There are legitimate medical reasons some people should not get a COVID-19 vaccine, or should delay getting one. These fall into a few categories: known allergies to vaccine ingredients, recent health events that require a waiting period, and rare but documented side effects that shift the risk calculus for specific groups. Here’s what the evidence actually shows.
Allergies to Vaccine Ingredients
The clearest reason to avoid a specific COVID-19 vaccine is a confirmed allergy to one of its ingredients. The mRNA vaccines from Pfizer and Moderna contain a small amount of polyethylene glycol (PEG), a compound also found in laxatives, skin creams, and some medications. Each dose of the Pfizer vaccine contains 0.05 mg of PEG linked to a lipid. For people with a documented PEG allergy, mRNA vaccines are generally considered contraindicated.
The now-discontinued Johnson & Johnson vaccine used a different formula without PEG but contained polysorbate 80, which can cross-react with PEG in some patients. If you’ve had a severe allergic reaction to PEG or polysorbate in any product, not just a vaccine, that’s worth discussing with an allergist before getting vaccinated. An international expert panel recommends shared decision-making between the patient and an allergist in these cases, since the risk of a reaction is uncertain and varies from person to person.
A severe allergic reaction (anaphylaxis) after a first dose of any COVID-19 vaccine is also a reason not to receive additional doses of the same vaccine type.
Heart Inflammation in Young Men
Myocarditis and pericarditis, forms of heart inflammation, are the most well-documented serious side effects of mRNA COVID-19 vaccines. The risk is highest in men ages 18 to 25 after their second dose. A large U.S. cohort study published in The Lancet found the incidence rate in this group was roughly 1.7 per 100,000 person-days for Pfizer and 2.2 per 100,000 person-days for Moderna in the week following the second shot. Women in the same age range had significantly fewer cases.
Most vaccine-associated myocarditis cases are mild and resolve within days to weeks, which distinguishes them from the more severe myocarditis that can follow a COVID-19 infection itself. Still, for young men who have already had myocarditis from any cause, or who developed heart inflammation after a previous dose, additional mRNA doses require careful consideration with a cardiologist.
Blood Clots With the J&J Vaccine
The Johnson & Johnson (Janssen) vaccine was linked to a rare but serious condition called thrombosis with thrombocytopenia syndrome (TTS), where unusual blood clots form alongside dangerously low platelet counts. U.S. data estimated the incidence at about 3.83 cases per million doses, with the risk concentrated among women under 50. The CDC recommended alternative vaccines for this group in April 2021.
For context, mRNA vaccines carried a far lower TTS signal: roughly 0.009 cases per million doses. The J&J vaccine is no longer available in the United States, and the AstraZeneca adenoviral vector vaccine (which carried similar clotting risks) has also been withdrawn in many countries. So this concern is largely historical for people getting vaccinated today, since the only vaccines currently offered in the U.S. are mRNA-based.
Guillain-Barré Syndrome
CDC studies found evidence of an increased risk of Guillain-Barré syndrome (GBS), a nerve disorder that causes muscle weakness and sometimes temporary paralysis, after the J&J vaccine. This signal was not found with Pfizer or Moderna vaccines. Since the J&J vaccine is no longer in use, this risk does not apply to currently available options. People with a history of GBS should still mention it before vaccination, but it is not listed as a contraindication for mRNA vaccines.
Recent COVID-19 Infection
If you recently tested positive for COVID-19, the CDC says you may delay vaccination for three months after your symptoms started, or three months after a positive test if you had no symptoms. This isn’t because the vaccine is dangerous during this window. Your immune system is already primed from the recent infection, so there’s little additional benefit from vaccinating right away. Waiting allows the immune response to mature and means the eventual vaccine dose acts more like a booster, producing stronger and longer-lasting protection.
After Multisystem Inflammatory Syndrome
Multisystem inflammatory syndrome (MIS-C in children, MIS-A in adults) is a rare but serious condition that can follow COVID-19 infection, causing widespread inflammation in the heart, lungs, kidneys, and other organs. The CDC recommends waiting at least 90 days after a MIS diagnosis before getting vaccinated, and only after clinical recovery, including a return to normal heart function. Vaccination can still be considered before those criteria are met, but that decision is best made with the clinical team managing recovery.
Stroke Concerns Were Investigated and Not Confirmed
In early 2023, the FDA flagged a preliminary safety signal suggesting people 65 and older might face a slightly higher risk of ischemic stroke in the three weeks after receiving the Pfizer bivalent booster. This got significant attention. However, follow-up investigations using Medicare data, Veterans Affairs records, the VAERS reporting system, Pfizer’s global safety database, and surveillance from other countries all found no increased stroke risk. The FDA shared the initial signal publicly out of transparency but concluded it was very unlikely to represent a true clinical risk.
What This Means in Practice
The list of people who genuinely should not receive a COVID-19 vaccine is short: those with confirmed allergies to specific vaccine ingredients, those who had anaphylaxis or myocarditis after a previous dose, and those in a temporary waiting period after certain health events. For everyone else, including young children (the American Academy of Pediatrics recommends vaccination for infants as young as 6 months), the documented benefits of vaccination outweigh the known risks.
If you’re weighing whether to get vaccinated and have a specific health condition that concerns you, the most productive step is identifying whether your situation falls into one of these defined categories. Many of the risks that generated headlines, like blood clots and GBS, were tied to vaccines that are no longer in use. The mRNA vaccines that remain available carry a well-characterized safety profile after billions of doses administered worldwide.