Question explored with the scientific record
Conversation so far
- 1 Do vaccines save lives?
- 2 What are the main risks or harms, and how common are they by vaccine?
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What are the main risks or harms, and how common are they by vaccine?
The short version: the harms are real and mostly rare, but the rates are wildly uneven by vaccine, and the most serious ones cluster in specific products and specific people.
The clearest numbers are anaphylaxis, which is the immediate severe allergic reaction. Across 61 million COVID-19 doses in Mexico it hit about 1 in 1 million overall, but the rate varied by product: roughly 5 in 1 million for Moderna, 2 in 1 million for Pfizer, and under 1 in 1 million for AstraZeneca and CoronaVac [7]. A Japanese Pfizer report found a much higher suspected rate of about 8 in 100,000 doses, though only a quarter of those were confirmed severe [6]. For older vaccines, anaphylaxis ran about 1 in 100,000 doses for DPT and influenza and under 1 in 100,000 for Japanese encephalitis [1].
| Vaccine / harm | Rate (as reported) | Source |
|---|---|---|
| COVID-19, anaphylaxis (all brands) | ~1 per 1,000,000 doses | [7] |
| Moderna, anaphylaxis | ~5 per 1,000,000 doses | [7] |
| DPT, anaphylaxis | ~1 per 100,000 doses | [1] |
| MMR, serious events | ~1 per 100,000 doses | [5] |
| Yellow fever, neurologic events | ~10 per 100,000 doses | [3] |
| mRNA, myocarditis (males 12–17) | 50–139 per 1,000,000 | [8] |
| Pandemrix flu, narcolepsy (kids) | ~1 in 16,000 (Finland) | [4] |
Myocarditis is the harm that matters most for young males after mRNA shots. The BMJ review puts it at 50 to 139 cases per million in boys 12 to 17, mostly after the second dose, and over half of a small followed group still had abnormalities at three months [8]. The Pandemrix flu vaccine caused narcolepsy in roughly 1 in 16,000 Finnish children [4]. Yellow fever's neurologic rate was about 10 per 100,000 doses [3]. MMR's serious events were about 1 per 100,000 [5].
Most of these figures come from passive reporting systems like VAERS, which catch only a fraction of what happens, so treat them as floors, not ceilings [2][5]. The long-term data does not exist. My call: the serious harms are uncommon but not evenly spread, and the myocarditis signal in young males is the one I would weigh hardest before a booster. Confidence: moderate on the rates, low on long-term harm.
Sources used 8
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Vaccine adverse events reported in post-marketing study of the Kitasato Institute from 1994 to 2004
This study analyzes vaccine adverse events reported in Japan from 1994 to 2004, categorizing them into allergic reactions and severe systemic illnesses, and highlights the low incidence of serious adverse events associated with vaccination.
DOI: 10.1016/j.vaccine.2006.05.130 -
Preparation for global introduction of inactivated poliovirus vaccine: safety evidence from the US Vaccine Adverse Event Reporting System, 2000–12
This study analyzes adverse events reported to the US Vaccine Adverse Event Reporting System (VAERS) following the administration of the inactivated poliovirus vaccine (IPV) from 2000 to 2012, finding a low incidence of serious adverse events and no new safety concerns associate…
DOI: 10.1016/s1473-3099(15)00059-6 -
Incidence of Yellow Fever Vaccine-Associated Neurotropic Disease
In a retrospective case series from a single French referral center, 4 serious neurologic events occurred within 30 days after 17D yellow fever vaccination among 40,404 doses, giving an estimated incidence of 9.9 per 100,000 doses, higher than previous estimates because acute me…
DOI: 10.4269/ajtmh.2009.09-0295 -
Adverse Events After Pandemic Flu A/H1N1 Vaccination In Children: Concern About Vaccine Safety
An integrative review of the literature examining adverse events following pandemic influenza A/H1N1 vaccination in children, with particular attention to vaccine safety and the rare but serious complication of narcolepsy with cataplexy.
DOI: 10.5200/sm-hs.2016.072 -
Surveillance and analysis of adverse events following immunization (AEFI) with measles, mumps, and rubella (MMR) vaccine in Zhejiang Province, China, 2020–2025
MMR vaccination in Zhejiang Province, China (2020-2025, n=5,909,900 doses) showed an overall AEFI incidence of 213.15 per 100,000 doses, with 95.49% of cases resolving spontaneously without intervention and only 68 serious events (1.15 per 100,000 doses), supporting a favorable …
DOI: 10.3389/fpubh.2026.1846354 -
Cumulative Adverse Event Reporting of Anaphylaxis After mRNA COVID-19 Vaccine (Pfizer-BioNTech) Injections in Japan: The First-Month Report
This study reports on the incidence of anaphylaxis and anaphylactoid symptoms following the administration of the Pfizer-BioNTech COVID-19 vaccine in Japan during the first month of the vaccination program, highlighting a total of 181 suspected cases with a reporting rate of 8.1…
DOI: 10.1007/s40264-021-01104-9 -
Anaphylaxis to SARS-CoV-2 Vaccines in the Setting of a Nationwide Passive Epidemiological Surveillance Program
This nationwide observational study in Mexico identified an overall incidence of anaphylaxis of 1.07 cases per 1,000,000 doses among recipients of various SARS-CoV-2 vaccines, highlighting the relative safety of these vaccines despite some cases of severe reactions.
DOI: 10.1007/s10875-022-01350-1 -
Incidence, risk factors, natural history, and hypothesised mechanisms of myocarditis and pericarditis following covid-19 vaccination: living evidence syntheses and review
This living systematic review found that myocarditis after mRNA COVID-19 vaccination occurs most frequently in male adolescents and young adults, with moderate-certainty evidence that Moderna is associated with higher incidence than Pfizer in 18-29 year olds and low-certainty ev…
DOI: 10.1136/bmj-2021-069445