FAQ – Vaccine Basics and Safety

What do vaccines do?

Vaccines help your immune system recognize and fight germs to prevent serious illness. They help your body respond faster if you’re exposed again, and they can also protect others by reducing spread.

Are vaccines safe?

Yes. Vaccines used in the United States go through careful testing before they are approved, and safety is monitored continuously after they are in use. Most side effects are mild and short-lived, like a sore arm or a low fever. Serious reactions are rare.

How do we know vaccines are safe over time?

Vaccine safety is monitored through reporting systems and ongoing studies that look for rare side effects. If concerns are found, recommendations can change and vaccines can be updated. This is why vaccines are closely tracked over time.

Do vaccines work?

Yes. Vaccines reduce the risk of severe illness, hospitalization, and death. Some vaccines also reduce the chance of infection and spread, but no vaccine prevents every infection in every person. Protection can be especially important for babies, older adults, and people with certain health conditions.

Why do vaccinated people sometimes still get sick?

Because protection isn’t all-or-nothing. Protection can vary by age, health, time since vaccination, and changes in the germ. If a vaccinated person does get infected, vaccines often make the illness less severe and last shorter, with a lower risk of severe complications.

Can vaccines give you the disease?

No. Routine vaccines cannot give you the disease they protect against. Some vaccines use inactivated (killed) germs, and others use only a piece of the germ or instructions that help your body recognize it. You might feel tired, achy, or feverish afterward, but that is your immune system responding.

What is “immunity,” and how is it different from being sick?

Immunity means your immune system is prepared to fight a germ. You can gain immunity after an infection, but that can come with serious risks. Vaccination helps build protection without having to get sick first.

Are vaccines safer than getting the disease?

For most people, yes. Many vaccine-preventable diseases can cause severe illness, long-term complications, or hospitalization. Vaccine side effects are usually mild and temporary, while the diseases they prevent can be much more serious.

Who should not get certain vaccines?

Some people should delay or avoid specific vaccines, such as:

• People who have had a severe allergic reaction to a previous dose or vaccine ingredient
• People with certain immune system conditions, depending on the vaccine
• People who are pregnant, for certain vaccines (others are strongly recommended during pregnancy)

If you have questions about your situation, talk with your health care provider or contact the Immunization Program.

What if I’m worried about an allergic reaction?

Severe allergic reactions are rare. Vaccine clinics are prepared to respond, and people are usually asked to wait briefly after vaccination. If you have a history of severe allergies or have had a reaction to a vaccine before, talk with your health care provider before getting vaccinated.

What does “recommended” vs “required” mean?

“Recommended” vaccines are advised because they protect health and help prevent outbreaks. “Required” vaccines are those California requires for school or childcare entry (with limited exceptions). Even if a vaccine isn’t required for school, it may still be strongly recommended. Some workplaces may also have requirements.

Who decides which vaccines are recommended?

Recommendations are based on scientific evidence about safety and effectiveness, and they are updated as new data becomes available. In the United States, national recommendations are developed by expert committees and used by health departments and health care systems to guide immunization schedules.

Are vaccines tested on children?

Yes. Vaccines are studied carefully, including age-specific studies, before they are authorized for children. Dosing and schedules are based on research about what produces the best protection with the fewest side effects for each age group.

What should I do if I have more questions?

If you have a medical condition, are pregnant, have questions about your child’s schedule, or have had a reaction to a vaccine, talk with your health care provider. You can also contact the Alameda County Immunization Program for general questions and help finding resources.