The mRNA COVID-19 vaccines are usually argued about as if there were only one thing to say, and there are several, so this page states each at the weight the evidence supports. In two large placebo-controlled trials they were about 95% effective against symptomatic COVID-19, and in early national rollout they cut hospitalization and death by around 97% in older and high-risk people. Protection against catching the virus and passing it on was present at first and then waned within months, so the 2021 message that vaccination would reliably stop transmission did not hold.
They also carry specific risks: myocarditis concentrated in young men and highest after the second dose, usually mild; rare anaphylaxis; and a reanalysis of the manufacturers' own trials that found about 1 in 800 excess serious adverse events. Two widely shared alarms, that the vaccine spike crosses into the brain and does harm and that embalmers are finding a novel vaccine-caused clot, are not established, and this page shows exactly how it reaches that. This is education about the evidence, not medical advice or a recommendation for or against.
Findings & Outcomes
This page follows the evidence in both directions and grades each finding at the strength the studies support. It shows where the science is settled, where it is still open, and where a widely repeated claim does not hold up. Nothing here is a treatment recommendation. Each finding states what the studies show, who was measured, and what the limits are, with a source for every one.
The science and the question of trust became tangled during the pandemic, and this page keeps both, because separating them would misrepresent what happened. The benefit, the risk, and the uncertainty sit in one place, each at the weight the evidence supports.
The treatment-politics story of the pandemic, remdesivir and the sidelining of cheaper drugs, has its own page, COVID-19 treatments. This page stays on the vaccines.
How mRNA Vaccines Work
An mRNA vaccine is a set of instructions, not a piece of the virus. It delivers a strand of messenger RNA, wrapped in a protective lipid nanoparticle, that instructs a person's own cells to make one part of the SARS-CoV-2 virus, the spike protein. The immune system meets that spike protein, learns to recognize it, and is then prepared if the real virus arrives. The mRNA itself is short-lived, broken down by the body within days, and it does not enter the cell nucleus and does not alter DNA. This is the mechanism the technology rests on, and it was worked out over roughly two decades of research before the pandemic made it famous.
Because the vaccine makes the body produce spike protein for a short time, how much spike is made, where it goes, and how long it lasts are measurable questions, and the answers appear below. The mechanism is well established. What is debated is the size of specific benefits and harms, which the trials and surveillance systems were built to measure.
What The Trials Showed
The pivotal evidence is strong and was produced quickly by the standards of vaccine development. Two large randomized, placebo-controlled trials tested the mRNA vaccines against the original virus, and efficacy in both was around 95% against symptomatic COVID-19 over the weeks that followed. In the Pfizer-BioNTech trial of about 43,000 people there were 8 cases in the vaccine group against 162 in placebo, for 95.0% efficacy; in the Moderna trial of about 30,000 people, 11 cases against 185, for 94.1%. These enrolled tens of thousands of people each, they were placebo-controlled, and their primary result was a hard, prespecified outcome. On the question they were designed to answer, preventing symptomatic disease from the original strain, the trials were about as strong as vaccine trials get.
No trial could measure how long protection would last, how it would hold against variants that had not yet emerged, or how the rarer safety signals would look at population scale. Those questions passed to real-world surveillance once hundreds of millions of doses were given, and the answers there are more mixed than the trial result, in both the reassuring and the cautionary direction.
Where The Benefit Held And Where It Waned
The clearest benefit in real-world use was against severe disease. In Israel's nationwide rollout in early 2021, two doses were associated with very high protection against the outcomes that matter most:
- around 97% against hospitalization,
- 97.5% against severe or critical disease,
- and 96.7% against death, in that early period.
Benefit of that size, concentrated in older and higher-risk people, is the core of the case for these vaccines, and it is well supported.
Protection against catching the virus and passing it on is more measured, and public messaging outran the evidence here. Early on, the vaccines did reduce infection, but that protection waned substantially with time and with the arrival of the Omicron variant. Against symptomatic Omicron, two doses were about 65.5% effective at 2 to 4 weeks and fell below 10% by around 25 weeks, with a booster restoring protection to about two thirds before it too declined. The 2021 message that being vaccinated would reliably stop a person from catching or spreading the virus did not hold as the variants arrived. Protection against severe illness lasted far better than protection against infection, and the transmission question turns on that difference.
