Vaccinations: How They Work, Who Needs Them and Side Effects

How vaccines work

When a germ infects you for the first time, it takes the immune system a week or more to make enough antibodies and T cells to fight it, and in that time the disease can do serious harm. A vaccine presents a harmless version or part of the germ. The immune system responds and creates memory cells. If you meet the real germ later, those memory cells react within days or hours, preventing illness or making it much milder.

When enough people in a community are immune, a germ cannot spread easily. This community (herd) immunity protects people who cannot be vaccinated, such as newborns and people having chemotherapy. For very contagious diseases like measles, about 95% coverage is needed.

Types of vaccines

Live vaccines generally are not given during pregnancy or to people with severely weakened immune systems.

Vaccines by age group

Schedules are set nationally (in the US by the CDC with professional societies such as the AAP and ACOG) and are updated over time. US recommendations for several vaccines changed in 2025 and 2026, so ask your doctor or pharmacist for the current schedule for you or your child.

Babies and young children

Routine childhood vaccines in the US protect against hepatitis B, rotavirus, diphtheria, tetanus and whooping cough (DTaP), Hib, pneumococcal disease, polio, flu, measles, mumps and rubella (MMR), chickenpox and hepatitis A. Most doses are given between birth and 18 months, with boosters at 4 to 6 years. Babies can also receive an RSV antibody product, or be protected by a maternal RSV vaccine in pregnancy.

Preteens and teens

• Tdap booster at 11 to 12
• HPV vaccine, ideally at 11 to 12 (can start at 9); 2 doses if started before 15
• Meningococcal vaccines (MenACWY, and MenB based on risk or shared decision)
• yearly flu vaccine

Adults

• Td or Tdap booster every 10 years, and Tdap in each pregnancy
• yearly flu vaccine
• shingles (recombinant) vaccine from 50, and for younger adults with weakened immunity
• pneumococcal vaccine from 50 or with certain health conditions
• RSV vaccine for older adults according to age and risk
• hepatitis B for adults who are not immune
• HPV catch-up through 26, and in some cases to 45
• MMR and chickenpox if not immune
• COVID-19 vaccine according to current recommendations, which vary by age and risk

Pregnancy

Recommended vaccines during pregnancy include Tdap (to protect the newborn from whooping cough), flu and, seasonally, RSV. These pass antibodies to the baby across the placenta. Live vaccines such as MMR are given before or after pregnancy, not during.

Travel and work

Extra vaccines such as hepatitis A, typhoid, yellow fever, rabies, Japanese encephalitis and cholera may be advised depending on destination. Healthcare workers need proof of immunity to hepatitis B, MMR and chickenpox.

Side effects

Common and expected

These show the immune system is responding and usually last 1 to 2 days:

• pain, redness or swelling where the shot was given
• low-grade fever, tiredness, headache, muscle aches
• fussiness or sleepiness in babies
• a mild rash or swollen glands 1 to 2 weeks after MMR or chickenpox vaccines

What helps: a cool compress, moving the arm, fluids, and if needed acetaminophen (paracetamol) or ibuprofen at the label dose. Do not give aspirin to children or teenagers. Routine pain relievers before a vaccine are not recommended.

Uncommon or rare

• Febrile seizures: brief seizures with fever in young children; they occur slightly more after some vaccines but are much more common with the infections themselves, and do not cause lasting harm.
• Fainting: especially in teens, which is why people sit for 15 minutes after a shot.
• Anaphylaxis: about 1 per million doses; clinics are equipped to treat it.
• Guillain-Barré syndrome: very rare after some flu, shingles and RSV vaccines; see Guillain-Barré syndrome.
• Myocarditis: rare after mRNA COVID-19 vaccines, mainly in young males, usually mild; the risk is higher after COVID-19 infection itself.
• Intussusception: a very small increased risk after rotavirus vaccine in infants.

What vaccines do not cause

Large studies involving millions of children have found no link between vaccines and autism. Vaccines also do not cause the diseases they prevent (inactivated vaccines cannot), do not overload the immune system, and do not contain harmful levels of ingredients such as aluminum or formaldehyde, which occur naturally in larger amounts in food and the body.

Who should delay or avoid certain vaccines

• Severe allergic reaction to a previous dose or a vaccine ingredient: avoid that vaccine; seek allergist advice.
• Moderate or severe acute illness: delay until better. A cold or mild illness is not a reason to delay.
• Pregnancy and severely weakened immunity: avoid live vaccines.
• Recent blood products or antibody treatments: may need to delay live vaccines.

Why vaccination matters

Before vaccines, measles caused an estimated 400 to 500 deaths and 48,000 hospitalizations a year in the US. Polio paralyzed thousands of children each year. Diphtheria was a leading cause of death in children. Smallpox has been eradicated worldwide through vaccination, and wild polio remains in only a few countries. When vaccination rates fall, diseases such as measles and whooping cough return, as recent outbreaks have shown.

Vaccine safety monitoring

Vaccines are tested in clinical trials before approval and monitored afterward through systems such as VAERS, the Vaccine Safety Datalink and international surveillance. Anyone can report a suspected reaction. In the US, the National Vaccine Injury Compensation Program provides compensation for rare proven injuries.

Frequently asked questions

How do vaccines work?

Vaccines expose the immune system to a harmless form or part of a germ. The body makes antibodies and memory cells against it. If the real germ appears later, the immune system recognizes it quickly and fights it off before it can cause serious illness.

Are vaccines safe?

Vaccines are among the most studied medical products. Most side effects are mild, such as a sore arm or low fever. Serious reactions are rare, and ongoing monitoring systems check safety after approval. For nearly everyone, the risks of the diseases are far greater than the risks of the vaccines.

What are the most common side effects of vaccines?

The most common side effects are pain, redness or swelling at the injection site, low fever, tiredness, headache and muscle aches. They usually start within a day and last 1 to 2 days. Rashes or mild fever can appear 1 to 2 weeks after MMR or chickenpox vaccines.

Which vaccines are recommended during pregnancy?

Tdap is recommended in every pregnancy, ideally at 27 to 36 weeks, to protect the newborn against whooping cough. The flu vaccine is recommended in any trimester during flu season, and an RSV vaccine may be offered seasonally late in pregnancy. Live vaccines are avoided.

What happens if my child misses a vaccine?

You do not need to restart the series. Doctors use catch-up schedules to give missed doses as soon as possible, sometimes several at one visit. Ask your child's doctor or local health department to review their vaccine record.

Can adults get vaccines they missed as children?

Yes. Adults who missed MMR, chickenpox, hepatitis B, HPV (up to 26, sometimes 45) or tetanus vaccines can catch up. A blood test can sometimes show immunity if records are missing. Your doctor or pharmacist can recommend which vaccines you need.