{ "slug": "vaccine-serious-adverse-event", "question": "What are the odds of a serious adverse event from a routine vaccine?", "category": "health", "no_reliable_estimate": false, "perceived": { "description": "Public perception of vaccine serious adverse event (SAE) rates runs orders of magnitude above the measured numbers, driven by a combination of very vivid individual case reports, the 2020-2022 COVID-mRNA myocarditis signal, and the asymmetry between a population-wide intervention and a background disease most readers never directly see. We have not found a recent rigorous poll that isolates “fear of a serious adverse event from a routine vaccine” from the much broader category of general vaccine hesitancy, so the perceived entry here is editorial intuition, not survey. The directionally robust observation is that when asked to guess SAE rates, readers typically name figures in the low single-digit percent range per dose — roughly three to four orders of magnitude above the measured per-dose rate from the Vaccine Safety Datalink.\n", "rough_estimate": "most readers guess the risk at 1 in 100 or worse per dose; measured rates are ~1 in a million for anaphylaxis", "kind": "intuition" }, "native": { "display": "~1.31 anaphylaxis cases per million vaccine doses (Vaccine Safety Datalink, 2009-2011)", "numerator": 33, "denominator": 25173965, "unit": "per vaccine dose", "population": "US patients in the Vaccine Safety Datalink, 2009-2011, all routine vaccines" }, "normalized": { "lifetime_us_adult": 0.00000131, "display": "~1 in 760,000 per dose (anaphylaxis, all routine vaccines)", "log_value": -5.88, "assumptions": "The headline figure is the anaphylaxis rate from McNeil et al. (JACI 2016), the largest prospective US study of vaccine-triggered anaphylaxis: 33 confirmed cases across 25,173,965 doses in the Vaccine Safety Datalink, for a rate of 1.31 per million doses (95% CI 0.90 to 1.84). We use anaphylaxis as the canonical SAE anchor because it is the best-measured, most consistently defined serious adverse event across the routine vaccine schedule, and because it is the specific “serious” outcome most often invoked in pre-vaccination consent discussions. Scope is declared as activity_specific_lifetime: this is a per-dose probability for the specific activity of receiving a routine vaccine, not a general US-adult-lifetime number, and is not directly comparable to the population-lifetime figures on other Likelier pages. Other measurable SAE signals across the routine schedule sit within the same order of magnitude: Guillain-Barre after influenza vaccine at approximately 1 to 2 additional cases per million doses, historical oral polio vaccine-associated paralytic polio at approximately 1 per 2 to 3 million doses. The notable outlier is post-2021 myocarditis after mRNA COVID-19 second doses in young males, measured at approximately 40.6 per million second doses in males 12 to 29 per the CDC ACIP / Shimabukuro presentation to MMWR in June 2021, with higher rates (62.8 per million) in the 12 to 17 male subgroup. That signal is real and is the subject of the regional_breakdown and caveats sections below; it is also an order of magnitude above the all-vaccine anaphylaxis baseline, which is why Likelier reports both. Almost all measured SAEs resolve without lasting harm: McNeil et al. report zero deaths and only one hospitalization among the 33 anaphylaxis cases.\n", "uncertainty": { "low": 9e-7, "high": 0.00000184 }, "scope": "activity_specific_lifetime" }, "sources": [ { "url": "https://pubmed.ncbi.nlm.nih.gov/26452420/", "title": "Risk of anaphylaxis after vaccination in children and adults", "publisher": "Journal of Allergy and Clinical Immunology / McNeil MM, Weintraub ES, Duffy J, et al. (PubMed / NLM)", "source_type": "peer_reviewed", "statistic": "33 confirmed vaccine-triggered anaphylaxis cases across 25,173,965 doses in the Vaccine Safety Datalink, 2009-2011; rate of 1.31 per million doses (95% CI 0.90-1.84); zero deaths, one hospitalization", "excerpt": "\"We identified 33 confirmed vaccine-triggered anaphylaxis cases that occurred after 25,173,965 vaccine doses. The rate of anaphylaxis was 1.31 (95% CI, 0.90-1.84) per million vaccine doses.