Immunity from a prior infection is substantial, especially against severe disease, and some public communication sidelined that fact, which drew fair criticism. Pooled across many studies, previous infection gave about 74.6% protection against hospitalization or severe disease at a year, while protection against catching the virus again waned to around 24.7%. The most durable protection of all, about 97.4% against severe disease at a year, came from hybrid immunity, infection plus vaccination together.
The Documented Safety Signals
The most important established harm is myocarditis, inflammation of the heart muscle, acknowledged by regulators including the CDC and FDA. It is concentrated in young males, is highest after the second dose, and appears somewhat more often after the Moderna product than the Pfizer-BioNTech one. Three independent surveillance systems measured it:
- United States surveillance confirmed 1,626 cases, median age 21, most of them in males, with reporting rates highest in males 16 to 17 at about 105.9 per million second doses of the Pfizer-BioNTech vaccine.
- Israeli active surveillance measured the risk in young men as several times the expected background rate.
- A Nordic cohort of about 23 million residents of Denmark, Finland, Norway and Sweden put the excess squarely in young men after the second dose, and higher after the Moderna product than the Pfizer-BioNTech one.
Scandinavia also acted on the signal early. In October 2021, before the question was settled, Sweden, Denmark, Finland and Norway paused or restricted the Moderna vaccine for younger age groups, a precautionary move made before the evidence was in.
The reassuring part sits alongside the signal, because a partial reading of either would mislead: the great majority of cases were mild and resolved, with most patients under 30 discharged after a short admission. Myocarditis is uncommon, it falls mostly on young men after the second dose, and the great majority of cases were mild and resolved. All of that is true at once.
The broader safety record has been examined at very large scale. A cohort of about 99 million vaccinated people across eight countries, assembled by the Global Vaccine Data Network, confirmed the myocarditis and pericarditis signals after the mRNA vaccines and flagged a rare inflammatory brain condition, acute disseminated encephalomyelitis, after a first Moderna dose. The same study reported signals for Guillain-Barré syndrome and for cerebral venous sinus thrombosis, but those followed the ChAdOx1 adenovirus-vector vaccine, a different technology and not an mRNA vaccine.
These events are rare in absolute terms, and a raised signal in that kind of analysis marks an event to investigate before any causal verdict. The study is the sort of independent, multi-country surveillance a serious safety picture needs, and it did not dismiss the signals. Severe allergic reactions, anaphylaxis, also occur, and are rare and treatable, on the order of a few per ten thousand vaccinations in one careful study, with the affected people recovering. The precise figures, the populations, and the caveats are in the cautions section below.
The Serious-Event Signal In The Trials, And What Surveillance Misses
The trials emphasized efficacy and reported the serious-harm side less prominently, and a later reanalysis measured the gap. Fraiman and colleagues, in a 2022 paper in Vaccine, pooled the serious adverse events of special interest, a prespecified watch-list of conditions, from the manufacturers' own randomized trial data. In the vaccinated groups they found an excess of about 12.5 per 10,000, roughly 1 in 800 (95% CI 2.1 to 22.9, risk ratio 1.43).
For every 10,000 people vaccinated in the trials, about 12 to 13 more had a serious adverse event of that kind than in the placebo groups. The signal is uncommon in absolute terms and larger than the trials' own framing suggested, which is why the authors called for formal harm-benefit analyses that account for a person's actual risk of severe COVID-19 instead of a single figure applied to everyone.
The picture after rollout is shaped by a limit in how harm is counted. VAERS, the main US system, is passive: someone has to notice an event, connect it to a vaccine, and file a report, so it captures a fraction of what happens, not a full census. An analysis funded by the Agency for Healthcare Research and Quality, run at Harvard Pilgrim Health Care, estimated that fewer than 1% of vaccine adverse events are reported to VAERS.
That figure deserves care in both directions. Treated as an estimate, not a precise constant, it means the raw VAERS counts are a floor and the true number of events is higher by a factor the passive system cannot pin down. It does not license multiplying a reported count by a hundred and presenting the result as fact, because the under-reporting factor is uncertain and varies by how serious and how recognizable an event is. The defensible reading is directional: reported totals understate the true number of events, by an amount no passive system can measure precisely, so a raw VAERS tally should never be read as the whole of what occurred.
The Research & Studies
Everything here is based on the research we have collected and checked, sorted into groups and ordered with the strongest evidence first. Click any claim to open the studies behind it.
How it works
mRNA tells your cells to make the spike, then clears within days without altering DNA
The vaccine is a set of instructions, not a piece of the virus. It tells your cells to make one part of the virus, the spike, so your immune system learns it in advance. The instructions are short-lived and do not change your DNA.
