False contraindications: the bias that delays vaccines

A mother arrives at the clinic with her 6-month-old son for the scheduled vaccines. The boy has a runny nose, no fever, and took amoxicillin for an ear infection that ended yesterday. The professional hesitates, asks whether he has already eaten egg and suggests coming back "when he is well". The family leaves without the vaccine. Maybe they will return in a month, maybe in a year.

This scene looks like care. In practice, it is a missed opportunity. Runny nose without fever, antibiotic use and egg allergy are among the situations that guidelines list as false contraindications (UKHSA, 2017; Kelso, 2022). Postponement, on the other hand, has consequences: the child is left unprotected during the interval, and each extra return is a new chance for the family to give up.

This text shows the difference between contraindication, precaution and myth, reviews the most studied myths, measures how often professionals get it wrong and explains why postponing seems safer than it is.

Key points

• Almost everyone can safely receive almost every vaccine; true contraindications are few (UKHSA, 2017).
• Mild illness without fever, antibiotic use, prematurity, asthma and a family history of reaction are false contraindications (UKHSA, 2017).
• Egg allergy, even severe, requires no extra precautions with the influenza vaccine (Grohskopf, 2024).
• In Quito, 98 out of every 100 professionals denied at least one indicated vaccine in simulated scenarios (Andrade-Guerrero, 2022).
• Children under-vaccinated against pertussis had up to 4.8 times higher risk of the disease (Rane, 2021).

Summary

• What a false contraindication is
• Contraindication, precaution and postponement
• The most common myths in the vaccination room
• Egg allergy: the most studied myth
• How often professionals get it wrong
• The risk of postponement: the missed opportunity
• Why postponing seems prudent
• How NEXO helps train this reasoning
• Frequently asked questions
• Take-home messages

What a false contraindication is

A contraindication is a condition that greatly increases the risk of a serious adverse event with a specific vaccine. A false contraindication is a condition that the professional or the family treats as an impediment, but that guidelines consider compatible with vaccination. The distinction seems bureaucratic. It decides whether the child leaves the appointment protected or unprotected.

The contraindications chapter of the Green Book, the United Kingdom immunization manual, opens with a sentence that sums up the topic: almost everyone can be safely vaccinated with all vaccines, and only in very few is vaccination contraindicated or should be postponed. When in doubt, the recommendation is to seek guidance from a specialist instead of withholding the vaccine (UKHSA, 2017).

The World Health Organization and national technical groups arrive at similar rules through similar paths, as the text on how WHO and countries decide the vaccination schedule shows. The problem is rarely in the rule. It is in the distance between the rule and the decision made at the vaccination room counter.

The Green Book list

The Green Book explicitly lists what does not contraindicate routine vaccination (UKHSA, 2017):

• family history of adverse reaction to a vaccine;
• previous history of the disease, except BCG in those with evidence of prior exposure to tuberculosis;
• contact with an infectious disease;
• premature birth and weight below a certain value;
• asthma, eczema or allergic rhinitis;
• mild, self-limited illness without fever, such as a runny nose;
• treatment with antibiotics and topical or inhaled corticosteroids;
• a pregnant mother or other household member, and breastfeeding;
• previous neonatal jaundice;
• age above the recommended in the schedule, except for rotavirus;
• personal or family history of febrile seizure or epilepsy;
• recent or upcoming elective surgery, and upcoming general anesthesia;
• unknown or poorly documented vaccination history;
• G6PD deficiency and food intolerances.

Several items on this list are exactly the ones that appear as reasons for postponement in surveys with professionals. Some require additional precautions described in the chapters for each vaccine, but none, on its own, justifies sending the person home without a vaccine.

Contraindication, precaution and postponement

Three categories organize the decision. The first is a true contraindication, which rules out that vaccine. The second is a situation that calls for temporary postponement. The third is everything that falls outside the first two, that is, most routine situations.

What actually contraindicates

General contraindications are few. Confirmed anaphylaxis to a previous dose of the same vaccine or to one of its components rules out that specific vaccine, but other vaccines can and should be given. Severe immunodeficiencies, primary or acquired, and current or recent immunosuppressive therapy contraindicate live attenuated vaccines, which can replicate uncontrolled. Pregnancy also contraindicates live vaccines, as a theoretical precaution (UKHSA, 2017).