\"\n", "source_date": "2016-03-01", "source_accessed": "2026-04-11", "archive_url": "http://web.archive.org/web/20260421201335/https://pubmed.ncbi.nlm.nih.gov/26452420/", "calculation_notes": "McNeil et al. is a prospective cohort analysis across five Vaccine Safety Datalink sites over 2009-2011. The 33-case numerator divided by the 25,173,965-dose denominator gives the headline rate of ~1.31 per million doses used directly as the native value. Normalized value = numerator / denominator = 33 / 25,173,965 ≈ 1.31 × 10^-6. Uncertainty band uses the paper’s own 95% confidence interval of 0.90 to 1.84 per million, i.e. 9.0 × 10^-7 to 1.84 × 10^-6. Note: incorporating the mRNA myocarditis signal (40.6 per million = 4.06 × 10^-5) would widen the band considerably, but that signal is subgroup- and vaccine-specific and is reported separately in the regional_breakdown. The paper separately reports that all 33 cases recovered, with no deaths and only a single hospitalization, which is the basis for the “resolves without lasting harm in most cases” framing in the long-form body.\n", "independence_note": "VSD-based. Draws on the same five-site cohort that CDC uses for most of its per-dose vaccine safety surveillance. Subsequent reviews (e.g., Dreskin et al. in Current Treatment Options in Allergy 2019) cite this paper as the canonical US per-dose anaphylaxis rate.\n" }, { "url": "https://www.cdc.gov/mmwr/volumes/70/wr/mm7027e2.htm", "title": "Use of mRNA COVID-19 Vaccine After Reports of Myocarditis Among Vaccine Recipients: Update from the Advisory Committee on Immunization Practices — United States, June 2021", "publisher": "Morbidity and Mortality Weekly Report / Centers for Disease Control and Prevention", "source_type": "govt_report", "statistic": "40.6 myocarditis cases reported per million second doses of mRNA COVID-19 vaccine in males aged 12-29; 62.8 per million in males 12-17 and 50.5 per million in males 18-24", "excerpt": "\"Myocarditis reporting rates were 40.6 cases per million second doses of mRNA COVID-19 vaccines administered to males aged 12−29 years.\"\n", "source_date": "2021-07-09", "source_accessed": "2026-04-11", "archive_url": "http://web.archive.org/web/20260405172824/https://www.cdc.gov/mmwr/volumes/70/wr/mm7027e2.htm", "calculation_notes": "The MMWR report is the ACIP / Shimabukuro presentation that crystallized the myocarditis signal. Used here not as the headline number but as the counter-example that explains why Likelier reports real SAE signals even when they are small. The 40.6 per million figure is approximately 30 times the all-vaccine anaphylaxis baseline of 1.31 per million, which is why the long-form body singles it out: it is the one recent routine-schedule vaccine SAE signal that is meaningfully above the century-average background rate. The same MMWR and follow-up CDC clinical guidance emphasize that the majority of reported myocarditis cases were mild and self-resolving with conservative management, which is what determines how Likelier reports the signal: a real elevated rate, a real clinical follow-up burden, but overwhelmingly mild outcomes.\n", "independence_note": "This is the CDC / ACIP primary surveillance report, drawing on VAERS and V-Safe data which are the US federal vaccine safety monitoring systems. The clinical and cardiology follow-up literature (Mevorach et al. NEJM 2021, Witberg et al. NEJM 2021, Patone et al. Nature Medicine 2022) replicates the signal in independent Israeli and UK datasets, so the effect itself is not a single-source artefact even though the MMWR number is cited here as the US authoritative figure.\n" } ], "comparison_anchors": [ { "label": "Fatal anaphylaxis from any cause (lifetime, US adult)", "lifetime_us_adult": 0.0000363 }, { "label": "Death in a plane crash (lifetime, US adult)", "lifetime_us_adult": 0.000017 }, { "label": "Death by lightning strike (lifetime, US)", "lifetime_us_adult": 0.00000354 }, { "label": "Death from measles (per measles case, pre-vaccine era)", "lifetime_us_adult": 0.001 } ], "regional_breakdown": [ { "region": "All routine vaccines, anaphylaxis (VSD, 2009-2011)", "probability": 0.00000131, "notes": "Headline figure from McNeil et al. JACI 2016. 33 confirmed cases / 25.2 million doses. Zero deaths, 1 hospitalization across all cases." }, { "region": "Influenza vaccine, anaphylaxis (inactivated trivalent, alone)", "probability": 0.00000135, "notes": "10 cases / 7,434,628 doses given alone, per McNeil et al. 1.35 per million, not meaningfully different from the all-vaccine average." }, { "region": "mRNA COVID-19 vaccine, myocarditis, males 12-29 (CDC ACIP, June 2021, 2nd dose)", "probability": 0.0000406, "notes": "40.6 per million second doses per MMWR 70(27). The one routine-schedule SAE signal in the past decade that is meaningfully above the all-vaccine anaphylaxis baseline. Clinical outcomes overwhelmingly mild and self-resolving per follow-up literature." }, { "region": "mRNA COVID-19 vaccine, myocarditis, males 12-17 (CDC ACIP, June 2021, 2nd dose)", "probability": 0.0000628, "notes": "62.8 per million second doses in the highest-signal subgroup. Still below 1 in 10,000 per dose." }, { "region": "Influenza vaccine, Guillain-Barre syndrome (excess over background)", "probability": 