Science: mRNA-Impfstoffe: Was die Evidenz zeigt

What an mRNA Vaccine Is

An mRNA vaccine is a strand of messenger RNA wrapped in a protective lipid nanoparticle. It carries a set of instructions, not a piece of the virus. Those instructions tell a person's own cells to make one part of SARS-CoV-2, the spike protein. The mRNA is short-lived. The body breaks it down within days, and it never enters the cell nucleus or alters DNA. The approach grew out of about 20 years of research before the pandemic.

What the Trials Found

The Pfizer-BioNTech trial enrolled about 43,000 people. Eight cases of COVID-19 appeared in the vaccine group against 162 in the placebo group, a 95.0% efficacy against symptomatic disease from the original virus (Polack 2020). The Moderna trial enrolled about 30,000 people. Eleven cases appeared in the vaccine group against 185 in placebo, a 94.1% efficacy.

When the vaccines reached the whole of Israel in early 2021, two doses cut hospitalization by about 97%. They cut severe or critical disease by 97.5% and death by 96.7% in that early period. The benefit fell most heavily on older and higher-risk people.

Protection against catching the virus did not last. Against symptomatic Omicron, two doses were about 65.5% effective at 2 to 4 weeks, then dropped below 10% by around 25 weeks (Andrews 2022). A booster restored protection to about two thirds before it too declined. The 2021 message that vaccination would reliably stop transmission did not hold.

Immunity from a prior infection is real. It gave about 74.6% protection against hospitalization or severe disease at one year, though protection against catching the virus again waned to around 24.7%. Hybrid immunity, infection plus vaccination, proved the most durable, at about 97.4% against severe disease over a year.

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

Die mRNA weist Ihre Zellen an, das Spike-Protein herzustellen, und wird dann innerhalb weniger Tage abgebaut, ohne die DNA zu verändernEstablished

In plain terms

Der Impfstoff ist eine Anleitung, kein Teil des Virus. Er weist Ihre Zellen an, einen Teil des Virus herzustellen, das Spike-Protein, damit Ihr Immunsystem es im Voraus kennenlernt. Die Anleitung ist kurzlebig und verändert Ihre DNA nicht.

In detail

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 riseEstablished

In plain terms

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.

In detail

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, not 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 peopleSpeculative

In plain terms

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.

In detail

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 trialsEstablished

In plain terms

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.

In detail

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 rolloutEstablished

In plain terms

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.

In detail

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 weeksEstablished

In plain terms

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.

In detail

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%Established

In plain terms

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.

In detail

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 erfasst weniger als 1 % der Nebenwirkungen, sodass die Zahlen eine Untergrenze darstellenEstablished

In plain terms

VAERS ist ein passives System, das darauf angewiesen ist, dass jemand ein Ereignis bemerkt und einen Bericht einreicht, sodass es das meiste Geschehen verpasst. Ein von der AHRQ finanziertes Projekt schätzte, dass weniger als 1 % der Impfstoff-Nebenwirkungen dort gemeldet werden, was bedeutet, dass die Rohzahlen eine Untergrenze sind, nicht die wahre Zahl.

In detail

Das ESP:VAERS-Projekt (Lazarus, Klompas und Kollegen, AHRQ Grant Final Report R18 HS 017045, Harvard Pilgrim Health Care, 2010) baute ein automatisiertes elektronisches System auf, um Impfstoff-Nebenwirkungen bei einer großen Patientenpopulation zu erkennen und zu melden. Der Bericht stellt fest, dass weniger als 1 % der Impfstoff-Nebenwirkungen an VAERS gemeldet werden, und rahmt die geringe Sensitivität des passiven Systems als das Problem ein, das das automatisierte Werkzeug lösen sollte. Die konkrete Zahl von unter 1 % ist eine Schätzung, und das automatisierte System wurde letztlich nicht für den Routinebetrieb übernommen. Gut belegt, und das, was diese Zeile trägt, ist die Richtung: passive Spontanmeldesysteme untererfassen Nebenwirkungen erheblich, sodass rohe VAERS-Gesamtzahlen eine Untergrenze sind, keine vollständige Erhebung. Diese Zeile erhält keine Richtung, da sie eine Messeinschränkung beschreibt, keinen Nutzen oder Schaden.

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

Myocarditis in Young Men

Myocarditis, inflammation of the heart muscle, sits at the top of the documented harms, acknowledged by regulators including the CDC and FDA. It concentrates in young males and peaks after the second dose, somewhat more often after Moderna than Pfizer-BioNTech. US surveillance confirmed 1,626 cases at a median age of 21, most of them male. Reporting rates ran highest in males 16 to 17, at about 105.9 per million second doses of the Pfizer-BioNTech vaccine (Oster 2022).

Israeli active surveillance put the risk in young men at several times the expected background rate. A Nordic cohort of about 23 million residents of Denmark, Finland, Norway and Sweden placed the excess squarely in young men after a second Moderna or Pfizer-BioNTech dose. It ran higher after Moderna. In October 2021, before that evidence was in, Sweden, Denmark, Finland and Norway paused or restricted the Moderna vaccine for younger age groups.

The reassuring part is real. The great majority of cases were mild and resolved, and most patients under 30 went home after a short admission.

