Anaphylaxis after vaccine: rarity, signs and management

A 15-year-old girl receives a vaccine and, two minutes later, turns pale, feels cold and slips off the chair. In the next room, a 40-year-old man with a history of drug allergy starts scratching his neck eight minutes after the injection, his voice becomes hoarse and red patches appear on his trunk. Both scenes are frightening, but they call for different responses.

Anaphylaxis after a vaccine is rare. It is also fast, can be fatal and has effective treatment when recognized early. The vasovagal reaction, on the other hand, is far more common, is usually benign and can cause falls and injury. Knowing how to tell the two apart, and acting without hesitation when doubt persists, is a basic skill for anyone who vaccinates.

This text shows what surveillance systems report about frequency, how anaphylaxis presents, why intramuscular adrenaline comes before everything else and what to do about the next vaccine.

Key points

• In a large American database, post-vaccine anaphylaxis occurred in about 1.3 per million injections (McNeil, 2019).
• Most cases begin in the first 15 to 30 minutes, which supports the observation period (Shimabukuro, 2021).
• Stress responses to the injection, such as vasovagal syncope, are far more frequent than anaphylaxis (Imai, 2022).
• Intramuscular adrenaline is the first-line treatment and has no contraindication in this situation (Shimabukuro, 2021; Dribin, 2023).
• The patient's position matters: standing during circulatory collapse is associated with worsening and death (Pouessel, 2024).
• Most people with suspected allergy to a vaccine can be revaccinated after evaluation (Rasmussen, 2021; Barth, 2023).

Summary

• How rare is anaphylaxis after a vaccine
• How anaphylaxis presents
• Who is at higher risk
• Anaphylaxis or vasovagal reaction
• The first hour: what to do
• Revaccinating after a reaction
• Reasoning errors that show up in the vaccination room
• How NEXO helps train this reasoning
• Frequently asked questions
• Take-home messages

How rare is anaphylaxis after a vaccine

The most cited estimate comes from the Vaccine Safety Datalink in the United States. Between 2009 and 2011, researchers reviewed records of people who received 25,178,965 vaccine injections and confirmed 33 anaphylaxis cases by the Brighton Collaboration definition. The incidence was 1.31 per million injections, and 1.59 per million for the trivalent influenza vaccine. There were no deaths, and 85 of every 100 affected people had a history of atopy (McNeil, 2019).

In South Korea, the national agency confirmed 13 anaphylaxis cases with a defined relationship to the vaccine between 2001 and 2016, with annual incidences between 0.036 and 0.188 per million injections. The median age was 9 years, and the median onset was 11 minutes. Notably, adrenaline was used in fewer than half of these confirmed anaphylaxis cases, 46 of every 100, which suggests undertreatment even in recognized cases (Roh, 2020).

Covid vaccines

Covid vaccination campaigns allowed anaphylaxis to be measured with unprecedented speed. In the first weeks of mRNA vaccine use in the United States, VAERS identified 21 anaphylaxis cases after the first injection, which corresponds to 11.1 per million injections. There were no deaths (Shimabukuro, 2021).

In South Korea, between February and April 2021, there were 3.8 million injections, 173 reported suspected cases and 44 confirmed anaphylaxis cases. The rates were 18.2 per million for the adenoviral vector vaccine and 6.2 per million for the mRNA vaccine (Lee, 2021). In Germany, the reporting rate was higher in women on the first injection of mRNA vaccines, around 1 per 100,000 vaccinations (Barth, 2023).

Why the numbers vary so much

Differences between studies reflect the vaccine, the population and, above all, the method. A database with record review, such as the VSD, measures risk better. A reporting system, such as VAERS, captures more reports, but without a reliable denominator. And the case definition used changes what gets counted, as the next section shows. The reasoning is the same as for any basic rate: the number only makes sense when you know where it came from.

How anaphylaxis presents

Anaphylaxis is an acute, systemic hypersensitivity reaction that is potentially fatal and involves several organs. The systems most affected are the skin, the cardiovascular, respiratory and gastrointestinal systems, with generalized urticaria, airway edema, difficulty breathing, hypotension, nausea and vomiting (McNeil, 2019).