0.0000015, "notes": "Approximately 1-2 additional GBS cases per million doses above background, per CDC and post-2009-H1N1 pharmacovigilance studies. Order of magnitude matches the anaphylaxis rate." }, { "region": "Oral polio vaccine, vaccine-associated paralytic polio (historical)", "probability": 4e-7, "notes": "Approximately 1 case per 2-3 million doses of the oral (Sabin) vaccine, the reason the US switched to inactivated (Salk) vaccine in 2000. Included as historical context; the currently-used inactivated polio vaccine does not carry this risk." } ], "personal_factor_multipliers": [ { "factor": "known prior vaccine reaction", "multiplier": 5, "notes": "A documented prior anaphylactic or other serious reaction to a vaccine component is the strongest individual predictor of repeat-event risk, and is the standard indication for allergist-supervised administration. Order-of-magnitude estimate.\n" }, { "factor": "no prior issues, routine schedule", "multiplier": 1, "notes": "The population baseline from McNeil et al. applies: roughly 1.31 per million doses for anaphylaxis.\n" }, { "factor": "immunocompromised", "multiplier": 2, "notes": "For some live attenuated vaccines (MMR, varicella, yellow fever, oral typhoid, BCG, live attenuated influenza), immunocompromise is generally a contraindication rather than a quantified multiplier, because the dominant concern is strain replication rather than anaphylaxis. The multiplier here is a rough order-of-magnitude for inactivated-vaccine SAE elevation in immunocompromised patients, not a clinical recommendation.\n" }, { "factor": "young male, mRNA COVID-19 second dose", "multiplier": 30, "notes": "Specific to the myocarditis signal, not anaphylaxis. 40.6 per million in males 12-29 divided by the 1.31 per million all-vaccine anaphylaxis baseline gives a rough 30x factor. This is the only routine-schedule subgroup currently above the century-average SAE background.\n" } ], "short_label": "Vaccine reaction", "myth_framing": "overrated", "outcome_severity": "moderate_harm", "exposure_pattern": "acute", "outcome_type": "recoverable_injury", "valence": "negative", "caveats": "The 1.31 per million headline covers anaphylaxis specifically, which is the best-measured SAE across the routine vaccine schedule and the outcome most commonly cited in pre-vaccination consent. It is not a single all-inclusive “any serious adverse event” rate, because the definition of “serious” varies across surveillance systems. Broader aggregate rates of medically-attended serious adverse events across the full US routine schedule run roughly 1 in 100,000 to 1 in 1,000,000 per dose depending on definition, which brackets the anaphylaxis figure. Second caveat: post-2021 mRNA COVID-19 vaccines produced a real, measured elevated myocarditis signal in young males receiving second doses, at approximately 40.6 per million second doses in males 12-29 and up to 62.8 per million in males 12-17 per the CDC ACIP MMWR of June 2021. Likelier publishes real measured numbers, and that signal is real; the clinical follow-up literature (Mevorach et al., Witberg et al., Patone et al.) confirms that the observed cases were overwhelmingly mild and self-resolving with conservative management, but the elevated incidence itself is not in dispute. Third caveat, which is probably the most important comparison Likelier can offer: for every vaccine with a measurable SAE rate, the disease the vaccine prevents has a higher rate of the same SAE. Measles causes myocarditis more often than MMR. SARS-CoV-2 infection causes myocarditis more often than an mRNA second dose, including in young males. Measles case fatality runs roughly 1 per 1,000 in high-income settings, approximately 770 times the MMR anaphylaxis rate. The denominator on “risk of a serious event from a vaccine” is always best understood next to the denominator on “risk of the same serious event from the disease the vaccine prevents.” See anaphylaxis-fatal for the related entry on fatal anaphylaxis from all causes.\n", "quality_score": { "d1": 5, "d2": 5, "d3": 5, "d4": 4, "d5": 5, "d6": 4, "d7": 4, "d8": 5, "avg": 4.625, "scored_by": "claude-code-8d", "scored_at": "2026-05-25", "methodology_version": "1.2" }, "reviewer": "likelier-phase-5-agent", "last_reviewed": "2026-04-11", "reviewed": true, "generated_at": "2026-04-11", "image": { "alt": "A single small glass vial resting on a pale neutral surface, flat vector illustration in muted greys and soft blue." }, "attribution": "Likelier — https://likelier.app", "license": "https://creativecommons.org/licenses/by-sa/4.0/", "support": "https://buymeacoffee.com/kgluszczyk?via=likelier&utm_content=api-fear-single", "canonical_url": "https://likelier.app/vaccine-serious-adverse-event" }