Messenger-RNA vaccine technology was developed over roughly two decades before the pandemic. A strand of mRNA encoding the SARS-CoV-2 spike protein is encapsulated in a lipid nanoparticle that protects it and helps it enter cells. Ribosomes in the cytoplasm translate the mRNA into spike protein, which is displayed to the immune system, generating antibody and T-cell responses. The mRNA acts in the cytoplasm, is not reverse-transcribed into the genome in this platform, and is degraded by normal cellular processes within days. Pardi and colleagues review the platform, its history and its immunology.
The study · 1
Pardi et al., mRNA vaccines: a new era in vaccinology · Nat Rev Drug Discov 2018;17(4):261-279
Vaccine spike antigen shows up within a day and clears as antibodies rise
The spike protein the vaccine makes does not stick around. In a small study, it showed up in the blood within a day and then cleared as the body built antibodies. This argues against the idea that vaccine spike lingers and circulates for a long time.
Ogata and colleagues measured SARS-CoV-2 proteins in longitudinal plasma samples from 13 recipients of two doses of mRNA-1273. Eleven of 13 had detectable SARS-CoV-2 antigen as early as day 1 after the first injection, and clearance of detectable antigen correlated with the production of IgG and IgA. The study is small and measures antigen presence and kinetics rather than any clinical outcome, but it is direct human evidence that vaccine-produced spike antigen is transient. It is given no direction because it is a biodistribution finding, not a benefit or a harm.
The study · 1
Ogata et al., circulating SARS-CoV-2 vaccine antigen detected in the plasma of mRNA-1273 recipients · Clin Infect Dis 2022;74(4):715-718
Spike crossing the blood-brain barrier is shown in mice, not in vaccinated people
The worry that the vaccine spike gets into the brain and causes damage is not established in humans. The study behind it used the virus's spike protein injected into mice, showing it can cross into a mouse brain. That is about the virus in an animal, not about vaccination causing brain harm in people.
Rhea and colleagues showed that radioiodinated S1, the S1 subunit of the SARS-CoV-2 spike protein, injected intravenously into mice, readily crossed the blood-brain barrier and entered brain tissue, and was also taken up by lung, spleen, kidney and liver. This is legitimate mechanistic work about the virus's protein in a mouse model. It does not measure vaccination, it does not measure humans, and it does not establish a clinical neurological injury from vaccination. It is graded preliminary and given no direction because it neither demonstrates harm from vaccination nor rules it out.
The study · 1
Rhea et al., the S1 protein of SARS-CoV-2 crosses the blood-brain barrier in mice · Nat Neurosci 2021;24(3):368-378
Respiratory Infection
About 95% effective against symptomatic COVID-19 in the pivotal trials
In the big trials, the vaccines cut the chance of getting sick with COVID-19 from the original virus by about 95%. These were large, placebo-controlled studies with tens of thousands of people each.
Polack and colleagues randomized about 43,548 participants to two doses of BNT162b2 or placebo and reported 95.0% efficacy against symptomatic, PCR-confirmed COVID-19 (95% CI 90.3 to 97.6), with 8 cases in the vaccine group versus 162 in placebo. Baden and colleagues randomized about 30,420 participants to mRNA-1273 or placebo and reported 94.1% efficacy (95% CI 89.3 to 96.8), with 11 versus 185 cases. Both trials measured a prespecified hard outcome against the ancestral strain over a period of weeks to a few months; neither could measure long-term durability or protection against variants that had not yet emerged.
The studies · 2
Polack et al., safety and efficacy of the BNT162b2 mRNA Covid-19 vaccine · N Engl J Med 2020;383(27):2603-2615
Baden et al., efficacy and safety of the mRNA-1273 SARS-CoV-2 vaccine · N Engl J Med 2021;384(5):403-416
About 97% less hospitalisation and death in Israel's early rollout
When the vaccine was rolled out across a whole country early on, fully vaccinated people were far less likely to be hospitalised or to die of COVID-19, by roughly 97%. This protection against severe illness is the strongest part of the case for the vaccines.
Haas and colleagues used national surveillance data from Israel's Ministry of Health covering the first four months of the BNT162b2 campaign. Seven days or more after the second dose, effectiveness estimates were 95.3% against infection, 97.0% against symptomatic COVID-19, 97.2% against hospitalisation, 97.5% against severe or critical hospitalisation and 96.7% against death. The benefit was largest in older and higher-risk groups, who bear most of the severe-disease burden. This is an observational, real-world estimate from a period dominated by the Alpha variant, not a randomized trial.
