox vaccine developed by Edward Jenner in 1796. If you aren’t familiar with these illnesses, that’s because vaccines allow most of us to forget just how devastating some infectious diseases truly are.

According to Baystate’s Dr. Andrew Artenstein, “Vaccines are widely recognized to be one of humankind’s greatest public health achievements of all time. They are responsible for eradicating the scourge of smallpox—the first and only human disease ever eradicated—and eliminating numerous diseases as threats against children and adults. Their importance in preserving the health and well being of all people cannot be overstated.”

Flu and COVID-19

In addition to getting the COVID-19 vaccine, it’s important to keep up with routine immunizations for your entire family.

In 2020, the state of Massachusetts announced that flu vaccines are required for all children 6 months of age or older who are attending Massachusetts child care, pre-school, kindergarten, K-12, and colleges and universities.

While some people are hesitant to return to doctor’s offices, experts say the benefits of immunizations make immunization appointments important to keep.

The Immune System is the Body’s Defense against Infection

Illness is caused by germs (bacteria or viruses) invading the body. That invasion is called an infection, and the body’s immune system fights infection with white blood cells. Vaccines use antigens – molecules present on all viruses – to train the immune system to identify threats, produce antibodies, and learn.

According to the CDC, “The first time the body encounters a germ, it can take several days to make and use all the germ-fighting tools needed to get over the infection. After the infection, the immune system remembers what it learned about how to protect the body against that disease.”

How Vaccines Work

Vaccines work by training the immune system to recognize and fight viruses or bacteria. They develop immunity, the ability to resist an infection, by imitating infections.

“This type of infection almost never causes illness. In rare cases, the imitation infection can cause minor symptoms, such as fever,” the CDC says.

Some vaccines, including the Pfizer and Moderna COVID-19 vaccines, use a technology called mRNA to prompt an immune response in the body. Instead of using a weakened or live virus, mRNA vaccines mimic how viruses work. Think of mRNA like a recipe card or instruction sheet teaching your body about the COVID-19 spike protein. The mRNA then breaks down and disappears.

Vaccines protect us from infection and create herd immunity, which can severely depress or extinguish disease within society. These broad effects help to protect those who cannot be vaccinated including those with compromised immune systems, pregnant women, infants, and those with severe allergies.

Who should get vaccinated?

The CDC recommends that children, adolescents, and adults (including pregnant women) receive routine vaccines in order to protect individuals and communities from vaccine-preventable diseases and outbreaks. During the COVID-19 pandemic, preventive care, including routine vaccinations, is important to reduce unnecessary medical office visits and the strain on the healthcare system. Vaccines are safe and effective.

Expectant Mothers

  • You have the power to protect yourself and your baby each pregnancy from serious diseases like whooping cough and flu.
  • If you are pregnant, getting vaccinated can help protect your baby after birth by passing on antibodies. These antibodies can give your baby short-term protection from flu and whooping cough until it is time for their own vaccines.
  • Ask your doctor or nurse about the vaccines you need during pregnancy to protect yourself and your baby.
  • The COVID-19 vaccine is also recommended during pregnancy.

Children and Adults

COVID-19 has caused some parents to avoid the doctor. Please know that postponing your child’s visit can do more harm than good.

  • You have the power to protect your children against serious diseases like measles, cancers caused by HPV, and whooping cough. Vaccinations can save your child’s life, protect those you care about, and protect future generations. Learn more.
  • Preteens and teens need four vaccines to protect against serious diseases: meningococcal conjugate vaccine to protect against meningitis and bloodstream infections; HPV vaccine to protect against cancers caused by HPV; Tdap vaccine to protect against tetanus, diphtheria and whooping cough; and a yearly flu vaccine to protect against seasonal flu.
  • View CDC’s parent-friendly immunization schedule to see which vaccines your children need, whether they are babies or teenagers.


Vaccines are not only for children.

  • Vaccines work with your body’s natural defenses to help safely develop protection from diseases.
  • Adults need protection from a variety of diseases including hepatitis, HPV, measles, influenza, tetanus, and shingles.
  • Even if you have been vaccinated in the past you may need a titer (a test to find out if you have antibodies). If no antibodies are present, or a low level, you may need a booster or new vaccination.

The most common side effects are mild and go away quickly.

Are Vaccines Safe?

Despite the fact that vaccines are safe and effective, concerns about vaccine safety and their unsubstantiated relationship to autism continue to circulate. Often celebrities or viral online content will stoke fear – but there is no scientific evidence to support vaccine skeptics.

  • The safety of vaccines is thoroughly studied before they are licensed for public use.
  • There is a strong system in place to help scientists monitor the safety of vaccines.
  • Like any medicine, vaccines can cause side effects but serious adverse events are rare.

It is often safe to receive combination vaccines or several different vaccines during one visit. Your doctor will talk with you about which vaccines are needed on a given visit. Vaccines are tested before licensing and carefully monitored afterwards to ensure their safety. Learn more about the safeguards that ensure the vaccines we need are safe.

Still have questions about vaccines?

Talk to your primary care provider or pediatrician.

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