In anaphylaxis after mRNA vaccine, the most common signs were urticaria, angioedema, rash and a sensation of throat closing. The median onset was 13 minutes; 71 of every 100 people had symptoms within the first 15 minutes and 86 of every 100 within the first 30 (Shimabukuro, 2021). In Korea, the median was 14 minutes, and most people had already recovered at the time of review (Lee, 2021). In the American series of anaphylaxis after mRNA vaccine, most were treated in an emergency department and nearly all were discharged or recovered (Shimabukuro, 2021).

The mechanism in a few words

In the classic form, the allergen binds to IgE antibodies on the surface of mast cells and basophils, which release histamine and other mediators within seconds. These mediators dilate vessels, increase capillary permeability and contract bronchial muscle. The same sequence appears in the pathophysiology of food anaphylaxis.

With vaccines, the mechanism is not always this one. In Germany, only 22 of every 100 anaphylaxis cases investigated after covid vaccines showed an IgE-mediated mechanism, and most remained without a clear explanation (Barth, 2023).

The suspected components

Among the components implicated in acute reactions to vaccines are egg protein, gelatin, milk proteins and residues from the manufacturing process. Natural latex, present in some syringe plungers and vial stoppers, is another possible trigger (McNeil, 2019). In covid vaccines, polyethylene glycol and polysorbate 80 were the most investigated excipients (Wolfson, 2021).

Who is at higher risk

Most people with anaphylaxis after vaccination have a history of allergy. Among anaphylaxis cases after mRNA vaccine, 81 of every 100 had documented allergy, and one third had had anaphylaxis before (Shimabukuro, 2021). In the VSD, 85 of every 100 had atopy (McNeil, 2019).

This association, however, does not protect those without a history. In the 26-year VAERS review, with 828 reports of anaphylaxis, 41 of every 100 described people with no history of hypersensitivity. There were 8 reports of death, some in people with no prior allergy (McNeil, 2019). Therefore, every service that vaccinates must be ready to treat anaphylaxis, not only those that care for allergic patients.

At a large vaccination center in Japan, the factors associated with immediate hypersensitivity reactions were female sex, asthma, atopic dermatitis, thyroid disease and a history of allergy. The authors concluded that the increase in risk was too small to justify special measures for these people (Imai, 2022). Asthma appears again as a marker of more reactive airways.

Anaphylaxis or vasovagal reaction

Vasovagal syncope and other stress responses linked to the injection are far more common than anaphylaxis. At the same Japanese center, with 1,201,688 injections, stress responses totaled 2,129 per million, vasovagal syncope 72 per million and anaphylaxis only 2 per million. Risk factors for the stress response included younger age, female sex, mental disorders and a history of vasovagal reflex (Imai, 2022).

In another Japanese survey, reported vasovagal reaction was 9.6 per million injections, more frequent between 10 and 29 years of age. In this age range, the rate was 57.4 per million at the first injection and 8.8 at the second. The prognosis is good, but the fall can cause injury (Okuyama, 2022).

How to tell them apart at the bedside

The table summarizes the findings that help separate the two situations. It serves as guidance, not as an absolute rule: when doubt persists and there are signs of severity, treat as anaphylaxis.

Aspect	Vasovagal reaction	Anaphylaxis
Onset	Seconds to a few minutes, often before or during the injection	Usually within minutes of administration, most within the first 30 minutes
Skin	Pallor, cold sweating	Urticaria, flushing, pruritus, angioedema
Breathing	There may be hyperventilation, without wheezing	Hoarseness, stridor, wheezing, cough, sensation of the throat closing
Pulse	Slow, sometimes very slow	Fast and weak
Blood pressure	Low for a short time, improves on lying down	Low and persistent, may worsen on standing up
Course	Improves within minutes with the patient lying down	Progresses without treatment
Management	Lie down, raise the legs, observe	Immediate intramuscular adrenaline and call for help

Fainting can come with brief jerks, which causes confusion with a seizure. The first step is the same as for syncope in the emergency department: confirm what happened and look for the cause. Dissociative or functional symptoms after administration also enter the differential diagnosis, as discussed in the text on functional seizures.