The study · 1
Haas et al., impact and effectiveness of BNT162b2 following a nationwide campaign in Israel · Lancet 2021;397(10287):1819-1829
Protection against infection fell from about 65% to under 10% by 25 weeks
The vaccines reduced infection at first, but that protection faded within months and dropped sharply once Omicron arrived. The early message that being vaccinated would reliably stop you catching or spreading the virus did not hold up over time.
Andrews and colleagues used a test-negative case-control design in England to estimate effectiveness against symptomatic disease from the Omicron variant. After two doses of BNT162b2, effectiveness against symptomatic Omicron was about 65.5% at 2 to 4 weeks, waning to under 10% by 25 or more weeks. A booster raised it to roughly 67% at 2 to 4 weeks, again declining over the following weeks. Effectiveness against severe outcomes held up considerably better than against symptomatic infection. The finding here is specifically about durable prevention of infection and transmission, which was not achieved, distinct from protection against severe disease.
The study · 1
Andrews et al., Covid-19 vaccine effectiveness against the Omicron variant · N Engl J Med 2022;386(16):1532-1546
Prior infection about 74.6% against severe disease; hybrid immunity most durable at 97.4%
Immunity from having had COVID is real and substantial, especially against severe illness, and lasts reasonably well. Protection from being infected again fades faster. The strongest, most lasting protection came from having both an infection and a vaccination.
Bobrovitz and colleagues systematically reviewed and meta-analyzed studies of protection from previous SARS-CoV-2 infection and from hybrid immunity, mostly against the Omicron variant. Previous infection alone provided about 74.6% protection against hospital admission or severe disease at 12 months and 24.7% against reinfection at 12 months. Hybrid immunity gave about 97.4% protection against severe disease at 12 months. This is included because the strength of infection-acquired immunity was relevant to the trust story, where its role in public messaging drew criticism.
The study · 1
Bobrovitz et al., protective effectiveness of previous infection and hybrid immunity against Omicron and severe disease · Lancet Infect Dis 2023;23(5):556-567
Evidence And Methods
VAERS captures fewer than 1% of adverse events, so counts are a floor
VAERS is a passive system that relies on someone noticing an event and filing a report, so it misses most of what happens. An AHRQ-funded project estimated that fewer than 1% of vaccine adverse events are reported to it, which means the raw counts are a floor, not the true number.
The ESP:VAERS project (Lazarus, Klompas and colleagues, AHRQ Grant Final Report R18 HS 017045, Harvard Pilgrim Health Care, 2010) built an automated electronic system to detect and report vaccine adverse events across a large patient population. The report states that fewer than 1% of vaccine adverse events are reported to VAERS, framing the passive system's low sensitivity as the problem the automated tool was meant to solve. The specific fewer-than-1% figure is an estimate and the automated system was not ultimately adopted for routine use. What is well established, and what this row carries, is directional: passive spontaneous-reporting systems substantially undercount adverse events, so raw VAERS totals are a floor rather than a full census. This row is given no direction because it describes a measurement limitation, not a benefit or a harm.
The study · 1
Lazarus, Klompas et al., electronic support for public health, vaccine adverse event reporting system (ESP:VAERS), AHRQ Grant Final Report R18 HS 017045 · Agency for Healthcare Research and Quality, 2010
The rows above are graded unevenly on purpose. The trial-efficacy and severe-disease rows are strong; the waning, myocarditis and natural-immunity rows are moderate, resting on large observational and surveillance data with the confounds named; the spike-protein rows are preliminary or emerging and point in no clear direction, because that is what the evidence supports. Different questions have different amounts of evidence behind them, and flattening them to a single verdict would lose the information a careful reader needs. The documented harms and the two contested safety claims are gathered in one place, in the cautions section near the foot of the page.
What Is Not Established (spike/BBB, the embalmer clots)
Two claims circulate widely and deserve a precise answer instead of a slogan in either direction.
The first is that the vaccine spike protein crosses the blood-brain barrier and causes neurological harm. In people, that harm is not established. The research behind the concern is mechanistic and mostly about the virus, not the vaccine: purified viral spike protein, injected into mice, can cross the mouse blood-brain barrier, a finding in an animal model and a legitimate thing to study. It does not show that vaccination causes brain injury in humans.