Even within these categories, contraindication is rarely automatic. In some situations, the benefit of the vaccine outweighs the risk; in others, vaccination should be postponed, not abandoned (UKHSA, 2017). Post VC04 in this series, on anaphylaxis after vaccination, covers the investigation of people who had a severe reaction and the decision to revaccinate.

When postponing makes sense

Legitimate postponement is rare. Mild illness without fever or systemic involvement does not justify postponing. If the person has an acute illness, for example with fever above 38.5 °C, the vaccine can wait until full recovery. The main reason is to avoid a new symptom or the progression of the illness being attributed to the vaccine (UKHSA, 2017).

The same logic applies to neurological diseases. Having a stable neurological condition is compatible with vaccination; ongoing neurological deterioration may justify postponing, so as not to confuse the progression of the disease with a vaccine effect. In that case, the Green Book asks that the risk of postponing be weighed against the risk of infection and that the vaccine be given as soon as the picture becomes clear (UKHSA, 2017).

Another expected postponement involves immunoglobulin. Donor antibodies can neutralize live virus vaccines, which, when there is no urgency, should be given at least three weeks before or three months after immunoglobulin. There are exceptions, such as yellow fever and BCG (UKHSA, 2017).

Corticosteroids and immunosuppression

Corticosteroids are one of the most common reasons for mistaken refusal. Inhaled, topical or intra-articular corticosteroids do not cause relevant systemic immunosuppression and do not contraindicate even live vaccines. Replacement therapy in people with adrenal insufficiency does not either. The limit that matters for live vaccines is systemic corticosteroid at a high daily amount for more than one week, or at a lower amount for more than 14 days, according to the manual's own criteria (UKHSA, 2017).

It is worth remembering that inactivated vaccines do not replicate and can be given to immunosuppressed people, even though the response may be lower (UKHSA, 2017). Refusing an inactivated vaccine because of corticosteroids is confusing two different rules.

The most common myths in the vaccination room

Some reasons for postponement appear in practically every study. The table summarizes what the sources say about each one.

Situation	What usually happens	What the sources say
Runny nose, otitis or mild diarrhea without fever	Postpone "until it gets better"	Mild illness without fever is a false contraindication (UKHSA, 2017)
Antibiotic use	Wait for the treatment to end	Antibiotics are listed among false contraindications (UKHSA, 2017)
Egg allergy	Refuse influenza or MMR, or send to the hospital	Egg allergy requires no extra measures for influenza (Grohskopf, 2024); children with egg allergy should receive MMR in primary care (UKHSA, 2017)
Fever or local pain after a previous dose	Suspend the next one	Local reactions and fever are common and do not contraindicate the next dose (Dreskin, 2016)
Prematurity or low weight	Wait to gain weight	Prematurity and low weight are false contraindications (UKHSA, 2017)
Febrile seizure or epilepsy in the family	Postpone the pertussis vaccine	A personal or family history of febrile seizure or epilepsy is a false contraindication (UKHSA, 2017)
Inhaled or topical corticosteroid	Refuse live vaccines	Non-systemic corticosteroid is compatible even with live vaccines (UKHSA, 2017)
Pregnant person in the household	Postpone the child's vaccine	Pregnancy of someone in the household is a false contraindication (UKHSA, 2017)

Previous reaction that was not allergy

Many families and professionals treat anything that happened after a vaccine as an allergy. Pain, redness, swelling at the site and fever are common and expected reactions, and the international consensus on allergic reactions to vaccines states that they do not contraindicate future doses (Dreskin, 2016).

Even when the reaction resembles an allergy, the assessment usually clears the vaccine. In Rome, 153 children referred for a "reaction to a previous dose" underwent specialist assessment and skin testing. Only 13 had symptoms suggestive of an IgE-mediated reaction, 151 had a negative test and completed the schedule, and none had a post-vaccination reaction during the reported follow-up (Nicolosi, 2014). A review of ten years of practice warns that anyone with possible anaphylaxis should be assessed by an allergist instead of simply being labeled allergic, because most can receive the following doses (Kelso, 2022).

