Vaccine Safety: What the Evidence Shows
Medically reviewed by Dr. Michael Okonkwo, MD, FAAP · Board-Certified Neonatologist
Content reviewed against published CDC, AAP, NIH guidelines
Last reviewed:
If your baby has been diagnosed with or you suspect vaccine safety: what the evidence shows, here is what the evidence says.
The short answer
Vaccines are among the most rigorously tested medical products in existence, undergoing years of clinical trials before approval and continuous safety monitoring afterward. Large-scale studies involving millions of children have consistently confirmed that childhood vaccines are safe and do not cause autism or other developmental disorders. The benefits of vaccination vastly outweigh the small risk of mild, temporary side effects.
Key takeaways
- Vaccines are among the most rigorously tested medical products in existence, undergoing years of clinical trials before approval and continuous safety monitoring afterward. Large-scale studies involving millions of children have consistently confirmed that childhood vaccines are safe and do not cause autism or other developmental disorders. The benefits of vaccination vastly outweigh the small risk of mild, temporary side effects.
- Usually normal when: Mild, temporary side effects like soreness, low-grade fever, or fussiness after vaccination
- Call your doctor if: Your child develops signs of a severe allergic reaction within minutes to hours of vaccination, including difficulty breathing, swelling of the face or throat, rapid heartbeat, or widespread hives -- call 911
- Varies by age — see the age-by-age breakdown below
“Fever itself is not an illness — rather, it is a sign or symptom that the body is fighting an infection. Fever stimulates certain defenses, such as the white blood cells, which attack and destroy invading bacteria.”
Thousands of parents search for this exact thing. You are not alone.
What Parents Should Know
According to CDC, AAP, NIH guidelines, vaccines are among the most rigorously tested medical products in existence, undergoing years of clinical trials before approval and continuous safety monitoring afterward. Large-scale studies involving millions of children have consistently confirmed that childhood vaccines are safe and do not cause autism or other developmental disorders. The benefits of vaccination vastly outweigh the small risk of mild, temporary side effects. At 0-3 months, the vaccines given at birth and 2 months have been studied in millions of newborns and young infants. Research shows that a baby's immune system, while immature, is fully capable of responding to vaccine antigens. In fact, babies encounter thousands of antigens daily from their environment. The few antigens in vaccines represent a tiny fraction of what their immune system already handles. It is generally considered normal when mild, temporary side effects like soreness, low-grade fever, or fussiness after vaccination. However, you should contact your pediatrician promptly if your child develops signs of a severe allergic reaction within minutes to hours of vaccination, including difficulty breathing, swelling of the face or throat, rapid heartbeat, or widespread hives -- call 911.
Normal vs. Concerning
By Age
What to expect by age
0-3 months
The vaccines given at birth and 2 months have been studied in millions of newborns and young infants. Research shows that a baby's immune system, while immature, is fully capable of responding to vaccine antigens. In fact, babies encounter thousands of antigens daily from their environment. The few antigens in vaccines represent a tiny fraction of what their immune system already handles.
3-6 months
Multiple studies have examined the safety of giving several vaccines at the same visit and have found no increased risk of adverse events. Combination vaccines reduce the total number of injections while providing the same protection. The safety monitoring systems in place, including VAERS and the Vaccine Safety Datalink, continuously track millions of vaccine doses for any safety signals.
6-12 months
By this age, your baby has likely received several rounds of vaccines. A landmark 2004 Institute of Medicine review and numerous subsequent studies have conclusively found no link between vaccines and autism. The original study claiming such a link was retracted due to fraud. Ongoing research continues to confirm the safety of the childhood vaccine schedule.
12-24 months
Live vaccines such as MMR and varicella, given around 12 months, have been studied extensively. A 2019 study of over 650,000 children in Denmark reaffirmed that the MMR vaccine does not increase the risk of autism, even in children with risk factors. Vaccine ingredients like aluminum salts and trace preservatives have been studied at the amounts present in vaccines and are safe.
What to Tell Your Pediatrician
- Describe when you first noticed vaccine safety: what the evidence shows and how it has changed over time.
- Note your baby's current age and which age-specific patterns you are seeing.
- Mention if you have specific concerns about vaccine ingredients or the number of vaccines and want to discuss the research.
- Mention if your child or a family member has a history of a severe allergic reaction and you want to understand which vaccines are safe.
- Bring a list of any questions or observations you want to discuss at the appointment.
What Should You Do?