What has been measured on the vaccine side points the other way on persistence. In a small study of people given an mRNA vaccine, spike antigen appeared in the blood within a day and then cleared as antibodies rose, which argues against the idea that vaccine spike lingers and circulates for long periods. Spike biology is studied, a clinical neurological harm from vaccination is not established, and the small human data on where spike goes and how long it lasts do not support the alarming version.
The second is the embalmer-clot claim: that embalmers, including one who has spoken publicly since 2021, are finding unusual large white fibrous clots, and that these were caused by vaccination. The 2022 film Died Suddenly amplified it, and the film was widely criticized for how it presented its footage and data. No peer-reviewed study has established a new clot phenomenon or tied one to vaccination at population scale.
Pale, fibrin-rich clots that form after death, long called chicken-fat clots, are a well-documented normal finding in forensic pathology, described in the literature well before the pandemic and reviewed systematically since. The most parsimonious reading is that the reports describe a mundane, long-known postmortem process. The reports themselves are uninvestigated anecdotes with no denominator and no comparison group, so they cannot show whether anything unusual is happening.
Two points follow. There is no established causal link to vaccination. The claim is an open anecdote set against a well-established ordinary explanation, which is neither evidence of harm nor a case that study has closed.
An observation that has not been systematically studied is not the same as one a study has ruled out.
Trust And The Institutions
Some of what damaged confidence during the pandemic was not about the vaccines at all, and it is part of an accurate account of the period. Guidance shifted, sometimes sharply, on masks and on whether vaccination would stop transmission. Mandates were introduced. Discussion of prior-infection immunity and of the possibility that the virus emerged from a laboratory was marginalized early and treated as more plausible later, which taught many people that a question being called settled was not the same as its being settled. These are documented features of how the response was communicated, and naming them is not an argument against vaccination. It is an argument for the kind of reading this whole site is built on, where the money and the incentives are kept in view.
The sharpest illustration of that pattern is not the vaccines at all. It is the pandemic's approved antiviral, remdesivir, which became a patented, expensive standard of care on trial evidence that did not support a survival benefit, while a cheaper generic was dismissed before its own trials were run. That story has its own page, COVID-19 treatments, because it is a drug story rather than a vaccine one and deserves the room to be told at full strength. The same reading applies here: what becomes standard, and what gets the benefit of the doubt, tracks incentives as well as outcomes.
For how these incentives operate in general, from funding to publication to the framing of a result, see how industry shapes science. For a companion case where the public story went wrong in both directions at once, promoted as a miracle and dismissed as only a veterinary product, see ivermectin.
How To Read This
The evidence supports a set of statements that are usually presented as if you had to choose among them, and you do not. The mRNA vaccines had strong trial efficacy, about 95% against symptomatic COVID-19, and they cut severe disease and death, most of all early and in high-risk people, while protection against infection and transmission was present at first and then waned substantially with time and variants, which the early messaging did not convey.
They also carry specific risks: a reanalysis of the trials found an excess of serious adverse events; there is a regulator-acknowledged myocarditis signal concentrated in young men, usually mild; anaphylaxis is rare and treatable; and the systems that watch for harm after rollout are passive and undercount, so reported totals are a floor. Public trust was strained by shifting guidance, by mandates, and by how dissent and prior-infection immunity were handled. The two alarming claims, spike crossing into the brain to cause harm and a novel vaccine-caused embalmer clot, are not established, the second an uninvestigated anecdote set against an ordinary explanation.
Holding all of that at once, at the weight the evidence supports, is the accurate position.
Go Deeper
- COVID-19 treatments: remdesivir, the approved drug whose survival benefit was never established, and the cheaper drug dismissed before its trials, told at full strength.
- How industry shapes science: the method for reading past funding, framing and buried data.
- Ivermectin: the pandemic's other contested drug, told in full, and a model for holding both directions of a story at once.
- Expectancy and belief: how conviction and a compelling narrative shape what people feel a treatment did, on all sides of a debate.
- The biology of aging: another field where solid science sits alongside claims that outrun the evidence.
Common Questions
Did the mRNA vaccines stop transmission?
Not durably, and the early messaging that suggested they would was more confident than the evidence. At the start the vaccines did reduce infection, but that protection waned substantially over months and with the Omicron variant. Against symptomatic Omicron, two doses were about 65.5% effective a few weeks out and fell below 10% by around 25 weeks, with boosters giving a temporary lift. Protection against severe disease held up much better than protection against catching the virus, and those two outcomes are what the transmission question actually turns on.
