How Vaccines Work

By: Alia Hoyt  | 

Infant Vaccinations

Vaccines are most effective at preventing disease when the recommended schedule is followed. Over the next three sections, we'll go over the schedules for infants, children and adults in the U.S.

Infants receive the lion's share of vaccines. First, babies have very immature immune systems and are at greater risk of complications and death if exposed to disease. Second, experts believe that a strict vaccine schedule from birth on helps to ensure that these diseases don't pop back up and cause large-scale epidemics. It is recommended that babies from birth to 15 months receive the following vaccines (see the CDC vaccination schedule for exact ages):


HepB: Hepatitis B is transmitted through blood transfusions, direct contact with infected bodily fluids, sexual contact, body piercing and tattoos. According to the CDC, of the children who acquire lifelong hepatitis B, 25 percent die of liver disease in adulthood.

DTaP: Combination vaccine that prevents three diseases (diphtheria, tetanus and pertussis) in one shot.

  • Diphtheria is serious infection of the nose and throat that is spread through person-to-person contact. It can cause difficulty breathing, heart failure, paralysis and even death. Diphtheria once took the lives of more than 15,000 children a year in the U.S., according to the CDC. Now, thanks to vaccinations, only two people were recorded as having the disease in the U.S. between 2004 and 2017.
  • Tetanus (lockjaw) is transmitted via contact with infected soil to cuts and punctures in the skin. It causes muscle spasms, often of the jaw (hence the name lockjaw) and can cause major complications, including lung infections, heart damage and death. A booster shot for tetanus is recommended every 10 years because it wears off over time and poses risk to people of all ages, causing death in 10 to 20 percent of cases.
  • Pertussis (whooping cough) is a contagious disease transmitted through coughing and sneezing fits. Pertussis is especially dangerous to the very young, the elderly and those with compromised immune systems, such as cancer patients.

Hib: The Haemophilus influenzae type b (Hib) vaccine protects against meningitis (infection of the covering around the brain and spinal cord). Meningitis once caused terrible complications in survivors, including mental retardation, deafness and seizures.

IPV (inactivated poliovirus): Spread through direct contact with fecal matter, polio once caused 13,000 to 20,000 cases annually in the United States alone. Paralytic polio left many victims relegated to wheelchairs, crutches and leg braces.

PCV (Pneumococcus): Causes complications such as meningitis, pneumonia, blood infection (sepsis) and brain damage. It is spread through coughing and sneezing and is especially dangerous to the elderly.

MMR (Measles, mumps and rubella): Another combination vaccine. These three diseases are spread through coughing and sneezing.

  • Measles is highly infectious. The trademark rash begins at the hairline and progresses downward. It can cause brain swelling and even death. Before widespread vaccination against this disease in the U.S., starting in the 1970s, 3-4 million Americans would get it each year. In 2000, measles was considered eliminated from the U.S. thanks to vaccination. However, that number has been creeping up lately.
  • Mumps causes swollen glands and cheeks and complications like deafness and brain damage.
  • Rubella (German measles) is often transmitted from pregnant women to their unborn fetuses. It caused many premature births, miscarriages and birth defects.

Varicella (chickenpox): Chickenpox used to cause approximately 4 million cases, 11,000 hospitalizations and 100 deaths per year before the vaccine became available in 1995. Complications are worse for teens and adults and include lung and brain damage and death. The trademark itchy rash usually covers the entire body.

HepA: Hepatitis A is usually spread via contaminated food and water, often in situations with bad food preparation habits and in developing countries. It can cause very low energy for as long as a year and liver failure, joint pain and kidney and blood disorders.

Rota (rotavirus): A common disease that infects almost all children in the United States before school age. The disease usually causes fever, nausea and diarrhea. In less developed countries it causes more than 500,000 deaths annually.

Influenza (the flu): The disease, which is spread through coughing, sneezing and other direct contact, can cause complications like brain swelling, pneumonia and death in severe cases. The Spanish flu pandemic of 1918 caused 675,000 deaths in the United States and 50 million deaths across the globe, according to the CDC. Because the flu virus changes each year, this vaccine is given annually.

"There is absolutely no chance that you can catch the flu from the flu shot," says Dr. Bradley. Sometimes the flu vaccine is mismatched to the particular virus so the protection isn't there, or these people were already coming down with the flu when they got the vaccine.

After all these vaccines, babies get a few years to rest before their next round of immunizations. Next we'll talk about childhood vaccinations.