When to take action
- Mild, temporary side effects like soreness, low-grade fever, or fussiness after vaccination
- Feeling uncertain about vaccines and wanting to review the evidence with your pediatrician
- A small hard lump at the injection site that gradually resolves over weeks
- You have specific concerns about vaccine ingredients or the number of vaccines and want to discuss the research
- Your child or a family member has a history of a severe allergic reaction and you want to understand which vaccines are safe
- You have encountered information online that worries you and want your pediatrician to help you evaluate it
- Your child has an immune system disorder and you want guidance on which vaccines are appropriate
- Your child develops signs of a severe allergic reaction within minutes to hours of vaccination, including difficulty breathing, swelling of the face or throat, rapid heartbeat, or widespread hives -- call 911
- Your child has a seizure or becomes unresponsive after receiving a vaccine
What You Can Do at Home
- Keep track of when you notice vaccine safety: what the evidence shows — noting the time of day, duration, and any triggers can help your pediatrician.
- Remember that mild, temporary side effects like soreness, low-grade fever, or fussiness after vaccination — this is generally within the range of normal.
- At 0-3 months, focus on observation rather than intervention unless your pediatrician advises otherwise.
- Follow any care instructions from your pediatrician. Keep a written log of symptoms to bring to appointments.
- While monitoring at home, seek immediate care if your child develops signs of a severe allergic reaction within minutes to hours of vaccination, including difficulty breathing, swelling of the face or throat, rapid heartbeat, or widespread hives -- call 911.
Related Resources
Vaccine Schedule Tracker
Track vaccinations, see what's due, and understand each vaccine.
Month-by-Month Development
Detailed monthly development guides from birth through 24 months.
When to Call the Doctor
General guide on when to call the pediatrician, visit urgent care, or go to the ER.
Frequently asked questions
Is vaccine safety: what the evidence shows normal?
When should I call the doctor about vaccine safety: what the evidence shows?
When is vaccine safety: what the evidence shows normal?
What causes vaccine safety: what the evidence shows?
What should I mention to my pediatrician about vaccine safety: what the evidence shows?
Is vaccine safety: what the evidence shows normal at 0-3 months?
Is vaccine safety: what the evidence shows normal at 3-6 months?
Should I go to the ER for vaccine safety: what the evidence shows?
Does vaccine safety: what the evidence shows go away on its own?
References
- [1]Centers for Disease Control and Prevention. Vaccine Safety: Overview, History, and How the Safety Process Works. CDC
- [2]American Academy of Pediatrics. Vaccine Safety: Examine the Evidence. HealthyChildren.org. AAP
- [3]Hviid, A. et al. Measles, Mumps, Rubella Vaccination and Autism: A Nationwide Cohort Study. Annals of Internal Medicine, 2019. NIH
Doctor Visit Checklist
Bring this checklist to your next pediatrician visit to discuss Vaccine Safety: What the Evidence Shows.
Things to mention
- Describe when you first noticed vaccine safety: what the evidence shows and how it has changed over time.
- Note your baby's current age and which age-specific patterns you are seeing.
- Mention if you have specific concerns about vaccine ingredients or the number of vaccines and want to discuss the research.
- Mention if your child or a family member has a history of a severe allergic reaction and you want to understand which vaccines are safe.
- Bring a list of any questions or observations you want to discuss at the appointment.
Observations to share
- You have specific concerns about vaccine ingredients or the number of vaccines and want to discuss the research
- Your child or a family member has a history of a severe allergic reaction and you want to understand which vaccines are safe
- You have encountered information online that worries you and want your pediatrician to help you evaluate it
Urgent signs to report immediately
- Your child develops signs of a severe allergic reaction within minutes to hours of vaccination, including difficulty breathing, swelling of the face or throat, rapid heartbeat, or widespread hives -- call 911
- Your child has a seizure or becomes unresponsive after receiving a vaccine
My notes
From ismybabyalright.com — free, evidence-based baby health guides
All content follows our editorial policy and is reviewed against published clinical guidelines.
2,705 evidence-based guides6 authoritative medical sources5 medical advisory board members
Related Resources
Bottom line
Most cases of vaccine safety: what the evidence shows are normal. Talk to your pediatrician if your child develops signs of a severe allergic reaction within minutes to hours of vaccination, including difficulty breathing, swelling of the face or throat, rapid heartbeat, or widespread hives -- call 911.
Trust your instincts. If something feels wrong, reach out to your pediatrician. Worrying about your baby means you care — that is a good thing.
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