The problem with the case definition

The Brighton Collaboration definition is the standard for surveillance, but it draws criticism. Hurley and colleagues show that it can classify non-allergic symptoms as anaphylaxis, such as vasovagal episodes, paresthesias, isolated urticaria and functional symptoms, by adding up major and minor criteria. The authors propose using simpler definitions, such as those of NIAID or the World Allergy Organization, also in vaccine surveillance (Hurley, 2022).

A 2024 review recalls that the different definitions are more alike than different and that each one adds a useful perspective (Shaker, 2024). In clinical practice, the criterion that matters is the one that leads to adrenaline without delay.

The first hour: what to do

The guidance for vaccination sites sums up preparation in five points: have material to treat anaphylaxis, especially adrenaline in a prefilled syringe or autoinjector; screen for contraindications and precautions; observe everyone vaccinated for 15 or 30 minutes according to allergy history; train the team to recognize the signs early; and treat the suspicion immediately with intramuscular adrenaline, which has no contraindication in this situation (Shimabukuro, 2021).

Intramuscular adrenaline first

International guidelines place intramuscular adrenaline as the first-line treatment, with a good safety profile. Observational studies link delay in administration to more biphasic reactions, more hospitalizations and more deaths, although the definition of delay varies. A poor response after two doses of adrenaline is a useful marker of severity and indicates urgent escalation (Dribin, 2023).

The same authors discuss points that are still uncertain: which symptoms should trigger adrenaline, whether emergency services need to be called when the patient responds to a single dose, and whether the autoinjector reduces mortality. They recall that adrenaline, even when it does not prevent a fatal reaction, can buy time until resuscitation in a health care setting (Dribin, 2023).

The amount is adjusted to weight, according to the local protocol, and it is given on the lateral aspect of the thigh. Antihistamines relieve pruritus and urticaria, but they do not replace adrenaline, as shown in management of food anaphylaxis.

Patient position

Anyone with circulatory collapse should stay lying down, with the legs raised. Changing from lying to standing is associated with worsening collapse and even death. If respiratory symptoms predominate, the patient may prefer to sit with the legs raised (Pouessel, 2024).

When intramuscular adrenaline is not enough

The recommendation for intramuscular adrenaline rests on consensus, not on randomized trials. A 2026 trial induced hypotension with histamine infusion in healthy volunteers. Intramuscular adrenaline, 300 to 500 micrograms, did not reverse hypotension in most participants: only 5 of 20 had a transient recovery of blood pressure (Weiss-Tessbach, 2026).

The study is small and used an artificial model, but it reinforces a lesson from the guidelines: intramuscular adrenaline is the start of treatment, and lack of response requires rapid escalation. In refractory anaphylaxis, the guidelines recommend aggressive volume replacement and intravenous adrenaline in a monitored setting, with other vasopressors as an option. Which vasopressor to use second is still uncertain. Most guidelines suggest intravenous glucagon for people using beta-blockers, despite the lack of evidence, and some mention methylene blue or extracorporeal circulatory support as rescue (Pouessel, 2024).

The same review recalls that individual factors may explain the lack of response to initial adrenaline. Deficiency of the enzyme that degrades platelet-activating factor, hereditary alpha-tryptasemia and mastocytosis may modulate the severity of the reaction or the response to treatment (Pouessel, 2024). In a cohort of suspected anaphylaxis cases seen in the emergency department, almost 8 in every 100 adults had mastocytosis, associated with more severe reactions (Pouessel, 2024). This is one more reason to refer anyone who had severe anaphylaxis to an allergist.

After the crisis

Patients should be advised to seek immediate care if they have signs of allergy after leaving the vaccination site (Shimabukuro, 2021). Observation after treatment and tryptase collection follow the same logic described in after the reaction. Reporting to the surveillance system is part of the care.

Observation after administration

The observation period exists for two reasons. The first is anaphylaxis, which most often begins in the first 15 to 30 minutes (Shimabukuro, 2021). The second is syncope: the American guidance recommends considering 15 minutes of observation after any vaccine to reduce the risk of injury from falls (McNeil, 2019).