Preterm and low birth weight

Preterm infants are vaccinated by chronological age, not corrected age; in the Israeli schedule, the exception is hepatitis B, which requires a minimum weight of 2,000 grams (Bary-Weisberg, 2020). Even so, delay is common. In an Israeli national cohort of 181,543 babies born in 2016, low-weight infants started the schedule later, with a chance of delay about a quarter higher for the first dose of the vaccine combined with pertussis (OR 1.26), and delays were greater below 1,000 grams. By 24 months, coverage had equalized (Bary-Weisberg, 2020).

The final data is reassuring, but it hides the problem. The delay is concentrated precisely in the first months, when the preterm infant is most vulnerable to severe infections such as pertussis. Waiting for the baby to "gain weight" is one of the false contraindications listed in the Green Book (UKHSA, 2017).

Egg allergy: the most studied myth

Some vaccines are produced in embryonated eggs or in cultures derived from chicken embryo, and this led to decades of caution. Most influenza vaccines are grown in eggs and contain small amounts of ovalbumin, which vary between manufacturers and batches (Dreskin, 2016). The question "does your child have an egg allergy?" became routine in screening.

Influenza

The evidence dismantled this caution. In a multicenter study in the United Kingdom, 779 children and adolescents aged 2 to 18 with egg allergy received the live attenuated nasal influenza vaccine, which contains egg protein. Of the total, 270 had already had anaphylaxis to egg, and 157 had respiratory or cardiovascular symptoms. No participant had a systemic allergic reaction; 9 had mild symptoms consistent with a local reaction (Turner, 2015).

ACIP, the United States immunization committee, now recommends that everyone aged 6 months and older with egg allergy receive the influenza vaccine, with any vaccine appropriate for age, egg-based or not. Egg allergy alone requires no safety measure beyond those recommended for any vaccine, whatever the severity of the previous reaction to egg (Grohskopf, 2024). The Kelso review goes further: there is no need to ask about egg allergy before the influenza vaccine, not even on screening forms (Kelso, 2022).

MMR

With MMR, the myth persists strongly. The Green Book states that anaphylactic reactions to MMR are not associated with egg hypersensitivity and that all children with egg allergy should receive it as routine, in primary care (UKHSA, 2017).

A 2026 Polish study shows the effect of the myth. In 39 children with egg allergy admitted to receive MMR, 15 with previous anaphylaxis to egg, none had a systemic reaction; one had a rash that improved with an antihistamine. Even so, 63 out of 100 had already had the vaccine postponed, and the longest delay reached 113 months (Balas, 2026). Almost ten years without protection against measles, because of a precaution that the evidence had already ruled out.

The gelatin exception

A general rule emerges from these studies: almost all patients tolerate vaccines that contain substances to which they are allergic. The known exception is gelatin, present in milligrams in some vaccines and linked to most of the anaphylactic reactions described for them. For those with confirmed gelatin allergy, the review proposes skin testing with the vaccine and, if positive, fractionated dosing (Kelso, 2022).

How often professionals get it wrong

False contraindications survive because health professionals also believe in them. Surveys that test this knowledge with clinical scenarios find similar results in very different contexts.

Ecuador

In Quito, 273 health professionals, including specialists, general practitioners, nurses and technicians, responded to 18 clinical scenarios. Almost all of them, 98 out of 100, withheld the vaccine in at least one scenario where it was indicated, and only five got the entire questionnaire right. Mild or moderate fever was the most frequent reason for refusal, cited by 89 out of 100. Corticosteroids, autoimmune disease and egg allergy also led to improper refusals in 71, 73 and 59 out of 100 responses, and just over half would vaccinate a child taking an antibiotic (Andrade-Guerrero, 2022).

Italy

In Italy, 892 pediatricians classified 11 situations as a permanent contraindication, a temporary one or a false one for the second dose of the hexavalent vaccine. Only 48, about 5 out of 100, got them all right. Otitis without fever was seen as a temporary contraindication by 37 out of 100; upper airway infection without fever, by 27; severe acute gastroenteritis split the pediatricians almost in half. A third of the pediatricians said they did not systematically check whether patients were up to date with their schedule (Filia, 2019).

These numbers matter because the professional's recommendation is among the strongest factors in the family's decision. The text on vaccine hesitancy and the 5C model shows how the way of recommending changes the parents' response. A postponement suggested by the professional sends the family the message that the vaccine carries some hidden risk.