Is the myocarditis risk real?
Yes, and it is acknowledged by regulators. Myocarditis after mRNA vaccination is concentrated in young males, is highest after the second dose, and is somewhat more common after the Moderna product. Large surveillance systems in the United States, Israel and the Nordic countries all measured it clearly. The risk is uncommon, it falls mostly on young men, and the great majority of cases were mild and resolved after a short hospital stay. All of those are true together, which is why a description that gives only the signal or only the reassurance is a partial reading.
What about the embalmer clots?
The precise answer is that the reports are uninvestigated anecdotes set against a well-established ordinary explanation. Some embalmers have reported unusual white fibrous clots since 2021, a claim amplified by a 2022 film that was widely criticized. No peer-reviewed study has established a new clot phenomenon or linked one to vaccination. Meanwhile, pale, fibrin-rich clots that form after death, long called chicken-fat clots, are a normal finding documented in forensic pathology well before the pandemic. There is no established causal link to vaccination, and the observation has not been systematically studied, which is not the same as a study having ruled it out.
Does the vaccine spike protein cross into the brain and cause harm?
A clinical harm of that kind in people is not established. The research behind the worry is an animal study in which purified viral spike protein, injected into mice, crossed the mouse blood-brain barrier. That concerns the virus in a mouse, not vaccination in a person. What has been measured in people points toward vaccine spike being short-lived: in a small study it appeared in the blood within a day and cleared as antibodies rose. Spike biology is studied; the alarming version of the story is not supported by the human data.
How many serious adverse events did the trials actually find?
More than the trials' top-line efficacy figure suggested. A 2022 reanalysis of the manufacturers' own trial data, by Fraiman and colleagues in Vaccine, pooled the serious adverse events of special interest, a prespecified watch-list of conditions, and found an excess in the vaccinated groups of about 12.5 per 10,000, roughly 1 in 800 (risk ratio 1.43). For every 10,000 people vaccinated in the trials, about 12 to 13 more had a serious adverse event of that kind than in placebo. It is uncommon in absolute terms and real, and the authors argued the balance of benefit and risk should be worked out for a person's actual risk of severe COVID-19 instead of applied as one number to everyone.
Can you trust VAERS numbers, in either direction?
Only as a floor, not a full count. VAERS is a passive system: someone has to notice an event, connect it to a vaccine, and file a report. An analysis funded by the Agency for Healthcare Research and Quality estimated that fewer than 1% of vaccine adverse events are reported to it. Read as an estimate, that means the raw counts understate what happened, by a factor the passive system cannot pin down. It does not justify multiplying a reported number by a hundred and calling the result fact, because the under-reporting factor is uncertain and depends on how serious and recognizable the event is. The defensible statement is directional: reported totals are lower than real events, by an unknown amount.
Where did the remdesivir story go?
It has its own page, COVID-19 treatments, because it is a drug story rather than a vaccine one. In short, remdesivir became the first approved COVID-19 drug on trial evidence that did not support a survival benefit, and the WHO recommended against it weeks later, while a cheaper generic was dismissed before its own trials were run.
Documented Harms and Contested Safety Claims
Everything to be aware of is here, in one place. This practice suits most healthy people; a few situations call for real care.