During covid vaccination, 30 minutes was adopted for people with a history of allergic reaction and 15 minutes for the others (Shimabukuro, 2021). For those with severe egg allergy, the American recommendation became to vaccinate against influenza in a setting supervised by a professional able to treat severe reactions, without a special observation period (McNeil, 2019).

Observation is also for those who are afraid

Adolescents and young adults, especially at the first dose of a new vaccine, have the highest rates of vasovagal reaction (Okuyama, 2022). Vaccinating while seated or lying down, asking about previous fainting and keeping the person on site for the recommended time reduce the risk of falls. A student who worsens silently recalls what the text about the child who worsens discusses: reassessment is part of observation.

Revaccinating after a reaction

The next question is always the same: can the person receive the next vaccine? Data from covid vaccines showed that the answer is most often yes, after evaluation.

At an American center, 80 people with a reported allergic reaction after the first dose of an mRNA vaccine underwent skin tests with excipients. Of the 70 who received the second dose, 62 had no reaction or a mild reaction treated with an antihistamine, and 2 needed adrenaline. The skin test result did not predict who would tolerate the vaccine (Wolfson, 2021).

In southern Denmark, among 199,377 doses, 9 people met the Brighton criteria for anaphylaxis. Of the 55 people with a reaction to the first dose, 52 were revaccinated without an adverse reaction. The evaluation found three allergies to medication excipients and one case of mastocytosis, which shows the value of a consultation with an allergist (Rasmussen, 2021).

In Germany, 73 of every 100 patients with confirmed anaphylaxis after covid vaccines were revaccinated with precautions and without recurrence (Barth, 2023).

What weighs in the decision

The decision involves the certainty of the initial diagnosis, the suspected component, the availability of alternatives and the risk of the disease the vaccine prevents. Many reactions labeled as allergic were in fact stress responses or nonspecific symptoms, and a wrong label can leave the person without protection. At the same time, anyone with confirmed anaphylaxis should be evaluated before the next dose, in a prepared setting.

Reasoning errors that show up in the vaccination room

The first error is the availability shortcut: whoever has just seen a fainting episode tends to call the next case vasovagal, and whoever has just read about anaphylaxis tends to see allergy in every pale patient. Classic heuristics help recognize this bias.

The second is waiting for confirmation before treating. Because intramuscular adrenaline has no contraindication in anaphylaxis and delay worsens the outcome, giving it when there is reasonable doubt is the right decision (Dribin, 2023). Deciding without certainty is a skill that can be trained, as discussed in the text on diagnostic uncertainty.

The third is labeling forever. A note of "vaccine allergy" without a description of what happened can block future vaccines unnecessarily. Recording signs, times, treatment and response lets another professional assess the case later. In the conversation with the family, it is worth checking what was understood with teach-back and adjusting the confidence of the words used, as diagnostic calibration proposes.

How NEXO helps train this reasoning

In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues, and you propose the diagnosis by searching for the ICD-10 code. Separating pallor with a slow pulse from urticaria with a fast pulse is the kind of clue that changes the hypothesis from one round to the next.

The limits of use of the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information under curation and under creators, and institutions can learn about the proposal on the partnerships page. To train the search for the code, see how the ICD-10 code is built.

Frequently asked questions

How often does anaphylaxis occur after a vaccine?

In a large American database, about 1.3 per million doses (McNeil, 2019). For covid vaccines, rates ranged from a few to a few dozen per million, depending on the vaccine and the method (Shimabukuro, 2021; Lee, 2021).

How long after the vaccine does anaphylaxis start?

Most often within minutes. After the mRNA vaccine, the median was 13 minutes, and 86 out of every 100 people had symptoms in the first 30 minutes (Shimabukuro, 2021).

How do you tell anaphylaxis from fainting?

Vasovagal fainting usually comes with pallor, cold sweat and a slow pulse, and improves when the person lies down. Anaphylaxis brings skin, respiratory or circulatory signs that progress. The vasovagal reaction is far more common (Imai, 2022).

What is the first treatment?

