Are vaccines safe?
Written by AI from saved sources. A script checks every quote word for word, and a second AI compares each claim with its source. How we work and corrections
Yes, vaccines are very safe overall, but no vaccine is completely safe for every person. Side effects are usually very minor and brief; serious harms are possible but extremely rare. The answer depends on the vaccine, the dose and who receives it.
Checked against: WHO, FDA, CDC, October 2026
Why people believe it
Worry about side effects deserves a straight answer. A 2025 KFF/Washington Post survey covered US parents and guardians with children under 18 living in their household. Among respondents, 436 reported delaying or skipping childhood vaccines other than COVID-19 or flu vaccines. Among those 436 respondents, 67% named side-effect concerns as a major reason. In the same group, 53% named distrust of vaccine safety as a major reason. Those figures describe that subgroup, not all parents. CDC also argues that seeing less of the diseases vaccines prevent shifts attention towards vaccine risks.
“Have they really been tested?”
FDA says phase 3 trials generally give the vaccine to thousands of people. They compare people who get the vaccine with a comparison group. But these tests can miss rare reactions, delayed harms or problems affecting particular groups. After approval, CDC’s Vaccine Safety Datalink compares rates of health events in medical records every week. If the vaccinated group’s rate exceeds the comparison group’s rate by more than a certain limit, CDC investigates further.
“They can make you ill”
Yes: CDC lists several side effects after mRNA COVID vaccines, the Pfizer-BioNTech and Moderna type. Examples include pain where the injection went in, tiredness, headache and fever. In COVID vaccine trials, most reactions were mild to moderate and cleared within one to three days. For MMR, the measles, mumps and rubella vaccine, CDC reports a temporary rash in about 5% of recipients.
“Some people have serious reactions”
CDC reports about one fever-related seizure per 3,000–4,000 MMR doses, usually six to fourteen days afterwards. In children, a personal or family history of fever-related seizures, or a family history of epilepsy, may raise this risk. CDC reports that an expert review found evidence that MMR can cause these seizures.
After COVID vaccination, inflammation in or around the heart (myocarditis or pericarditis) has rarely been observed. It has been reported most often in adolescent and young adult males within seven days of a second mRNA dose. Cases have also been observed in females and after other doses. An analysis of commercial health-insurance claims examined the 2023–2024 mRNA COVID vaccine formulas. It estimated about 27 myocarditis or pericarditis cases per million doses in males aged 12–24. These cases were during days one to seven after vaccination. That estimate of cases after vaccination has not been adjusted. It does not mean 27 additional cases caused by vaccination.
“But look at all the injury reports”
The US reporting system VAERS looks for unusual patterns that need investigation. Its reports can be incomplete, inaccurate, coincidental or unverified. Report totals alone cannot show what a vaccine caused or how often it causes harm.
Thinking tool: compared with what?
WHO recommends giving risks as numbers of cases out of the same total, such as cases per 100 people. A subset, or smaller group, was monitored in RV5 rotavirus vaccine trials in infants. In that group, diarrhoea in the first week after any dose occurred in 18.1% of vaccine recipients. In the same period, it occurred in 15.3% of placebo recipients. So counting every illness in the vaccinated group as vaccine harm would include illnesses also seen in the comparison group. Rates of serious events, including deaths, were similar in the RV5 vaccine and comparison groups. This comparison covered the 42 days after any dose.
For other claims, ask: compared with whom, over how long, and out of how many?
Official positions, with dates
WHO, 23 September 2025: vaccines are very safe, with possible side effects. FDA, 14 December 2020: approval weighs benefits and risks for the intended population and disease. CDC, 24 April 2024: MMR and MMRV should not be given during pregnancy or with severe immune suppression.
What people also ask
What are the possible side effects of getting vaccinated?
Side effects depend on the vaccine. Examples include the mRNA COVID reactions above and joint symptoms after MMR. Those joint symptoms are reported in up to 25% of adult women.
What are the most serious side effects of vaccines?
One established example is anaphylaxis, a severe allergic reaction, after MMR. The heart inflammation discussed above is another safety concern after COVID vaccination. These examples do not rank every vaccine’s risks.
Do vaccines cause long-term side effects?
MMR-related fever seizures do not appear linked to lasting effects. FDA reports on a study of hospitalized patients with myocarditis associated with COVID-19 vaccination. At follow-up, abnormalities on heart MRI scans were commonly still present. The median follow-up, or middle follow-up time, was about five months after vaccination. What these findings mean for patients is unknown. What they mean for future health is also unknown.
Are vaccines 100% safe?
No: FDA explicitly says no vaccine is completely safe in every individual.
Check it yourself (5 minutes)
- Open CDC’s Chapter 4: Vaccine Safety.
- Use Find for “Further assessment”.
- Read the sentence: does an illness happening afterwards establish that vaccination caused it?
What you should have found
“Further assessment is needed to determine if an adverse event is caused by a vaccine.”
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