The Global Vaccine Data Network followed about 99 million vaccinated people across eight countries. It confirmed the myocarditis and pericarditis signals after mRNA vaccines, and flagged a rare inflammatory brain condition, acute disseminated encephalomyelitis, after a first Moderna dose. The same study's signals for Guillain-Barre syndrome and cerebral venous sinus thrombosis followed the ChAdOx1 adenovirus-vector vaccine, a different technology and not an mRNA product.

Anaphylaxis stays rare and treatable. One careful study put it on the order of a few per 10,000 vaccinations, with the affected people recovering.

The Excess of Serious Adverse Events in the Trials

Fraiman and colleagues, writing in Vaccine in 2022, pooled the serious adverse events that manufacturers had prespecified as events of special interest, drawn from their own randomized trial data. That watch-list showed an excess of about 12.5 per 10,000, roughly 1 in 800 (95% CI 2.1 to 22.9, risk ratio 1.43). Put plainly, about 12 to 13 more people per 10,000 vaccinated had a serious event of that kind than in placebo. The authors called for formal harm-benefit analyses tied to a person's actual risk of severe COVID-19, not one figure for everyone.

What Passive Reporting Can and Cannot Tell You

VAERS works passively: someone must notice an event, connect it to the vaccine, and file a report. 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 reach VAERS. Raw counts are therefore only a floor. The under-reporting factor is uncertain and shifts with how serious and how recognizable an event is. Multiplying a count by 100 and calling the result a fact is not supported.

The Spike-in-the-Brain Claim

The claim that the vaccine's spike protein crosses into the brain and does harm is not established in people. The research behind it is mechanistic and mostly about the virus. Purified viral spike protein injected into mice can cross the mouse blood-brain barrier, an animal model that does not show vaccination injuring a human brain. On the vaccine side, a small study of people given an mRNA vaccine found spike antigen in the blood within about 24 hours. It then cleared as antibodies rose, arguing against the vaccine spike lingering for long periods.

The Embalmer-Clot Claim

Some embalmers, one of them public since 2021, report finding unusual large white fibrous clots and attribute them to vaccination. The 2022 film Died Suddenly amplified the idea and drew wide criticism for 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 before the pandemic and reviewed systematically since. The reports remain uninvestigated anecdotes with no denominator and no comparison group.

An observation that has not been systematically studied is not the same as one a study has ruled out.

Reading the Evidence With the Incentives in View

Two companion pages bear on how this story was handled: COVID-19 treatments and how industry shapes science. Remdesivir became a patented, expensive standard of care on trial evidence that did not support a survival benefit, and the WHO recommended against it weeks later. A cheaper generic was dismissed before its own trials were run. What becomes standard, and what gets the benefit of the doubt, tracks incentives as much as outcomes.

Guidance shifted on masks and transmission through 2020 and 2021, mandates were introduced, and prior-infection immunity and the lab-leak possibility were marginalized early, then treated as more plausible later. A question being called settled was not the same as its being settled.

Go Deeper

• Ivermectin: the pandemic's other contested drug, promoted as a miracle and dismissed as a veterinary product, 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

Does the vaccine contain the virus, and can it give me COVID-19?

No. It carries instructions for a single viral protein, not the virus, so nothing in it can reproduce or start an infection. Your immune system learns to recognize that one protein before the real virus, the thing that causes COVID-19, ever arrives.

How do I weigh the myocarditis risk against the risk from COVID-19 itself?

It depends on the person. A healthy young man and an older, higher-risk adult face very different balances of harm and benefit. That is the balance Fraiman's reanalysis pointed to, arguing the calculation be made per person rather than as a single number.

The vaccine arrived fast, so was the science rushed?

The mRNA delivery platform was not invented in 2020; it grew out of research stretching back well before the pandemic. What moved quickly that year was large-scale testing and manufacturing, run in parallel instead of one stage after another. Those trials still enrolled tens of thousands of people before the authorizations at the end of 2020.

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

Eine erneute Analyse der Studien fand etwa 1 von 800 zusätzliche schwerwiegende unerwünschte Ereignisse

Fraiman und Kollegen analysierten schwerwiegende unerwünschte Ereignisse von besonderem Interesse (eine Prioritätsliste der Brighton Collaboration) aus den randomisierten Phase-III-Studien der mRNA-Impfstoffe von Pfizer-BioNTech und Moderna erneut. Über beide hinweg betrug das überschüssige Risiko schwerwiegender unerwünschter Ereignisse von besonderem Interesse 10.1 pro 10,000 für Pfizer-BioNTech und 15.1 pro 10,000 für Moderna, zusammen 12.5 pro 10,000 (95 %-KI 2.1 bis 22.9), ein Risk Ratio von 1.43 (95 %-KI 1.07 bis 1.92). Die Autoren stellten fest, dass die Konfidenzintervalle breit sind und die Analyse eine sekundäre erneute Analyse von Studiendaten mit eigenen Einschränkungen ist, und forderten formale Nutzen-Risiko-Analysen, stratifiziert nach dem individuellen Risiko einer schweren COVID-19-Erkrankung, statt einer einzigen Bevölkerungszahl. Dies wird als moderat eingestuft: Es beruht auf randomisierten Studiendaten, ist aber als post-hoc gepoolte erneute Analyse mit breiten Intervallen ein abzuwägendes Signal, keine feststehende Effektgröße.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

The safety findings are gathered here so the rest of the page can read plainly.