The risk of postponing: the missed opportunity

The World Health Organization calls a missed vaccination opportunity the situation in which an eligible person, with no contraindication, goes through a health service and leaves without all the vaccines they could have received. The Expanded Programme on Immunization has recommended since 1983 using every consultation as an opportunity to vaccinate, whatever the reason for the visit (Borras-Bermejo, 2022).

How many opportunities are missed

In 19 services supported by the organization Doctors Without Borders in six low-income countries, 2,738 children up to 5 years old had a vaccination card. Among them, 63 out of 100 were eligible for some vaccine, and 65 out of 100 of those left without it. The loss was greater in hospitals and in consultations for another reason, and much greater after the first year of life. The authors point to the belief that a sick child cannot be vaccinated as one of the explanations (Borras-Bermejo, 2022).

In Pakistan, an analysis of a household survey with more than 100,000 children estimated that about 1 in every 4 children aged 1 to 5 missed at least one dose of the pentavalent or measles vaccine during a mother's visit to a health service (Fatmee, 2026). The mother was at the service. The child was eligible. The vaccine was left for later.

What the delay causes

Whooping cough shows the effect clearly. In a population cohort of 316,404 children in the United States, those under-vaccinated in the primary series, in the second-year booster and in the preschool booster had a risk of whooping cough 4.8, 3.2 and 4.6 times higher than those fully vaccinated. Short delays were not associated with higher risk, but the authors warn that postponing on purpose is not recommended, because protection depends on the number, the timing and the interval between doses (Rane, 2021).

A meta-analysis of 22 studies reached a similar conclusion. Children from hesitant families, with no doses at all, had about four times the chance of whooping cough (pooled OR of 4.12) compared with those fully vaccinated (Wang, 2023). Postponement due to a false contraindication produces the same result as hesitancy, only on the professional's initiative.

Why postponing seems prudent

If the rules are clear, why do so many professionals postpone? Part of the answer is lack of knowledge, as the questionnaires show. Another part is the way the brain weighs action and omission.

Omission bias

Omission bias is the tendency to judge harm caused by an action as worse than equal or greater harm caused by not acting. If the vaccinated child with a runny nose has a fever the next day, the fever will be attributed to the vaccine and to whoever gave it. If the unvaccinated child catches whooping cough three months later, nobody will link the disease to the postponement. Action leaves a trace; omission almost never does.

The same mechanism appears in outcome bias, in which the decision is judged by the outcome and not by the quality of the reasoning. And it adds to affective bias: the fear of being blamed for an adverse event weighs more than an abstract and distant risk.

Heuristics and numbers

The availability heuristic also contributes. A serious adverse event seen once stays in memory for years, while the hundreds of problem-free vaccinations vanish. The antidote is to think in frequencies, as the text on base rate proposes: how many serious reactions happen per million vaccinations, compared with how many cases of disease happen in those who go without the vaccine.

Deciding with uncertainty

Some doubts persist until the end of the consultation. When that happens, the way out is the same one that applies to other clinical decisions: separate what is a real doubt from what is discomfort, use the written rule and, if needed, consult someone before sending the family away. The text on strategies for deciding without certainty and the one on benefits and harms in clinical decisions organize this reasoning.

In the conversation with the family, explaining why a runny nose does not prevent vaccination, and checking what was understood with teach-back, reduces the chance that the same doubt returns at the next visit.

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. Training how to read clues helps separate what changes management from what only seems important, the same skill that separates a real contraindication from a myth.

The limits on using 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 searching for the code, see how the ICD-10 code is built, and to adjust the safety of your own answers, see diagnostic calibration.

Frequently asked questions

Can a child with a mild cold get vaccinated?

Yes. A mild, self-limiting illness without fever, such as a runny nose, is a false contraindication. If there is an acute illness with fever above 38.5 °C, the vaccine can be postponed until full recovery (UKHSA, 2017).

Can someone taking antibiotics be vaccinated?

Yes. Treatment with antibiotics appears on the Green Book list of false contraindications (UKHSA, 2017).

Does an egg allergy prevent the influenza vaccine?

Only in the imagination. ACIP recommends vaccinating everyone with an egg allergy, with any vaccine appropriate for age and without extra measures, regardless of the severity of the reaction to egg (Grohskopf, 2024).

And the MMR vaccine in someone with an egg allergy?

It should be given as routine care in primary care, because anaphylactic reactions to MMR are not linked to egg allergy (UKHSA, 2017; Balas, 2026).

