Should You Get a Vaccine When You're Sick?

Why does mild illness not usually require a delay?

The underlying concern is understandable: if the immune system is busy fighting an infection, will the vaccine be less effective, or will it make you feel worse? The evidence-based answer is that mild illness does not meaningfully interfere with the immune response to most vaccines, and a mild cold does not put you at meaningful additional risk from the vaccination itself 1Ref 1Wodi AP, Issa AN, Moser CA, Cineas S (2025).Advisory Committee on Immunization Practices Recommended Immunization Schedule for Adults Aged 19 Years or Older — United States, 2025.ACIP general best-practice guidance that inactivated vaccines are safe in the context of mild illness; live-vaccine precautions in immunocompromised individuals.

ACIP's General Best Practice Guidelines state explicitly that children with mild acute illness — including low-grade fever, upper respiratory infection, ear infection, or mild diarrhea — should be vaccinated on schedule, and that low-grade fever alone is not a contraindication 2Ref 2Centers for Disease Control and Prevention (2024).Chapter 2: General Best Practice Guidance for Immunization (Epidemiology and Prevention of Vaccine-Preventable Diseases, The Pink Book).ACIP guidelines that mild illness — including low-grade fever, URI, ear infection, or mild diarrhea — is NOT a contraindication to vaccination; moderate or severe illness is a precaution warranting deferral. Postponing vaccines for every minor sniffle leads to missed and delayed doses, leaving people unprotected longer than necessary.

When is it reasonable to wait?

Moderate to severe illness. ACIP guidance lists moderate or severe acute illness as a precaution — vaccination should generally be deferred until the illness has improved 2Ref 2Centers for Disease Control and Prevention (2024).Chapter 2: General Best Practice Guidance for Immunization (Epidemiology and Prevention of Vaccine-Preventable Diseases, The Pink Book).ACIP guidelines that mild illness — including low-grade fever, URI, ear infection, or mild diarrhea — is NOT a contraindication to vaccination; moderate or severe illness is a precaution warranting deferral. A significant fever, inability to sit up, or signs of a serious infection are reasonable reasons to reschedule.

Conditions that suppress the immune system. People with immune-suppressing conditions or on relevant medications should discuss vaccine timing with a provider even when feeling well. Live vaccines (MMR, varicella, live-attenuated flu nasal spray) require particularly individualized timing in this group 1Ref 1Wodi AP, Issa AN, Moser CA, Cineas S (2025).Advisory Committee on Immunization Practices Recommended Immunization Schedule for Adults Aged 19 Years or Older — United States, 2025.ACIP general best-practice guidance that inactivated vaccines are safe in the context of mild illness; live-vaccine precautions in immunocompromised individuals.

Active, contagious illness. Going to a clinic or pharmacy while actively contagious may expose others — including immunocompromised patients in the waiting room. Call ahead.

Unable to participate in the informed consent discussion. If a child or adult is too ill to engage clearly, waiting until they feel better is reasonable practical judgment.

Does the type of vaccine matter?

Most vaccines given to healthy adults use inactivated or non-replicating ingredients — the flu shot, COVID-19 vaccine, Tdap, pneumococcal, shingles, and HPV vaccines all fall into this category. They do not contain live virus and pose no meaningful additional risk with a mild illness 1Ref 1Wodi AP, Issa AN, Moser CA, Cineas S (2025).Advisory Committee on Immunization Practices Recommended Immunization Schedule for Adults Aged 19 Years or Older — United States, 2025.ACIP general best-practice guidance that inactivated vaccines are safe in the context of mild illness; live-vaccine precautions in immunocompromised individuals.

Live attenuated vaccines (MMR, varicella, yellow fever, and the nasal-spray flu vaccine) contain a weakened but living virus. Even for these, mild illness is not typically a contraindication 2Ref 2Centers for Disease Control and Prevention (2024).Chapter 2: General Best Practice Guidance for Immunization (Epidemiology and Prevention of Vaccine-Preventable Diseases, The Pink Book).ACIP guidelines that mild illness — including low-grade fever, URI, ear infection, or mild diarrhea — is NOT a contraindication to vaccination; moderate or severe illness is a precaution warranting deferral. However, any immune-compromising condition warrants a more careful timing conversation with a provider — regardless of current illness.

If you are unsure which category your vaccine falls into, the pharmacist or nurse administering it can tell you.

What is the practical step to take?

If you are scheduled for vaccination and wake up feeling unwell, call the clinic or pharmacy, describe your symptoms, and ask whether to come in as planned, reschedule, or come in for an assessment first.

For routine adult vaccines like the flu shot, most pharmacies will proceed with a mild cold as long as there is no significant fever. For childhood vaccine visits, the pediatrician's office makes the call based on how the child presents that day. The key factors your clinician will weigh: - How significant is the illness? A runny nose is very different from influenza or a GI illness with vomiting. - Is there a fever, and how high? - Is this vaccine time-sensitive (e.g., flu shot in peak season) or more easily rescheduled (e.g., shingles series)? - Are you pregnant? Recommended pregnancy vaccines should not be indefinitely delayed due to mild illness 3Ref 3Centers for Disease Control and Prevention (2024).Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2024–25 Influenza Season.Guidance on flu vaccine timing during peak season and the importance of not indefinitely delaying vaccination for mild symptoms, including in pregnant individuals.