Intramuscular adrenaline, without delay and without contraindication in this situation (Shimabukuro, 2021; Dribin, 2023).

Does an antihistamine solve it?

An antihistamine relieves skin symptoms, but adrenaline comes first. Even adrenaline can fail in severe hypotension, which requires escalation (Weiss-Tessbach, 2026; Pouessel, 2024).

Can someone who had a reaction take the next vaccine?

Many people can, after assessment with an allergist. In series of covid vaccines, most were revaccinated without recurrence (Rasmussen, 2021; Barth, 2023).

Does egg allergy prevent the influenza vaccine?

The American recommendation allows vaccinating people with egg allergy; those who had a severe reaction should be vaccinated in a supervised setting (McNeil, 2019).

Take-home messages

• Post-vaccine anaphylaxis is rare, on the order of 1 to a few dozen per million doses depending on the vaccine.
• Most cases start in the first 15 to 30 minutes, which justifies observation.
• The vasovagal reaction is far more common and improves when the person lies down; anaphylaxis progresses.
• When in doubt with signs of severity, immediate intramuscular adrenaline.
• Lay the patient down with the legs raised; never stand a patient in collapse up.
• Lack of response after two doses of adrenaline requires escalation.
• Recording what happened and assessing with an allergist allows most people to be revaccinated.

Play the case of the day on NEXO.

References

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2. Shimabukuro T, Nair N. Allergic Reactions Including Anaphylaxis After Receipt of the First Dose of Pfizer-BioNTech COVID-19 Vaccine. JAMA. 2021. PMID 33475702. DOI 10.1001/jama.2021.0600.
3. Roh EJ, Lee MH, Song KB, Lee YK, et al. Vaccine-related Anaphylaxis Cases Confirmed by KCDC from 2001–2016. J Korean Med Sci. 2020. PMID 32989932. DOI 10.3346/jkms.2020.35.e337.
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5. Hurley S, Hourihane JO. We should abandon the Brighton Collaboration criteria for vaccine-associated anaphylaxis. Ann Allergy Asthma Immunol. 2022. PMID 35717131. DOI 10.1016/j.anai.2022.01.016.
6. Shaker MS, Golden D, Wang J, Waserman S, et al. Anaphylaxis: Definition and criteria. J Food Allergy. 2024. PMID 39257603. DOI 10.2500/jfa.2024.6.240002.
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9. Weiss-Tessbach M, Dizdarevic AM, Bischof T, Firbas C, et al. Effect of Intramuscular Adrenaline on Histamine-Induced Hypotension: A Randomised Placebo-Controlled Pilot Trial. Allergy. 2026. PMID 41771778. DOI 10.1111/all.70277.
10. Imai K, Tanaka F, Kawano S, Esaki K, et al. Incidence and Risk Factors of Immediate Hypersensitivity Reactions and Immunization Stress-Related Responses With COVID-19 mRNA Vaccine. J Allergy Clin Immunol Pract. 2022. PMID 35953016. DOI 10.1016/j.jaip.2022.07.027.
11. Okuyama M, Morino S, Tanaka K, Nakamura-Miwa H, et al. Vasovagal reactions after COVID-19 vaccination in Japan. Vaccine. 2022. PMID 36068111. DOI 10.1016/j.vaccine.2022.08.056.
12. Wolfson AR, Robinson LB, Li L, McMahon AE, et al. First-Dose mRNA COVID-19 Vaccine Allergic Reactions: Limited Role for Excipient Skin Testing. J Allergy Clin Immunol Pract. 2021. PMID 34166844. DOI 10.1016/j.jaip.2021.06.010.
13. Rasmussen TH, Mortz CG, Georgsen TK, Rasmussen HM, et al. Patients with suspected allergic reactions to COVID-19 vaccines can be safely revaccinated after diagnostic work-up. Clin Transl Allergy. 2021. PMID 34331522. DOI 10.1002/clt2.12044.
14. Barth I, Weißer K, Gaston-Tischberger D, Mahler V, et al. Anaphylactic reactions after COVID-19 vaccination in Germany. Allergol Select. 2023. PMID 37056447. DOI 10.5414/ALX02374E.