Should a premature infant wait to gain weight before vaccination?

Premature infants are vaccinated by chronological age. Prematurity and low weight are false contraindications (UKHSA, 2017; Bary-Weisberg, 2020).

Does fever after the last vaccine prevent the next one?

Local reactions and fever after vaccination are common and do not contraindicate the next dose (Dreskin, 2016).

What actually contraindicates a vaccine?

Confirmed anaphylaxis to a previous dose or to a component of that vaccine; for live vaccines, also severe immunodeficiency, relevant immunosuppression and pregnancy (UKHSA, 2017).

Take-home messages

• True contraindications are few and specific to each vaccine.
• Runny nose without fever, antibiotics, prematurity and asthma are false contraindications.
• High fever with acute illness allows postponing until recovery, without cancelling the vaccine.
• Egg allergy requires no extra precautions with influenza or MMR.
• A local reaction or fever after a vaccine does not prevent the next dose.
• Every visit is an opportunity to check and update the schedule.
• Postponing is also a decision, with risks that omission bias hides.

Play today's case on NEXO.

References

1. UK Health Security Agency. Contraindications and special considerations: the green book, chapter 6. Immunisation against infectious disease. Updated in August 2017. gov.uk.
2. Andrade-Guerrero F, Tapia A, Andrade V, Vásconez-González J, et al. False Contraindications for Vaccinations Result in Sub-Optimal Vaccination Coverage in Quito, Ecuador: A Cross-Sectional Study. Vaccines (Basel). 2022. PMID 36679905. DOI 10.3390/vaccines11010060.
3. Filia A, Bella A, D'Ancona F, Fabiani M, et al. Childhood vaccinations: knowledge, attitudes and practices of paediatricians and factors associated with their confidence in addressing parental concerns, Italy, 2016. Euro Surveill. 2019. PMID 30755294. DOI 10.2807/1560-7917.ES.2019.24.6.1800275.
4. Borras-Bermejo B, Panunzi I, Bachy C, Gil-Cuesta J. Missed opportunities for vaccination (MOV) in children up to 5 years old in 19 Médecins Sans Frontières-supported health facilities: a cross-sectional survey in six low-resource countries. BMJ Open. 2022. PMID 35882455. DOI 10.1136/bmjopen-2021-059900.
5. Fatmee A, Nisa ZU, Ibrahim M, Irum A, et al. Missed Opportunities for Childhood Immunization in Pakistan: Failure to Vaccinate Despite Health System Contact. Gates Open Res. 2026. PMID 42358653. DOI 10.12688/gatesopenres.16388.1.
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7. Grohskopf LA, Ferdinands JM, Blanton LH, Broder KR, et al. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices – United States, 2024–25 Influenza Season. MMWR Recomm Rep. 2024. PMID 39197095. DOI 10.15585/mmwr.rr7305a1. Original title with an em dash, replaced by an en dash.
8. Turner PJ, Southern J, Andrews NJ, Miller E, et al. Safety of live attenuated influenza vaccine in young people with egg allergy: multicentre prospective cohort study. BMJ. 2015. PMID 26645895. DOI 10.1136/bmj.h6291.
9. Balas WM, Kaczor M, Strzelecka J, Sybilski AJ. MMR Vaccination and Hen's Egg Allergy: Bridging the Gap Between Evidence and Clinical Practice. Vaccines (Basel). 2026. PMID 42347632. DOI 10.3390/vaccines14060511.
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12. Rane MS, Rohani P, Halloran ME. Association of Diphtheria-Tetanus–Acellular Pertussis Vaccine Timeliness and Number of Doses With Age-Specific Pertussis Risk in Infants and Young Children. JAMA Netw Open. 2021. PMID 34374773. DOI 10.1001/jamanetworkopen.2021.19118.
13. Wang Y, Shi N, Wang Q, Yang L, et al. The association between vaccine hesitancy and pertussis: a systematic review and meta-analysis. Ital J Pediatr. 2023. PMID 37443026. DOI 10.1186/s13052-023-01495-8.
14. Bary-Weisberg D, Stein-Zamir C. Vaccination timeliness and completeness among preterm and low birthweight infants: a national cohort study. Hum Vaccin Immunother. 2020. PMID 33325771. DOI 10.1080/21645515.2020.1840255.