Myocarditis in young men after the second dose, about 105.9 per million in males 16 to 17
Oster and colleagues reviewed reports to the US Vaccine Adverse Event Reporting System, confirming 1,626 myocarditis cases among people over 12 (median age 21, median onset 2 days, 82% male among cases with sex reported). Crude reporting rates within 7 days exceeded expected rates in multiple young age and sex strata, highest in males 16 to 17 (about 105.9 per million second BNT162b2 doses) and 12 to 15 (about 70.7 per million). About 96% of confirmed cases were hospitalized, and about 87% of those hospitalized had resolution of symptoms by discharge. Mevorach and colleagues, in Israeli active surveillance, found a standardized incidence ratio of 5.34 (95% CI 4.48 to 6.40) overall after the second dose, highest in males aged 16 to 19 (rate ratio about 8.96), with a clinical course that was generally mild. Regulators including the CDC and FDA acknowledge the signal.Oster et al., myocarditis cases reported after mRNA-based COVID-19 vaccination in the USMevorach et al., myocarditis after BNT162b2 mRNA vaccine against Covid-19 in IsraelKarlstad et al., SARS-CoV-2 vaccination and myocarditis in a Nordic cohort study of 23 million residents
A study of 99 million confirmed the myocarditis and pericarditis signals
Faksova and colleagues, in the Global Vaccine Data Network cohort, analyzed adverse events of special interest up to 42 days after vaccination across 99,068,901 people and 23.2 million person-years, using observed-versus-expected ratios. The analysis confirmed pre-established signals for myocarditis and pericarditis after mRNA vaccines and flagged acute disseminated encephalomyelitis after a first mRNA-1273 dose (observed-to-expected ratio 3.78). The Guillain-Barré and cerebral venous sinus thrombosis signals it reported followed the ChAdOx1 adenovirus-vector vaccine, not the mRNA vaccines. Signals with a lower confidence bound above 1.5 were treated as warranting further investigation.Faksova et al., COVID-19 vaccines and adverse events of special interest: a Global Vaccine Data Network cohort of 99 million
Anaphylaxis is rare and treatable, about 2.5 per 10,000 vaccinations
Blumenthal and colleagues examined acute allergic reactions to mRNA COVID-19 vaccines among 64,900 employees in a Massachusetts health-care system. Acute allergic reactions were self-reported in about 2.1% of vaccinations, and 16 cases met criteria for anaphylaxis, roughly 2.47 per 10,000, all of whom recovered. This single-system rate is higher than the few-per-million figures from national passive surveillance, a difference typical of active versus passive monitoring, but both agree the event is rare and manageable.Blumenthal et al., acute allergic reactions to mRNA COVID-19 vaccines
A reanalysis of the trials found about 1 in 800 excess serious adverse events
Fraiman and colleagues reanalyzed serious adverse events of special interest (a Brighton Collaboration priority list) from the phase III randomized trials of the Pfizer-BioNTech and Moderna mRNA vaccines. Across both, the excess risk of serious adverse events of special interest was 10.1 per 10,000 for Pfizer-BioNTech and 15.1 per 10,000 for Moderna, combined 12.5 per 10,000 (95% CI 2.1 to 22.9), a risk ratio of 1.43 (95% CI 1.07 to 1.92). The authors noted the confidence intervals are wide and the analysis is a secondary reanalysis of trial data with its own limits, and called for formal harm-benefit analyzes stratified by an individual's risk of severe COVID-19 rather than a single population figure. This is graded moderate: it rests on randomized-trial data, but as a post-hoc pooled reanalysis with wide intervals it is a signal to weigh, not a settled effect size.Fraiman et al., serious adverse events of special interest following mRNA COVID-19 vaccination in randomized trials in adults
The embalmer clot reports are anecdotes, not a studied new phenomenon
The claim traces to embalmer reports circulating since 2021 and the 2022 film Died Suddenly, which was widely criticized for how it presented footage and data. A search of the peer-reviewed literature finds no systematic investigation establishing a new clot phenomenon or a vaccine link at population scale. By contrast, pale, fibrin-rich clots forming after death, described as chicken-fat clots, are a recognized entity in forensic pathology: medico-legal case studies characterized them well before the pandemic (2008), and a 2025 systematic review of cadaver clots catalogues the antemortem, agonal and postmortem categories while noting that comparative science distinguishing them remains incomplete. The most parsimonious reading is that the embalmer observations describe a mundane, long-known postmortem process. The reports are uncontrolled anecdotes with no denominator, so they establish neither harm nor its absence.Solarino et al., cadaver clots: a systematic review of the literatureUekita et al., medico-legal investigation of chicken-fat clot in forensic cases
This section gathers the safety findings in one place so the rest of the page can read plainly. Read the research, weigh your own risk of severe COVID-19, and consult a professional if you have any concerns. This is here to inform your choice, not make it for you.
Explore Related
Other pages this one connects to, by the evidence they share, the outcomes they touch, and the ground they cover.
All 17 sources on this page independently checked and cross-referenced.
Thomas Dehli, Founder & Editor, Sacred Lotus
Sacred Lotus has published Chinese medicine reference material since 2001. Integrative pages are held to the same standard as the herb and formula library: cite the source, grade the claim at its real strength, and say where the research has not looked. This page is educational and it is not medical advice. Last reviewed and updated August 10, 2026.
Evidence strength
How confidently the research supports a claim. Strength describes the evidence, not our endorsement.